[v1,2/2] x86: Optimize memcmp-evex-movbe.S for frontend behavior and size

  No bug.

The frontend optimizations are to:
1. Reorganize logically connected basic blocks so they are either in
   the same cache line or adjacent cache lines.
2. Avoid cases when basic blocks unnecissarily cross cache lines.
3. Try and 32 byte align any basic blocks possible without sacrificing
   code size. Smaller / Less hot basic blocks are used for this.

Overall code size shrunk by 168 bytes. This should make up for any
extra costs due to aligning to 64 bytes.

In general performance before deviated a great deal dependending on
whether entry alignment % 64 was 0, 16, 32, or 48. These changes
essentially make it so that the current implementation is at least
equal to the best alignment of the original for any arguments.

The only additional optimization is in the page cross case. Branch on
equals case was removed from the size == [4, 7] case. As well the [4,
7] and [2, 3] case where swapped as [4, 7] is likely a more hot
argument size.

test-memcmp and test-wmemcmp are both passing.
---
Performance results attached in reply.

Benchmark CPU: https://ark.intel.com/content/www/us/en/ark/products/208921/intel-core-i7-1165g7-processor-12m-cache-up-to-4-70-ghz-with-ipu.html

Results are from geometric mean of N = 20 runs.

Time for new implemention shown.

Score(N) is the New Time / Old Time with entry alignment == N

In general there is one performance degregation for size range [2, 3]
in the page cross case. This is expected as the new code intentionally
gives the fallthrough path to the hotter size range [4, 7].


For the micro benchmarks there is no outright performance improvement
in the sense that for each benchmark configuration there is at least
one entry alignment where the old version had roughly the same
performance. But this should provide a great deal more performance
stability.

The hope, however, is that with the code size shrink and
reorganization of basic blocks for better icache behavior. Concerns
such as those brought up in this paper:

https://storage.googleapis.com/pub-tools-public-publication-data/pdf/e52f61fd2c51e8962305120548581efacbc06ffc.pdf 

should be better addressed.

 sysdeps/x86_64/multiarch/memcmp-evex-movbe.S | 434 +++++++++++--------
 1 file changed, 242 insertions(+), 192 deletions(-)
  

On Tue, Sep 21, 2021 at 7:51 PM Noah Goldstein <goldstein.w.n@gmail.com>
wrote:

> No bug.
>
> The frontend optimizations are to:
> 1. Reorganize logically connected basic blocks so they are either in
>    the same cache line or adjacent cache lines.
> 2. Avoid cases when basic blocks unnecissarily cross cache lines.
> 3. Try and 32 byte align any basic blocks possible without sacrificing
>    code size. Smaller / Less hot basic blocks are used for this.
>
> Overall code size shrunk by 168 bytes. This should make up for any
> extra costs due to aligning to 64 bytes.
>
> In general performance before deviated a great deal dependending on
> whether entry alignment % 64 was 0, 16, 32, or 48. These changes
> essentially make it so that the current implementation is at least
> equal to the best alignment of the original for any arguments.
>
> The only additional optimization is in the page cross case. Branch on
> equals case was removed from the size == [4, 7] case. As well the [4,
> 7] and [2, 3] case where swapped as [4, 7] is likely a more hot
> argument size.
>
> test-memcmp and test-wmemcmp are both passing.
> ---
> Performance results attached in reply.
>
Performance numbers.

>
> Benchmark CPU:
> https://ark.intel.com/content/www/us/en/ark/products/208921/intel-core-i7-1165g7-processor-12m-cache-up-to-4-70-ghz-with-ipu.html
>
> Results are from geometric mean of N = 20 runs.
>
> Time for new implemention shown.
>
> Score(N) is the New Time / Old Time with entry alignment == N
>
> In general there is one performance degregation for size range [2, 3]
> in the page cross case. This is expected as the new code intentionally
> gives the fallthrough path to the hotter size range [4, 7].
>
>
> For the micro benchmarks there is no outright performance improvement
> in the sense that for each benchmark configuration there is at least
> one entry alignment where the old version had roughly the same
> performance. But this should provide a great deal more performance
> stability.
>
> The hope, however, is that with the code size shrink and
> reorganization of basic blocks for better icache behavior. Concerns
> such as those brought up in this paper:
>
>
> https://storage.googleapis.com/pub-tools-public-publication-data/pdf/e52f61fd2c51e8962305120548581efacbc06ffc.pdf
>
> should be better addressed.
>
>  sysdeps/x86_64/multiarch/memcmp-evex-movbe.S | 434 +++++++++++--------
>  1 file changed, 242 insertions(+), 192 deletions(-)
>
> diff --git a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> index 654dc7ac8c..2761b54f2e 100644
> --- a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> +++ b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> @@ -34,7 +34,24 @@
>        area.
>     7. Use 2 vector compares when size is 2 * CHAR_PER_VEC or less.
>     8. Use 4 vector compares when size is 4 * CHAR_PER_VEC or less.
> -   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.  */
> +   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.
> +
> +When possible the implementation tries to optimize for frontend in the
> +following ways:
> +Throughput:
> +    1. All code sections that fit are able to run optimally out of the
> +       LSD.
> +    2. All code sections that fit are able to run optimally out of the
> +       DSB
> +    3. Basic blocks are contained in minimum number of fetch blocks
> +       necessary.
> +
> +Latency:
> +    1. Logically connected basic blocks are put in the same
> +       cache-line.
> +    2. Logically connected basic blocks that do not fit in the same
> +       cache-line are put in adjacent lines. This can get beneficial
> +       L2 spatial prefetching and L1 next-line prefetching.  */
>
>  # include <sysdep.h>
>
> @@ -47,9 +64,11 @@
>  # ifdef USE_AS_WMEMCMP
>  #  define CHAR_SIZE    4
>  #  define VPCMP        vpcmpd
> +#  define VPTEST       vptestmd
>  # else
>  #  define CHAR_SIZE    1
>  #  define VPCMP        vpcmpub
> +#  define VPTEST       vptestmb
>  # endif
>
>  # define VEC_SIZE      32
> @@ -75,7 +94,9 @@
>  */
>
>         .section .text.evex,"ax",@progbits
> -ENTRY (MEMCMP)
> +/* Cache align memcmp entry. This allows for much more thorough
> +   frontend optimization.  */
> +ENTRY_P2ALIGN (MEMCMP, 6)
>  # ifdef __ILP32__
>         /* Clear the upper 32 bits.  */
>         movl    %edx, %edx
> @@ -89,7 +110,7 @@ ENTRY (MEMCMP)
>         VPCMP   $4, (%rdi), %YMM1, %k1
>         kmovd   %k1, %eax
>         /* NB: eax must be destination register if going to
> -          L(return_vec_[0,2]). For L(return_vec_3 destination register
> +          L(return_vec_[0,2]). For L(return_vec_3) destination register
>            must be ecx.  */
>         testl   %eax, %eax
>         jnz     L(return_vec_0)
> @@ -121,10 +142,6 @@ ENTRY (MEMCMP)
>         testl   %ecx, %ecx
>         jnz     L(return_vec_3)
>
> -       /* Zero YMM0. 4x VEC reduction is done with vpxor + vtern so
> -          compare with zero to get a mask is needed.  */
> -       vpxorq  %XMM0, %XMM0, %XMM0
> -
>         /* Go to 4x VEC loop.  */
>         cmpq    $(CHAR_PER_VEC * 8), %rdx
>         ja      L(more_8x_vec)
> @@ -148,47 +165,61 @@ ENTRY (MEMCMP)
>
>         VMOVU   (VEC_SIZE * 2)(%rsi), %YMM3
>         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> -       /* Or together YMM1, YMM2, and YMM3 into YMM3.  */
> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
>
>         VMOVU   (VEC_SIZE * 3)(%rsi), %YMM4
>         /* Ternary logic to xor (VEC_SIZE * 3)(%rdi) with YMM4 while
> -          oring with YMM3. Result is stored in YMM4.  */
> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
> -       /* Compare YMM4 with 0. If any 1s s1 and s2 don't match.  */
> -       VPCMP   $4, %YMM4, %YMM0, %k1
> +          oring with YMM1. Result is stored in YMM4.  */
> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
> +
> +       /* Or together YMM2, YMM3, and YMM4 into YMM4.  */
> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> +
> +       /* Test YMM4 against itself. Store any CHAR mismatches in k1.
> +        */
> +       VPTEST  %YMM4, %YMM4, %k1
> +       /* k1 must go to ecx for L(return_vec_0_1_2_3).  */
>         kmovd   %k1, %ecx
>         testl   %ecx, %ecx
>         jnz     L(return_vec_0_1_2_3)
>         /* NB: eax must be zero to reach here.  */
>         ret
>
> -       /* NB: aligning 32 here allows for the rest of the jump targets
> -          to be tuned for 32 byte alignment. Most important this ensures
> -          the L(more_8x_vec) loop is 32 byte aligned.  */
> -       .p2align 5
> -L(less_vec):
> -       /* Check if one or less CHAR. This is necessary for size = 0 but
> -          is also faster for size = CHAR_SIZE.  */
> -       cmpl    $1, %edx
> -       jbe     L(one_or_less)
> +       .p2align 4
> +L(8x_end_return_vec_0_1_2_3):
> +       movq    %rdx, %rdi
> +L(8x_return_vec_0_1_2_3):
> +       addq    %rdi, %rsi
> +L(return_vec_0_1_2_3):
> +       VPTEST  %YMM1, %YMM1, %k0
> +       kmovd   %k0, %eax
> +       testl   %eax, %eax
> +       jnz     L(return_vec_0)
>
> -       /* Check if loading one VEC from either s1 or s2 could cause a
> -          page cross. This can have false positives but is by far the
> -          fastest method.  */
> -       movl    %edi, %eax
> -       orl     %esi, %eax
> -       andl    $(PAGE_SIZE - 1), %eax
> -       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
> -       jg      L(page_cross_less_vec)
> +       VPTEST  %YMM2, %YMM2, %k0
> +       kmovd   %k0, %eax
> +       testl   %eax, %eax
> +       jnz     L(return_vec_1)
>
> -       /* No page cross possible.  */
> -       VMOVU   (%rsi), %YMM2
> -       VPCMP   $4, (%rdi), %YMM2, %k1
> -       kmovd   %k1, %eax
> -       /* Create mask in ecx for potentially in bound matches.  */
> -       bzhil   %edx, %eax, %eax
> -       jnz     L(return_vec_0)
> +       VPTEST  %YMM3, %YMM3, %k0
> +       kmovd   %k0, %eax
> +       testl   %eax, %eax
> +       jnz     L(return_vec_2)
> +L(return_vec_3):
> +       /* bsf saves 1 byte from tzcnt. This keep L(return_vec_3) in one
> +          fetch block and the entire L(*return_vec_0_1_2_3) in 1 cache
> +          line.  */
> +       bsfl    %ecx, %ecx
> +# ifdef USE_AS_WMEMCMP
> +       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
> +       xorl    %edx, %edx
> +       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
> +       setg    %dl
> +       leal    -1(%rdx, %rdx), %eax
> +# else
> +       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
> +       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
> +       subl    %ecx, %eax
> +# endif
>         ret
>
>         .p2align 4
> @@ -209,10 +240,11 @@ L(return_vec_0):
>  # endif
>         ret
>
> -       /* NB: No p2align necessary. Alignment  % 16 is naturally 1
> -          which is good enough for a target not in a loop.  */
> +       .p2align 4
>  L(return_vec_1):
> -       tzcntl  %eax, %eax
> +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_1) in one
> +          fetch block.  */
> +       bsfl    %eax, %eax
>  # ifdef USE_AS_WMEMCMP
>         movl    VEC_SIZE(%rdi, %rax, CHAR_SIZE), %ecx
>         xorl    %edx, %edx
> @@ -226,10 +258,11 @@ L(return_vec_1):
>  # endif
>         ret
>
> -       /* NB: No p2align necessary. Alignment  % 16 is naturally 2
> -          which is good enough for a target not in a loop.  */
> +       .p2align 4,, 10
>  L(return_vec_2):
> -       tzcntl  %eax, %eax
> +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_2) in one
> +          fetch block.  */
> +       bsfl    %eax, %eax
>  # ifdef USE_AS_WMEMCMP
>         movl    (VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>         xorl    %edx, %edx
> @@ -243,40 +276,6 @@ L(return_vec_2):
>  # endif
>         ret
>
> -       .p2align 4
> -L(8x_return_vec_0_1_2_3):
> -       /* Returning from L(more_8x_vec) requires restoring rsi.  */
> -       addq    %rdi, %rsi
> -L(return_vec_0_1_2_3):
> -       VPCMP   $4, %YMM1, %YMM0, %k0
> -       kmovd   %k0, %eax
> -       testl   %eax, %eax
> -       jnz     L(return_vec_0)
> -
> -       VPCMP   $4, %YMM2, %YMM0, %k0
> -       kmovd   %k0, %eax
> -       testl   %eax, %eax
> -       jnz     L(return_vec_1)
> -
> -       VPCMP   $4, %YMM3, %YMM0, %k0
> -       kmovd   %k0, %eax
> -       testl   %eax, %eax
> -       jnz     L(return_vec_2)
> -L(return_vec_3):
> -       tzcntl  %ecx, %ecx
> -# ifdef USE_AS_WMEMCMP
> -       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
> -       xorl    %edx, %edx
> -       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
> -       setg    %dl
> -       leal    -1(%rdx, %rdx), %eax
> -# else
> -       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
> -       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
> -       subl    %ecx, %eax
> -# endif
> -       ret
> -
>         .p2align 4
>  L(more_8x_vec):
>         /* Set end of s1 in rdx.  */
> @@ -288,21 +287,19 @@ L(more_8x_vec):
>         andq    $-VEC_SIZE, %rdi
>         /* Adjust because first 4x vec where check already.  */
>         subq    $-(VEC_SIZE * 4), %rdi
> +
>         .p2align 4
>  L(loop_4x_vec):
>         VMOVU   (%rsi, %rdi), %YMM1
>         vpxorq  (%rdi), %YMM1, %YMM1
> -
>         VMOVU   VEC_SIZE(%rsi, %rdi), %YMM2
>         vpxorq  VEC_SIZE(%rdi), %YMM2, %YMM2
> -
>         VMOVU   (VEC_SIZE * 2)(%rsi, %rdi), %YMM3
>         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> -
>         VMOVU   (VEC_SIZE * 3)(%rsi, %rdi), %YMM4
> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
> -       VPCMP   $4, %YMM4, %YMM0, %k1
> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> +       VPTEST  %YMM4, %YMM4, %k1
>         kmovd   %k1, %ecx
>         testl   %ecx, %ecx
>         jnz     L(8x_return_vec_0_1_2_3)
> @@ -319,28 +316,25 @@ L(loop_4x_vec):
>         cmpl    $(VEC_SIZE * 2), %edi
>         jae     L(8x_last_2x_vec)
>
> +       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
> +
>         VMOVU   (%rsi, %rdx), %YMM1
>         vpxorq  (%rdx), %YMM1, %YMM1
>
>         VMOVU   VEC_SIZE(%rsi, %rdx), %YMM2
>         vpxorq  VEC_SIZE(%rdx), %YMM2, %YMM2
> -
> -       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> -
>         VMOVU   (VEC_SIZE * 3)(%rsi, %rdx), %YMM4
> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM3, %YMM4
> -       VPCMP   $4, %YMM4, %YMM0, %k1
> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM1, %YMM4
> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> +       VPTEST  %YMM4, %YMM4, %k1
>         kmovd   %k1, %ecx
> -       /* Restore s1 pointer to rdi.  */
> -       movq    %rdx, %rdi
>         testl   %ecx, %ecx
> -       jnz     L(8x_return_vec_0_1_2_3)
> +       jnz     L(8x_end_return_vec_0_1_2_3)
>         /* NB: eax must be zero to reach here.  */
>         ret
>
>         /* Only entry is from L(more_8x_vec).  */
> -       .p2align 4
> +       .p2align 4,, 10
>  L(8x_last_2x_vec):
>         VPCMP   $4, (VEC_SIZE * 2)(%rdx), %YMM3, %k1
>         kmovd   %k1, %eax
> @@ -355,7 +349,31 @@ L(8x_last_1x_vec):
>         jnz     L(8x_return_vec_3)
>         ret
>
> -       .p2align 4
> +       /* Not ideally aligned (at offset +9 bytes in fetch block) but
> +          not aligning keeps it in the same cache line as
> +          L(8x_last_1x/2x_vec) so likely worth it. As well, saves code
> +          size.  */
> +       .p2align 4,, 4
> +L(8x_return_vec_2):
> +       subq    $VEC_SIZE, %rdx
> +L(8x_return_vec_3):
> +       bsfl    %eax, %eax
> +# ifdef USE_AS_WMEMCMP
> +       leaq    (%rdx, %rax, CHAR_SIZE), %rax
> +       movl    (VEC_SIZE * 3)(%rax), %ecx
> +       xorl    %edx, %edx
> +       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
> +       setg    %dl
> +       leal    -1(%rdx, %rdx), %eax
> +# else
> +       addq    %rdx, %rax
> +       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
> +       movzbl  (VEC_SIZE * 3)(%rax), %eax
> +       subl    %ecx, %eax
> +# endif
> +       ret
> +
> +       .p2align 4,, 10
>  L(last_2x_vec):
>         /* Check second to last VEC.  */
>         VMOVU   -(VEC_SIZE * 2)(%rsi, %rdx, CHAR_SIZE), %YMM1
> @@ -374,26 +392,49 @@ L(last_1x_vec):
>         jnz     L(return_vec_0_end)
>         ret
>
> -       .p2align 4
> -L(8x_return_vec_2):
> -       subq    $VEC_SIZE, %rdx
> -L(8x_return_vec_3):
> -       tzcntl  %eax, %eax
> +       .p2align 4,, 10
> +L(return_vec_1_end):
> +       /* Use bsf to save code size. This is necessary to have
> +          L(one_or_less) fit in aligning bytes between.  */
> +       bsfl    %eax, %eax
> +       addl    %edx, %eax
>  # ifdef USE_AS_WMEMCMP
> -       leaq    (%rdx, %rax, CHAR_SIZE), %rax
> -       movl    (VEC_SIZE * 3)(%rax), %ecx
> +       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>         xorl    %edx, %edx
> -       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
> +       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
>         setg    %dl
>         leal    -1(%rdx, %rdx), %eax
>  # else
> -       addq    %rdx, %rax
> -       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
> -       movzbl  (VEC_SIZE * 3)(%rax), %eax
> +       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
> +       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
>         subl    %ecx, %eax
>  # endif
>         ret
>
> +       /* NB: L(one_or_less) fits in alignment padding between
> +          L(return_vec_1_end) and L(return_vec_0_end).  */
> +# ifdef USE_AS_WMEMCMP
> +L(one_or_less):
> +       jb      L(zero)
> +       movl    (%rdi), %ecx
> +       xorl    %edx, %edx
> +       cmpl    (%rsi), %ecx
> +       je      L(zero)
> +       setg    %dl
> +       leal    -1(%rdx, %rdx), %eax
> +       ret
> +# else
> +L(one_or_less):
> +       jb      L(zero)
> +       movzbl  (%rsi), %ecx
> +       movzbl  (%rdi), %eax
> +       subl    %ecx, %eax
> +       ret
> +# endif
> +L(zero):
> +       xorl    %eax, %eax
> +       ret
> +
>         .p2align 4
>  L(return_vec_0_end):
>         tzcntl  %eax, %eax
> @@ -412,23 +453,56 @@ L(return_vec_0_end):
>         ret
>
>         .p2align 4
> -L(return_vec_1_end):
> +L(less_vec):
> +       /* Check if one or less CHAR. This is necessary for size == 0
> +          but is also faster for size == CHAR_SIZE.  */
> +       cmpl    $1, %edx
> +       jbe     L(one_or_less)
> +
> +       /* Check if loading one VEC from either s1 or s2 could cause a
> +          page cross. This can have false positives but is by far the
> +          fastest method.  */
> +       movl    %edi, %eax
> +       orl     %esi, %eax
> +       andl    $(PAGE_SIZE - 1), %eax
> +       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
> +       jg      L(page_cross_less_vec)
> +
> +       /* No page cross possible.  */
> +       VMOVU   (%rsi), %YMM2
> +       VPCMP   $4, (%rdi), %YMM2, %k1
> +       kmovd   %k1, %eax
> +       /* Check if any matches where in bounds. Intentionally not
> +          storing result in eax to limit dependency chain if it goes to
> +          L(return_vec_0_lv).  */
> +       bzhil   %edx, %eax, %edx
> +       jnz     L(return_vec_0_lv)
> +       xorl    %eax, %eax
> +       ret
> +
> +       /* Essentially duplicate of L(return_vec_0). Ends up not costing
> +          any code as shrinks L(less_vec) by allowing 2-byte encoding of
> +          the jump and ends up fitting in aligning bytes. As well fits on
> +          same cache line as L(less_vec) so also saves a line from having
> +          to be fetched on cold calls to memcmp.  */
> +       .p2align 4,, 4
> +L(return_vec_0_lv):
>         tzcntl  %eax, %eax
> -       addl    %edx, %eax
>  # ifdef USE_AS_WMEMCMP
> -       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
> +       movl    (%rdi, %rax, CHAR_SIZE), %ecx
>         xorl    %edx, %edx
> -       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
> +       cmpl    (%rsi, %rax, CHAR_SIZE), %ecx
> +       /* NB: no partial register stall here because xorl zero idiom
> +          above.  */
>         setg    %dl
>         leal    -1(%rdx, %rdx), %eax
>  # else
> -       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
> -       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
> +       movzbl  (%rsi, %rax), %ecx
> +       movzbl  (%rdi, %rax), %eax
>         subl    %ecx, %eax
>  # endif
>         ret
>
> -
>         .p2align 4
>  L(page_cross_less_vec):
>         /* if USE_AS_WMEMCMP it can only be 0, 4, 8, 12, 16, 20, 24, 28
> @@ -439,108 +513,84 @@ L(page_cross_less_vec):
>         cmpl    $8, %edx
>         jae     L(between_8_15)
>         cmpl    $4, %edx
> -       jae     L(between_4_7)
> -L(between_2_3):
> -       /* Load as big endian to avoid branches.  */
> -       movzwl  (%rdi), %eax
> -       movzwl  (%rsi), %ecx
> -       shll    $8, %eax
> -       shll    $8, %ecx
> -       bswap   %eax
> -       bswap   %ecx
> -       movzbl  -1(%rdi, %rdx), %edi
> -       movzbl  -1(%rsi, %rdx), %esi
> -       orl     %edi, %eax
> -       orl     %esi, %ecx
> -       /* Subtraction is okay because the upper 8 bits are zero.  */
> -       subl    %ecx, %eax
> -       ret
> -       .p2align 4
> -L(one_or_less):
> -       jb      L(zero)
> -       movzbl  (%rsi), %ecx
> -       movzbl  (%rdi), %eax
> -       subl    %ecx, %eax
> +       jb      L(between_2_3)
> +
> +       /* Load as big endian with overlapping movbe to avoid branches.
> +        */
> +       movbe   (%rdi), %eax
> +       movbe   (%rsi), %ecx
> +       shlq    $32, %rax
> +       shlq    $32, %rcx
> +       movbe   -4(%rdi, %rdx), %edi
> +       movbe   -4(%rsi, %rdx), %esi
> +       orq     %rdi, %rax
> +       orq     %rsi, %rcx
> +       subq    %rcx, %rax
> +       /* edx is guranteed to be positive int32 in range [4, 7].  */
> +       cmovne  %edx, %eax
> +       /* ecx is -1 if rcx > rax. Otherwise 0.  */
> +       sbbl    %ecx, %ecx
> +       /* If rcx > rax, then ecx is 0 and eax is positive. If rcx ==
> +          rax then eax and ecx are zero. If rax < rax then ecx is -1 so
> +          eax doesn't matter.  */
> +       orl     %ecx, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 4,, 8
>  L(between_8_15):
>  # endif
>         /* If USE_AS_WMEMCMP fall through into 8-15 byte case.  */
> -       vmovq   (%rdi), %XMM1
> -       vmovq   (%rsi), %XMM2
> -       VPCMP   $4, %XMM1, %XMM2, %k1
> +       vmovq   (%rdi), %xmm1
> +       vmovq   (%rsi), %xmm2
> +       VPCMP   $4, %xmm1, %xmm2, %k1
>         kmovd   %k1, %eax
>         testl   %eax, %eax
> -       jnz     L(return_vec_0)
> +       jnz     L(return_vec_0_lv)
>         /* Use overlapping loads to avoid branches.  */
> -       leaq    -8(%rdi, %rdx, CHAR_SIZE), %rdi
> -       leaq    -8(%rsi, %rdx, CHAR_SIZE), %rsi
> -       vmovq   (%rdi), %XMM1
> -       vmovq   (%rsi), %XMM2
> -       VPCMP   $4, %XMM1, %XMM2, %k1
> +       vmovq   -8(%rdi, %rdx, CHAR_SIZE), %xmm1
> +       vmovq   -8(%rsi, %rdx, CHAR_SIZE), %xmm2
> +       VPCMP   $4, %xmm1, %xmm2, %k1
> +       addl    $(CHAR_PER_VEC - (8 / CHAR_SIZE)), %edx
>         kmovd   %k1, %eax
>         testl   %eax, %eax
> -       jnz     L(return_vec_0)
> -       ret
> -
> -       .p2align 4
> -L(zero):
> -       xorl    %eax, %eax
> +       jnz     L(return_vec_0_end)
>         ret
>
> -       .p2align 4
> +       .p2align 4,, 8
>  L(between_16_31):
>         /* From 16 to 31 bytes.  No branch when size == 16.  */
> -       VMOVU   (%rsi), %XMM2
> -       VPCMP   $4, (%rdi), %XMM2, %k1
> +
> +       /* Use movups to save code size.  */
> +       movups  (%rsi), %xmm2
> +       VPCMP   $4, (%rdi), %xmm2, %k1
>         kmovd   %k1, %eax
>         testl   %eax, %eax
> -       jnz     L(return_vec_0)
> -
> +       jnz     L(return_vec_0_lv)
>         /* Use overlapping loads to avoid branches.  */
> -
> -       VMOVU   -16(%rsi, %rdx, CHAR_SIZE), %XMM2
> -       leaq    -16(%rdi, %rdx, CHAR_SIZE), %rdi
> -       leaq    -16(%rsi, %rdx, CHAR_SIZE), %rsi
> -       VPCMP   $4, (%rdi), %XMM2, %k1
> +       movups  -16(%rsi, %rdx, CHAR_SIZE), %xmm2
> +       VPCMP   $4, -16(%rdi, %rdx, CHAR_SIZE), %xmm2, %k1
> +       addl    $(CHAR_PER_VEC - (16 / CHAR_SIZE)), %edx
>         kmovd   %k1, %eax
>         testl   %eax, %eax
> -       jnz     L(return_vec_0)
> -       ret
> -
> -# ifdef USE_AS_WMEMCMP
> -       .p2align 4
> -L(one_or_less):
> -       jb      L(zero)
> -       movl    (%rdi), %ecx
> -       xorl    %edx, %edx
> -       cmpl    (%rsi), %ecx
> -       je      L(zero)
> -       setg    %dl
> -       leal    -1(%rdx, %rdx), %eax
> +       jnz     L(return_vec_0_end)
>         ret
> -# else
>
> -       .p2align 4
> -L(between_4_7):
> -       /* Load as big endian with overlapping movbe to avoid branches.
> -        */
> -       movbe   (%rdi), %eax
> -       movbe   (%rsi), %ecx
> -       shlq    $32, %rax
> -       shlq    $32, %rcx
> -       movbe   -4(%rdi, %rdx), %edi
> -       movbe   -4(%rsi, %rdx), %esi
> -       orq     %rdi, %rax
> -       orq     %rsi, %rcx
> -       subq    %rcx, %rax
> -       jz      L(zero_4_7)
> -       sbbl    %eax, %eax
> -       orl     $1, %eax
> -L(zero_4_7):
> +# ifndef USE_AS_WMEMCMP
> +L(between_2_3):
> +       /* Load as big endian to avoid branches.  */
> +       movzwl  (%rdi), %eax
> +       movzwl  (%rsi), %ecx
> +       shll    $8, %eax
> +       shll    $8, %ecx
> +       bswap   %eax
> +       bswap   %ecx
> +       movzbl  -1(%rdi, %rdx), %edi
> +       movzbl  -1(%rsi, %rdx), %esi
> +       orl     %edi, %eax
> +       orl     %esi, %ecx
> +       /* Subtraction is okay because the upper 8 bits are zero.  */
> +       subl    %ecx, %eax
>         ret
>  # endif
> -
>  END (MEMCMP)
>  #endif
> --
> 2.25.1
>
>
  

On Tue, Sep 21, 2021 at 7:52 PM Noah Goldstein <goldstein.w.n@gmail.com>
wrote:

>
>
> On Tue, Sep 21, 2021 at 7:51 PM Noah Goldstein <goldstein.w.n@gmail.com>
> wrote:
>
>> No bug.
>>
>> The frontend optimizations are to:
>> 1. Reorganize logically connected basic blocks so they are either in
>>    the same cache line or adjacent cache lines.
>> 2. Avoid cases when basic blocks unnecissarily cross cache lines.
>> 3. Try and 32 byte align any basic blocks possible without sacrificing
>>    code size. Smaller / Less hot basic blocks are used for this.
>>
>> Overall code size shrunk by 168 bytes. This should make up for any
>> extra costs due to aligning to 64 bytes.
>>
>> In general performance before deviated a great deal dependending on
>> whether entry alignment % 64 was 0, 16, 32, or 48. These changes
>> essentially make it so that the current implementation is at least
>> equal to the best alignment of the original for any arguments.
>>
>> The only additional optimization is in the page cross case. Branch on
>> equals case was removed from the size == [4, 7] case. As well the [4,
>> 7] and [2, 3] case where swapped as [4, 7] is likely a more hot
>> argument size.
>>
>> test-memcmp and test-wmemcmp are both passing.
>> ---
>> Performance results attached in reply.
>>
> Performance numbers.
>
>>
>> Benchmark CPU:
>> https://ark.intel.com/content/www/us/en/ark/products/208921/intel-core-i7-1165g7-processor-12m-cache-up-to-4-70-ghz-with-ipu.html
>>
>> Results are from geometric mean of N = 20 runs.
>>
>> Time for new implemention shown.
>>
>> Score(N) is the New Time / Old Time with entry alignment == N
>>
>> In general there is one performance degregation for size range [2, 3]
>> in the page cross case. This is expected as the new code intentionally
>> gives the fallthrough path to the hotter size range [4, 7].
>>
>>
>> For the micro benchmarks there is no outright performance improvement
>> in the sense that for each benchmark configuration there is at least
>> one entry alignment where the old version had roughly the same
>> performance. But this should provide a great deal more performance
>> stability.
>>
>> The hope, however, is that with the code size shrink and
>> reorganization of basic blocks for better icache behavior. Concerns
>> such as those brought up in this paper:
>>
>>
>> https://storage.googleapis.com/pub-tools-public-publication-data/pdf/e52f61fd2c51e8962305120548581efacbc06ffc.pdf
>>
>> should be better addressed.
>>
>>  sysdeps/x86_64/multiarch/memcmp-evex-movbe.S | 434 +++++++++++--------
>>  1 file changed, 242 insertions(+), 192 deletions(-)
>>
>> diff --git a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
>> b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
>> index 654dc7ac8c..2761b54f2e 100644
>> --- a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
>> +++ b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
>> @@ -34,7 +34,24 @@
>>        area.
>>     7. Use 2 vector compares when size is 2 * CHAR_PER_VEC or less.
>>     8. Use 4 vector compares when size is 4 * CHAR_PER_VEC or less.
>> -   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.  */
>> +   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.
>> +
>> +When possible the implementation tries to optimize for frontend in the
>> +following ways:
>> +Throughput:
>> +    1. All code sections that fit are able to run optimally out of the
>> +       LSD.
>> +    2. All code sections that fit are able to run optimally out of the
>> +       DSB
>> +    3. Basic blocks are contained in minimum number of fetch blocks
>> +       necessary.
>> +
>> +Latency:
>> +    1. Logically connected basic blocks are put in the same
>> +       cache-line.
>> +    2. Logically connected basic blocks that do not fit in the same
>> +       cache-line are put in adjacent lines. This can get beneficial
>> +       L2 spatial prefetching and L1 next-line prefetching.  */
>>
>>  # include <sysdep.h>
>>
>> @@ -47,9 +64,11 @@
>>  # ifdef USE_AS_WMEMCMP
>>  #  define CHAR_SIZE    4
>>  #  define VPCMP        vpcmpd
>> +#  define VPTEST       vptestmd
>>  # else
>>  #  define CHAR_SIZE    1
>>  #  define VPCMP        vpcmpub
>> +#  define VPTEST       vptestmb
>>  # endif
>>
>>  # define VEC_SIZE      32
>> @@ -75,7 +94,9 @@
>>  */
>>
>>         .section .text.evex,"ax",@progbits
>> -ENTRY (MEMCMP)
>> +/* Cache align memcmp entry. This allows for much more thorough
>> +   frontend optimization.  */
>> +ENTRY_P2ALIGN (MEMCMP, 6)
>>  # ifdef __ILP32__
>>         /* Clear the upper 32 bits.  */
>>         movl    %edx, %edx
>> @@ -89,7 +110,7 @@ ENTRY (MEMCMP)
>>         VPCMP   $4, (%rdi), %YMM1, %k1
>>         kmovd   %k1, %eax
>>         /* NB: eax must be destination register if going to
>> -          L(return_vec_[0,2]). For L(return_vec_3 destination register
>> +          L(return_vec_[0,2]). For L(return_vec_3) destination register
>>            must be ecx.  */
>>         testl   %eax, %eax
>>         jnz     L(return_vec_0)
>> @@ -121,10 +142,6 @@ ENTRY (MEMCMP)
>>         testl   %ecx, %ecx
>>         jnz     L(return_vec_3)
>>
>> -       /* Zero YMM0. 4x VEC reduction is done with vpxor + vtern so
>> -          compare with zero to get a mask is needed.  */
>> -       vpxorq  %XMM0, %XMM0, %XMM0
>> -
>>         /* Go to 4x VEC loop.  */
>>         cmpq    $(CHAR_PER_VEC * 8), %rdx
>>         ja      L(more_8x_vec)
>> @@ -148,47 +165,61 @@ ENTRY (MEMCMP)
>>
>>         VMOVU   (VEC_SIZE * 2)(%rsi), %YMM3
>>         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
>> -       /* Or together YMM1, YMM2, and YMM3 into YMM3.  */
>> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
>>
>>         VMOVU   (VEC_SIZE * 3)(%rsi), %YMM4
>>         /* Ternary logic to xor (VEC_SIZE * 3)(%rdi) with YMM4 while
>> -          oring with YMM3. Result is stored in YMM4.  */
>> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
>> -       /* Compare YMM4 with 0. If any 1s s1 and s2 don't match.  */
>> -       VPCMP   $4, %YMM4, %YMM0, %k1
>> +          oring with YMM1. Result is stored in YMM4.  */
>> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
>> +
>> +       /* Or together YMM2, YMM3, and YMM4 into YMM4.  */
>> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
>> +
>> +       /* Test YMM4 against itself. Store any CHAR mismatches in k1.
>> +        */
>> +       VPTEST  %YMM4, %YMM4, %k1
>> +       /* k1 must go to ecx for L(return_vec_0_1_2_3).  */
>>         kmovd   %k1, %ecx
>>         testl   %ecx, %ecx
>>         jnz     L(return_vec_0_1_2_3)
>>         /* NB: eax must be zero to reach here.  */
>>         ret
>>
>> -       /* NB: aligning 32 here allows for the rest of the jump targets
>> -          to be tuned for 32 byte alignment. Most important this ensures
>> -          the L(more_8x_vec) loop is 32 byte aligned.  */
>> -       .p2align 5
>> -L(less_vec):
>> -       /* Check if one or less CHAR. This is necessary for size = 0 but
>> -          is also faster for size = CHAR_SIZE.  */
>> -       cmpl    $1, %edx
>> -       jbe     L(one_or_less)
>> +       .p2align 4
>> +L(8x_end_return_vec_0_1_2_3):
>> +       movq    %rdx, %rdi
>> +L(8x_return_vec_0_1_2_3):
>> +       addq    %rdi, %rsi
>> +L(return_vec_0_1_2_3):
>> +       VPTEST  %YMM1, %YMM1, %k0
>> +       kmovd   %k0, %eax
>> +       testl   %eax, %eax
>> +       jnz     L(return_vec_0)
>>
>> -       /* Check if loading one VEC from either s1 or s2 could cause a
>> -          page cross. This can have false positives but is by far the
>> -          fastest method.  */
>> -       movl    %edi, %eax
>> -       orl     %esi, %eax
>> -       andl    $(PAGE_SIZE - 1), %eax
>> -       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
>> -       jg      L(page_cross_less_vec)
>> +       VPTEST  %YMM2, %YMM2, %k0
>> +       kmovd   %k0, %eax
>> +       testl   %eax, %eax
>> +       jnz     L(return_vec_1)
>>
>> -       /* No page cross possible.  */
>> -       VMOVU   (%rsi), %YMM2
>> -       VPCMP   $4, (%rdi), %YMM2, %k1
>> -       kmovd   %k1, %eax
>> -       /* Create mask in ecx for potentially in bound matches.  */
>> -       bzhil   %edx, %eax, %eax
>> -       jnz     L(return_vec_0)
>> +       VPTEST  %YMM3, %YMM3, %k0
>> +       kmovd   %k0, %eax
>> +       testl   %eax, %eax
>> +       jnz     L(return_vec_2)
>> +L(return_vec_3):
>> +       /* bsf saves 1 byte from tzcnt. This keep L(return_vec_3) in one
>> +          fetch block and the entire L(*return_vec_0_1_2_3) in 1 cache
>> +          line.  */
>> +       bsfl    %ecx, %ecx
>> +# ifdef USE_AS_WMEMCMP
>> +       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
>> +       xorl    %edx, %edx
>> +       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
>> +       setg    %dl
>> +       leal    -1(%rdx, %rdx), %eax
>> +# else
>> +       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
>> +       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
>> +       subl    %ecx, %eax
>> +# endif
>>         ret
>>
>>         .p2align 4
>> @@ -209,10 +240,11 @@ L(return_vec_0):
>>  # endif
>>         ret
>>
>> -       /* NB: No p2align necessary. Alignment  % 16 is naturally 1
>> -          which is good enough for a target not in a loop.  */
>> +       .p2align 4
>>  L(return_vec_1):
>> -       tzcntl  %eax, %eax
>> +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_1) in one
>> +          fetch block.  */
>> +       bsfl    %eax, %eax
>>  # ifdef USE_AS_WMEMCMP
>>         movl    VEC_SIZE(%rdi, %rax, CHAR_SIZE), %ecx
>>         xorl    %edx, %edx
>> @@ -226,10 +258,11 @@ L(return_vec_1):
>>  # endif
>>         ret
>>
>> -       /* NB: No p2align necessary. Alignment  % 16 is naturally 2
>> -          which is good enough for a target not in a loop.  */
>> +       .p2align 4,, 10
>>  L(return_vec_2):
>> -       tzcntl  %eax, %eax
>> +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_2) in one
>> +          fetch block.  */
>> +       bsfl    %eax, %eax
>>  # ifdef USE_AS_WMEMCMP
>>         movl    (VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>>         xorl    %edx, %edx
>> @@ -243,40 +276,6 @@ L(return_vec_2):
>>  # endif
>>         ret
>>
>> -       .p2align 4
>> -L(8x_return_vec_0_1_2_3):
>> -       /* Returning from L(more_8x_vec) requires restoring rsi.  */
>> -       addq    %rdi, %rsi
>> -L(return_vec_0_1_2_3):
>> -       VPCMP   $4, %YMM1, %YMM0, %k0
>> -       kmovd   %k0, %eax
>> -       testl   %eax, %eax
>> -       jnz     L(return_vec_0)
>> -
>> -       VPCMP   $4, %YMM2, %YMM0, %k0
>> -       kmovd   %k0, %eax
>> -       testl   %eax, %eax
>> -       jnz     L(return_vec_1)
>> -
>> -       VPCMP   $4, %YMM3, %YMM0, %k0
>> -       kmovd   %k0, %eax
>> -       testl   %eax, %eax
>> -       jnz     L(return_vec_2)
>> -L(return_vec_3):
>> -       tzcntl  %ecx, %ecx
>> -# ifdef USE_AS_WMEMCMP
>> -       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
>> -       xorl    %edx, %edx
>> -       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
>> -       setg    %dl
>> -       leal    -1(%rdx, %rdx), %eax
>> -# else
>> -       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
>> -       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
>> -       subl    %ecx, %eax
>> -# endif
>> -       ret
>> -
>>         .p2align 4
>>  L(more_8x_vec):
>>         /* Set end of s1 in rdx.  */
>> @@ -288,21 +287,19 @@ L(more_8x_vec):
>>         andq    $-VEC_SIZE, %rdi
>>         /* Adjust because first 4x vec where check already.  */
>>         subq    $-(VEC_SIZE * 4), %rdi
>> +
>>         .p2align 4
>>  L(loop_4x_vec):
>>         VMOVU   (%rsi, %rdi), %YMM1
>>         vpxorq  (%rdi), %YMM1, %YMM1
>> -
>>         VMOVU   VEC_SIZE(%rsi, %rdi), %YMM2
>>         vpxorq  VEC_SIZE(%rdi), %YMM2, %YMM2
>> -
>>         VMOVU   (VEC_SIZE * 2)(%rsi, %rdi), %YMM3
>>         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
>> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
>> -
>>         VMOVU   (VEC_SIZE * 3)(%rsi, %rdi), %YMM4
>> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
>> -       VPCMP   $4, %YMM4, %YMM0, %k1
>> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
>> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
>> +       VPTEST  %YMM4, %YMM4, %k1
>>         kmovd   %k1, %ecx
>>         testl   %ecx, %ecx
>>         jnz     L(8x_return_vec_0_1_2_3)
>> @@ -319,28 +316,25 @@ L(loop_4x_vec):
>>         cmpl    $(VEC_SIZE * 2), %edi
>>         jae     L(8x_last_2x_vec)
>>
>> +       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
>> +
>>         VMOVU   (%rsi, %rdx), %YMM1
>>         vpxorq  (%rdx), %YMM1, %YMM1
>>
>>         VMOVU   VEC_SIZE(%rsi, %rdx), %YMM2
>>         vpxorq  VEC_SIZE(%rdx), %YMM2, %YMM2
>> -
>> -       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
>> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
>> -
>>         VMOVU   (VEC_SIZE * 3)(%rsi, %rdx), %YMM4
>> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM3, %YMM4
>> -       VPCMP   $4, %YMM4, %YMM0, %k1
>> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM1, %YMM4
>> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
>> +       VPTEST  %YMM4, %YMM4, %k1
>>         kmovd   %k1, %ecx
>> -       /* Restore s1 pointer to rdi.  */
>> -       movq    %rdx, %rdi
>>         testl   %ecx, %ecx
>> -       jnz     L(8x_return_vec_0_1_2_3)
>> +       jnz     L(8x_end_return_vec_0_1_2_3)
>>         /* NB: eax must be zero to reach here.  */
>>         ret
>>
>>         /* Only entry is from L(more_8x_vec).  */
>> -       .p2align 4
>> +       .p2align 4,, 10
>>  L(8x_last_2x_vec):
>>         VPCMP   $4, (VEC_SIZE * 2)(%rdx), %YMM3, %k1
>>         kmovd   %k1, %eax
>> @@ -355,7 +349,31 @@ L(8x_last_1x_vec):
>>         jnz     L(8x_return_vec_3)
>>         ret
>>
>> -       .p2align 4
>> +       /* Not ideally aligned (at offset +9 bytes in fetch block) but
>> +          not aligning keeps it in the same cache line as
>> +          L(8x_last_1x/2x_vec) so likely worth it. As well, saves code
>> +          size.  */
>> +       .p2align 4,, 4
>> +L(8x_return_vec_2):
>> +       subq    $VEC_SIZE, %rdx
>> +L(8x_return_vec_3):
>> +       bsfl    %eax, %eax
>> +# ifdef USE_AS_WMEMCMP
>> +       leaq    (%rdx, %rax, CHAR_SIZE), %rax
>> +       movl    (VEC_SIZE * 3)(%rax), %ecx
>> +       xorl    %edx, %edx
>> +       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
>> +       setg    %dl
>> +       leal    -1(%rdx, %rdx), %eax
>> +# else
>> +       addq    %rdx, %rax
>> +       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
>> +       movzbl  (VEC_SIZE * 3)(%rax), %eax
>> +       subl    %ecx, %eax
>> +# endif
>> +       ret
>> +
>> +       .p2align 4,, 10
>>  L(last_2x_vec):
>>         /* Check second to last VEC.  */
>>         VMOVU   -(VEC_SIZE * 2)(%rsi, %rdx, CHAR_SIZE), %YMM1
>> @@ -374,26 +392,49 @@ L(last_1x_vec):
>>         jnz     L(return_vec_0_end)
>>         ret
>>
>> -       .p2align 4
>> -L(8x_return_vec_2):
>> -       subq    $VEC_SIZE, %rdx
>> -L(8x_return_vec_3):
>> -       tzcntl  %eax, %eax
>> +       .p2align 4,, 10
>> +L(return_vec_1_end):
>> +       /* Use bsf to save code size. This is necessary to have
>> +          L(one_or_less) fit in aligning bytes between.  */
>> +       bsfl    %eax, %eax
>> +       addl    %edx, %eax
>>  # ifdef USE_AS_WMEMCMP
>> -       leaq    (%rdx, %rax, CHAR_SIZE), %rax
>> -       movl    (VEC_SIZE * 3)(%rax), %ecx
>> +       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>>         xorl    %edx, %edx
>> -       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
>> +       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
>>         setg    %dl
>>         leal    -1(%rdx, %rdx), %eax
>>  # else
>> -       addq    %rdx, %rax
>> -       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
>> -       movzbl  (VEC_SIZE * 3)(%rax), %eax
>> +       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
>> +       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
>>         subl    %ecx, %eax
>>  # endif
>>         ret
>>
>> +       /* NB: L(one_or_less) fits in alignment padding between
>> +          L(return_vec_1_end) and L(return_vec_0_end).  */
>> +# ifdef USE_AS_WMEMCMP
>> +L(one_or_less):
>> +       jb      L(zero)
>> +       movl    (%rdi), %ecx
>> +       xorl    %edx, %edx
>> +       cmpl    (%rsi), %ecx
>> +       je      L(zero)
>> +       setg    %dl
>> +       leal    -1(%rdx, %rdx), %eax
>> +       ret
>> +# else
>> +L(one_or_less):
>> +       jb      L(zero)
>> +       movzbl  (%rsi), %ecx
>> +       movzbl  (%rdi), %eax
>> +       subl    %ecx, %eax
>> +       ret
>> +# endif
>> +L(zero):
>> +       xorl    %eax, %eax
>> +       ret
>> +
>>         .p2align 4
>>  L(return_vec_0_end):
>>         tzcntl  %eax, %eax
>> @@ -412,23 +453,56 @@ L(return_vec_0_end):
>>         ret
>>
>>         .p2align 4
>> -L(return_vec_1_end):
>> +L(less_vec):
>> +       /* Check if one or less CHAR. This is necessary for size == 0
>> +          but is also faster for size == CHAR_SIZE.  */
>> +       cmpl    $1, %edx
>> +       jbe     L(one_or_less)
>> +
>> +       /* Check if loading one VEC from either s1 or s2 could cause a
>> +          page cross. This can have false positives but is by far the
>> +          fastest method.  */
>> +       movl    %edi, %eax
>> +       orl     %esi, %eax
>> +       andl    $(PAGE_SIZE - 1), %eax
>> +       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
>> +       jg      L(page_cross_less_vec)
>> +
>> +       /* No page cross possible.  */
>> +       VMOVU   (%rsi), %YMM2
>> +       VPCMP   $4, (%rdi), %YMM2, %k1
>> +       kmovd   %k1, %eax
>> +       /* Check if any matches where in bounds. Intentionally not
>> +          storing result in eax to limit dependency chain if it goes to
>> +          L(return_vec_0_lv).  */
>> +       bzhil   %edx, %eax, %edx
>> +       jnz     L(return_vec_0_lv)
>> +       xorl    %eax, %eax
>> +       ret
>> +
>> +       /* Essentially duplicate of L(return_vec_0). Ends up not costing
>> +          any code as shrinks L(less_vec) by allowing 2-byte encoding of
>> +          the jump and ends up fitting in aligning bytes. As well fits on
>> +          same cache line as L(less_vec) so also saves a line from having
>> +          to be fetched on cold calls to memcmp.  */
>> +       .p2align 4,, 4
>> +L(return_vec_0_lv):
>>         tzcntl  %eax, %eax
>> -       addl    %edx, %eax
>>  # ifdef USE_AS_WMEMCMP
>> -       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>> +       movl    (%rdi, %rax, CHAR_SIZE), %ecx
>>         xorl    %edx, %edx
>> -       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
>> +       cmpl    (%rsi, %rax, CHAR_SIZE), %ecx
>> +       /* NB: no partial register stall here because xorl zero idiom
>> +          above.  */
>>         setg    %dl
>>         leal    -1(%rdx, %rdx), %eax
>>  # else
>> -       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
>> -       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
>> +       movzbl  (%rsi, %rax), %ecx
>> +       movzbl  (%rdi, %rax), %eax
>>         subl    %ecx, %eax
>>  # endif
>>         ret
>>
>> -
>>         .p2align 4
>>  L(page_cross_less_vec):
>>         /* if USE_AS_WMEMCMP it can only be 0, 4, 8, 12, 16, 20, 24, 28
>> @@ -439,108 +513,84 @@ L(page_cross_less_vec):
>>         cmpl    $8, %edx
>>         jae     L(between_8_15)
>>         cmpl    $4, %edx
>> -       jae     L(between_4_7)
>> -L(between_2_3):
>> -       /* Load as big endian to avoid branches.  */
>> -       movzwl  (%rdi), %eax
>> -       movzwl  (%rsi), %ecx
>> -       shll    $8, %eax
>> -       shll    $8, %ecx
>> -       bswap   %eax
>> -       bswap   %ecx
>> -       movzbl  -1(%rdi, %rdx), %edi
>> -       movzbl  -1(%rsi, %rdx), %esi
>> -       orl     %edi, %eax
>> -       orl     %esi, %ecx
>> -       /* Subtraction is okay because the upper 8 bits are zero.  */
>> -       subl    %ecx, %eax
>> -       ret
>> -       .p2align 4
>> -L(one_or_less):
>> -       jb      L(zero)
>> -       movzbl  (%rsi), %ecx
>> -       movzbl  (%rdi), %eax
>> -       subl    %ecx, %eax
>> +       jb      L(between_2_3)
>> +
>> +       /* Load as big endian with overlapping movbe to avoid branches.
>> +        */
>> +       movbe   (%rdi), %eax
>> +       movbe   (%rsi), %ecx
>> +       shlq    $32, %rax
>> +       shlq    $32, %rcx
>> +       movbe   -4(%rdi, %rdx), %edi
>> +       movbe   -4(%rsi, %rdx), %esi
>> +       orq     %rdi, %rax
>> +       orq     %rsi, %rcx
>> +       subq    %rcx, %rax
>> +       /* edx is guranteed to be positive int32 in range [4, 7].  */
>> +       cmovne  %edx, %eax
>> +       /* ecx is -1 if rcx > rax. Otherwise 0.  */
>> +       sbbl    %ecx, %ecx
>> +       /* If rcx > rax, then ecx is 0 and eax is positive. If rcx ==
>> +          rax then eax and ecx are zero. If rax < rax then ecx is -1 so
>> +          eax doesn't matter.  */
>> +       orl     %ecx, %eax
>>         ret
>>
>> -       .p2align 4
>> +       .p2align 4,, 8
>>  L(between_8_15):
>>  # endif
>>         /* If USE_AS_WMEMCMP fall through into 8-15 byte case.  */
>> -       vmovq   (%rdi), %XMM1
>> -       vmovq   (%rsi), %XMM2
>> -       VPCMP   $4, %XMM1, %XMM2, %k1
>> +       vmovq   (%rdi), %xmm1
>> +       vmovq   (%rsi), %xmm2
>> +       VPCMP   $4, %xmm1, %xmm2, %k1
>>         kmovd   %k1, %eax
>>         testl   %eax, %eax
>> -       jnz     L(return_vec_0)
>> +       jnz     L(return_vec_0_lv)
>>         /* Use overlapping loads to avoid branches.  */
>> -       leaq    -8(%rdi, %rdx, CHAR_SIZE), %rdi
>> -       leaq    -8(%rsi, %rdx, CHAR_SIZE), %rsi
>> -       vmovq   (%rdi), %XMM1
>> -       vmovq   (%rsi), %XMM2
>> -       VPCMP   $4, %XMM1, %XMM2, %k1
>> +       vmovq   -8(%rdi, %rdx, CHAR_SIZE), %xmm1
>> +       vmovq   -8(%rsi, %rdx, CHAR_SIZE), %xmm2
>> +       VPCMP   $4, %xmm1, %xmm2, %k1
>> +       addl    $(CHAR_PER_VEC - (8 / CHAR_SIZE)), %edx
>>         kmovd   %k1, %eax
>>         testl   %eax, %eax
>> -       jnz     L(return_vec_0)
>> -       ret
>> -
>> -       .p2align 4
>> -L(zero):
>> -       xorl    %eax, %eax
>> +       jnz     L(return_vec_0_end)
>>         ret
>>
>> -       .p2align 4
>> +       .p2align 4,, 8
>>  L(between_16_31):
>>         /* From 16 to 31 bytes.  No branch when size == 16.  */
>> -       VMOVU   (%rsi), %XMM2
>> -       VPCMP   $4, (%rdi), %XMM2, %k1
>> +
>> +       /* Use movups to save code size.  */
>> +       movups  (%rsi), %xmm2
>> +       VPCMP   $4, (%rdi), %xmm2, %k1
>>         kmovd   %k1, %eax
>>         testl   %eax, %eax
>> -       jnz     L(return_vec_0)
>> -
>> +       jnz     L(return_vec_0_lv)
>>         /* Use overlapping loads to avoid branches.  */
>> -
>> -       VMOVU   -16(%rsi, %rdx, CHAR_SIZE), %XMM2
>> -       leaq    -16(%rdi, %rdx, CHAR_SIZE), %rdi
>> -       leaq    -16(%rsi, %rdx, CHAR_SIZE), %rsi
>> -       VPCMP   $4, (%rdi), %XMM2, %k1
>> +       movups  -16(%rsi, %rdx, CHAR_SIZE), %xmm2
>> +       VPCMP   $4, -16(%rdi, %rdx, CHAR_SIZE), %xmm2, %k1
>> +       addl    $(CHAR_PER_VEC - (16 / CHAR_SIZE)), %edx
>>         kmovd   %k1, %eax
>>         testl   %eax, %eax
>> -       jnz     L(return_vec_0)
>> -       ret
>> -
>> -# ifdef USE_AS_WMEMCMP
>> -       .p2align 4
>> -L(one_or_less):
>> -       jb      L(zero)
>> -       movl    (%rdi), %ecx
>> -       xorl    %edx, %edx
>> -       cmpl    (%rsi), %ecx
>> -       je      L(zero)
>> -       setg    %dl
>> -       leal    -1(%rdx, %rdx), %eax
>> +       jnz     L(return_vec_0_end)
>>         ret
>> -# else
>>
>> -       .p2align 4
>> -L(between_4_7):
>> -       /* Load as big endian with overlapping movbe to avoid branches.
>> -        */
>> -       movbe   (%rdi), %eax
>> -       movbe   (%rsi), %ecx
>> -       shlq    $32, %rax
>> -       shlq    $32, %rcx
>> -       movbe   -4(%rdi, %rdx), %edi
>> -       movbe   -4(%rsi, %rdx), %esi
>> -       orq     %rdi, %rax
>> -       orq     %rsi, %rcx
>> -       subq    %rcx, %rax
>> -       jz      L(zero_4_7)
>> -       sbbl    %eax, %eax
>> -       orl     $1, %eax
>> -L(zero_4_7):
>> +# ifndef USE_AS_WMEMCMP
>> +L(between_2_3):
>> +       /* Load as big endian to avoid branches.  */
>> +       movzwl  (%rdi), %eax
>> +       movzwl  (%rsi), %ecx
>> +       shll    $8, %eax
>> +       shll    $8, %ecx
>> +       bswap   %eax
>> +       bswap   %ecx
>> +       movzbl  -1(%rdi, %rdx), %edi
>> +       movzbl  -1(%rsi, %rdx), %esi
>> +       orl     %edi, %eax
>> +       orl     %esi, %ecx
>> +       /* Subtraction is okay because the upper 8 bits are zero.  */
>> +       subl    %ecx, %eax
>>         ret
>>  # endif
>> -
>>  END (MEMCMP)
>>  #endif
>> --
>> 2.25.1
>>
>> ping
  

On Mon, Sep 27, 2021 at 11:07 AM Noah Goldstein <goldstein.w.n@gmail.com>
wrote:

>
>
> On Tue, Sep 21, 2021 at 7:52 PM Noah Goldstein <goldstein.w.n@gmail.com>
> wrote:
>
>>
>>
>> On Tue, Sep 21, 2021 at 7:51 PM Noah Goldstein <goldstein.w.n@gmail.com>
>> wrote:
>>
>>> No bug.
>>>
>>> The frontend optimizations are to:
>>> 1. Reorganize logically connected basic blocks so they are either in
>>>    the same cache line or adjacent cache lines.
>>> 2. Avoid cases when basic blocks unnecissarily cross cache lines.
>>> 3. Try and 32 byte align any basic blocks possible without sacrificing
>>>    code size. Smaller / Less hot basic blocks are used for this.
>>>
>>> Overall code size shrunk by 168 bytes. This should make up for any
>>> extra costs due to aligning to 64 bytes.
>>>
>>> In general performance before deviated a great deal dependending on
>>> whether entry alignment % 64 was 0, 16, 32, or 48. These changes
>>> essentially make it so that the current implementation is at least
>>> equal to the best alignment of the original for any arguments.
>>>
>>> The only additional optimization is in the page cross case. Branch on
>>> equals case was removed from the size == [4, 7] case. As well the [4,
>>> 7] and [2, 3] case where swapped as [4, 7] is likely a more hot
>>> argument size.
>>>
>>> test-memcmp and test-wmemcmp are both passing.
>>> ---
>>> Performance results attached in reply.
>>>
>> Performance numbers.
>>
>>>
>>> Benchmark CPU:
>>> https://ark.intel.com/content/www/us/en/ark/products/208921/intel-core-i7-1165g7-processor-12m-cache-up-to-4-70-ghz-with-ipu.html
>>>
>>> Results are from geometric mean of N = 20 runs.
>>>
>>> Time for new implemention shown.
>>>
>>> Score(N) is the New Time / Old Time with entry alignment == N
>>>
>>> In general there is one performance degregation for size range [2, 3]
>>> in the page cross case. This is expected as the new code intentionally
>>> gives the fallthrough path to the hotter size range [4, 7].
>>>
>>>
>>> For the micro benchmarks there is no outright performance improvement
>>> in the sense that for each benchmark configuration there is at least
>>> one entry alignment where the old version had roughly the same
>>> performance. But this should provide a great deal more performance
>>> stability.
>>>
>>> The hope, however, is that with the code size shrink and
>>> reorganization of basic blocks for better icache behavior. Concerns
>>> such as those brought up in this paper:
>>>
>>>
>>> https://storage.googleapis.com/pub-tools-public-publication-data/pdf/e52f61fd2c51e8962305120548581efacbc06ffc.pdf
>>>
>>> should be better addressed.
>>>
>>>  sysdeps/x86_64/multiarch/memcmp-evex-movbe.S | 434 +++++++++++--------
>>>  1 file changed, 242 insertions(+), 192 deletions(-)
>>>
>>> diff --git a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
>>> b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
>>> index 654dc7ac8c..2761b54f2e 100644
>>> --- a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
>>> +++ b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
>>> @@ -34,7 +34,24 @@
>>>        area.
>>>     7. Use 2 vector compares when size is 2 * CHAR_PER_VEC or less.
>>>     8. Use 4 vector compares when size is 4 * CHAR_PER_VEC or less.
>>> -   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.  */
>>> +   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.
>>> +
>>> +When possible the implementation tries to optimize for frontend in the
>>> +following ways:
>>> +Throughput:
>>> +    1. All code sections that fit are able to run optimally out of the
>>> +       LSD.
>>> +    2. All code sections that fit are able to run optimally out of the
>>> +       DSB
>>> +    3. Basic blocks are contained in minimum number of fetch blocks
>>> +       necessary.
>>> +
>>> +Latency:
>>> +    1. Logically connected basic blocks are put in the same
>>> +       cache-line.
>>> +    2. Logically connected basic blocks that do not fit in the same
>>> +       cache-line are put in adjacent lines. This can get beneficial
>>> +       L2 spatial prefetching and L1 next-line prefetching.  */
>>>
>>>  # include <sysdep.h>
>>>
>>> @@ -47,9 +64,11 @@
>>>  # ifdef USE_AS_WMEMCMP
>>>  #  define CHAR_SIZE    4
>>>  #  define VPCMP        vpcmpd
>>> +#  define VPTEST       vptestmd
>>>  # else
>>>  #  define CHAR_SIZE    1
>>>  #  define VPCMP        vpcmpub
>>> +#  define VPTEST       vptestmb
>>>  # endif
>>>
>>>  # define VEC_SIZE      32
>>> @@ -75,7 +94,9 @@
>>>  */
>>>
>>>         .section .text.evex,"ax",@progbits
>>> -ENTRY (MEMCMP)
>>> +/* Cache align memcmp entry. This allows for much more thorough
>>> +   frontend optimization.  */
>>> +ENTRY_P2ALIGN (MEMCMP, 6)
>>>  # ifdef __ILP32__
>>>         /* Clear the upper 32 bits.  */
>>>         movl    %edx, %edx
>>> @@ -89,7 +110,7 @@ ENTRY (MEMCMP)
>>>         VPCMP   $4, (%rdi), %YMM1, %k1
>>>         kmovd   %k1, %eax
>>>         /* NB: eax must be destination register if going to
>>> -          L(return_vec_[0,2]). For L(return_vec_3 destination register
>>> +          L(return_vec_[0,2]). For L(return_vec_3) destination register
>>>            must be ecx.  */
>>>         testl   %eax, %eax
>>>         jnz     L(return_vec_0)
>>> @@ -121,10 +142,6 @@ ENTRY (MEMCMP)
>>>         testl   %ecx, %ecx
>>>         jnz     L(return_vec_3)
>>>
>>> -       /* Zero YMM0. 4x VEC reduction is done with vpxor + vtern so
>>> -          compare with zero to get a mask is needed.  */
>>> -       vpxorq  %XMM0, %XMM0, %XMM0
>>> -
>>>         /* Go to 4x VEC loop.  */
>>>         cmpq    $(CHAR_PER_VEC * 8), %rdx
>>>         ja      L(more_8x_vec)
>>> @@ -148,47 +165,61 @@ ENTRY (MEMCMP)
>>>
>>>         VMOVU   (VEC_SIZE * 2)(%rsi), %YMM3
>>>         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
>>> -       /* Or together YMM1, YMM2, and YMM3 into YMM3.  */
>>> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
>>>
>>>         VMOVU   (VEC_SIZE * 3)(%rsi), %YMM4
>>>         /* Ternary logic to xor (VEC_SIZE * 3)(%rdi) with YMM4 while
>>> -          oring with YMM3. Result is stored in YMM4.  */
>>> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
>>> -       /* Compare YMM4 with 0. If any 1s s1 and s2 don't match.  */
>>> -       VPCMP   $4, %YMM4, %YMM0, %k1
>>> +          oring with YMM1. Result is stored in YMM4.  */
>>> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
>>> +
>>> +       /* Or together YMM2, YMM3, and YMM4 into YMM4.  */
>>> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
>>> +
>>> +       /* Test YMM4 against itself. Store any CHAR mismatches in k1.
>>> +        */
>>> +       VPTEST  %YMM4, %YMM4, %k1
>>> +       /* k1 must go to ecx for L(return_vec_0_1_2_3).  */
>>>         kmovd   %k1, %ecx
>>>         testl   %ecx, %ecx
>>>         jnz     L(return_vec_0_1_2_3)
>>>         /* NB: eax must be zero to reach here.  */
>>>         ret
>>>
>>> -       /* NB: aligning 32 here allows for the rest of the jump targets
>>> -          to be tuned for 32 byte alignment. Most important this ensures
>>> -          the L(more_8x_vec) loop is 32 byte aligned.  */
>>> -       .p2align 5
>>> -L(less_vec):
>>> -       /* Check if one or less CHAR. This is necessary for size = 0 but
>>> -          is also faster for size = CHAR_SIZE.  */
>>> -       cmpl    $1, %edx
>>> -       jbe     L(one_or_less)
>>> +       .p2align 4
>>> +L(8x_end_return_vec_0_1_2_3):
>>> +       movq    %rdx, %rdi
>>> +L(8x_return_vec_0_1_2_3):
>>> +       addq    %rdi, %rsi
>>> +L(return_vec_0_1_2_3):
>>> +       VPTEST  %YMM1, %YMM1, %k0
>>> +       kmovd   %k0, %eax
>>> +       testl   %eax, %eax
>>> +       jnz     L(return_vec_0)
>>>
>>> -       /* Check if loading one VEC from either s1 or s2 could cause a
>>> -          page cross. This can have false positives but is by far the
>>> -          fastest method.  */
>>> -       movl    %edi, %eax
>>> -       orl     %esi, %eax
>>> -       andl    $(PAGE_SIZE - 1), %eax
>>> -       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
>>> -       jg      L(page_cross_less_vec)
>>> +       VPTEST  %YMM2, %YMM2, %k0
>>> +       kmovd   %k0, %eax
>>> +       testl   %eax, %eax
>>> +       jnz     L(return_vec_1)
>>>
>>> -       /* No page cross possible.  */
>>> -       VMOVU   (%rsi), %YMM2
>>> -       VPCMP   $4, (%rdi), %YMM2, %k1
>>> -       kmovd   %k1, %eax
>>> -       /* Create mask in ecx for potentially in bound matches.  */
>>> -       bzhil   %edx, %eax, %eax
>>> -       jnz     L(return_vec_0)
>>> +       VPTEST  %YMM3, %YMM3, %k0
>>> +       kmovd   %k0, %eax
>>> +       testl   %eax, %eax
>>> +       jnz     L(return_vec_2)
>>> +L(return_vec_3):
>>> +       /* bsf saves 1 byte from tzcnt. This keep L(return_vec_3) in one
>>> +          fetch block and the entire L(*return_vec_0_1_2_3) in 1 cache
>>> +          line.  */
>>> +       bsfl    %ecx, %ecx
>>> +# ifdef USE_AS_WMEMCMP
>>> +       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
>>> +       xorl    %edx, %edx
>>> +       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
>>> +       setg    %dl
>>> +       leal    -1(%rdx, %rdx), %eax
>>> +# else
>>> +       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
>>> +       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
>>> +       subl    %ecx, %eax
>>> +# endif
>>>         ret
>>>
>>>         .p2align 4
>>> @@ -209,10 +240,11 @@ L(return_vec_0):
>>>  # endif
>>>         ret
>>>
>>> -       /* NB: No p2align necessary. Alignment  % 16 is naturally 1
>>> -          which is good enough for a target not in a loop.  */
>>> +       .p2align 4
>>>  L(return_vec_1):
>>> -       tzcntl  %eax, %eax
>>> +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_1) in one
>>> +          fetch block.  */
>>> +       bsfl    %eax, %eax
>>>  # ifdef USE_AS_WMEMCMP
>>>         movl    VEC_SIZE(%rdi, %rax, CHAR_SIZE), %ecx
>>>         xorl    %edx, %edx
>>> @@ -226,10 +258,11 @@ L(return_vec_1):
>>>  # endif
>>>         ret
>>>
>>> -       /* NB: No p2align necessary. Alignment  % 16 is naturally 2
>>> -          which is good enough for a target not in a loop.  */
>>> +       .p2align 4,, 10
>>>  L(return_vec_2):
>>> -       tzcntl  %eax, %eax
>>> +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_2) in one
>>> +          fetch block.  */
>>> +       bsfl    %eax, %eax
>>>  # ifdef USE_AS_WMEMCMP
>>>         movl    (VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>>>         xorl    %edx, %edx
>>> @@ -243,40 +276,6 @@ L(return_vec_2):
>>>  # endif
>>>         ret
>>>
>>> -       .p2align 4
>>> -L(8x_return_vec_0_1_2_3):
>>> -       /* Returning from L(more_8x_vec) requires restoring rsi.  */
>>> -       addq    %rdi, %rsi
>>> -L(return_vec_0_1_2_3):
>>> -       VPCMP   $4, %YMM1, %YMM0, %k0
>>> -       kmovd   %k0, %eax
>>> -       testl   %eax, %eax
>>> -       jnz     L(return_vec_0)
>>> -
>>> -       VPCMP   $4, %YMM2, %YMM0, %k0
>>> -       kmovd   %k0, %eax
>>> -       testl   %eax, %eax
>>> -       jnz     L(return_vec_1)
>>> -
>>> -       VPCMP   $4, %YMM3, %YMM0, %k0
>>> -       kmovd   %k0, %eax
>>> -       testl   %eax, %eax
>>> -       jnz     L(return_vec_2)
>>> -L(return_vec_3):
>>> -       tzcntl  %ecx, %ecx
>>> -# ifdef USE_AS_WMEMCMP
>>> -       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
>>> -       xorl    %edx, %edx
>>> -       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
>>> -       setg    %dl
>>> -       leal    -1(%rdx, %rdx), %eax
>>> -# else
>>> -       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
>>> -       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
>>> -       subl    %ecx, %eax
>>> -# endif
>>> -       ret
>>> -
>>>         .p2align 4
>>>  L(more_8x_vec):
>>>         /* Set end of s1 in rdx.  */
>>> @@ -288,21 +287,19 @@ L(more_8x_vec):
>>>         andq    $-VEC_SIZE, %rdi
>>>         /* Adjust because first 4x vec where check already.  */
>>>         subq    $-(VEC_SIZE * 4), %rdi
>>> +
>>>         .p2align 4
>>>  L(loop_4x_vec):
>>>         VMOVU   (%rsi, %rdi), %YMM1
>>>         vpxorq  (%rdi), %YMM1, %YMM1
>>> -
>>>         VMOVU   VEC_SIZE(%rsi, %rdi), %YMM2
>>>         vpxorq  VEC_SIZE(%rdi), %YMM2, %YMM2
>>> -
>>>         VMOVU   (VEC_SIZE * 2)(%rsi, %rdi), %YMM3
>>>         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
>>> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
>>> -
>>>         VMOVU   (VEC_SIZE * 3)(%rsi, %rdi), %YMM4
>>> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
>>> -       VPCMP   $4, %YMM4, %YMM0, %k1
>>> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
>>> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
>>> +       VPTEST  %YMM4, %YMM4, %k1
>>>         kmovd   %k1, %ecx
>>>         testl   %ecx, %ecx
>>>         jnz     L(8x_return_vec_0_1_2_3)
>>> @@ -319,28 +316,25 @@ L(loop_4x_vec):
>>>         cmpl    $(VEC_SIZE * 2), %edi
>>>         jae     L(8x_last_2x_vec)
>>>
>>> +       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
>>> +
>>>         VMOVU   (%rsi, %rdx), %YMM1
>>>         vpxorq  (%rdx), %YMM1, %YMM1
>>>
>>>         VMOVU   VEC_SIZE(%rsi, %rdx), %YMM2
>>>         vpxorq  VEC_SIZE(%rdx), %YMM2, %YMM2
>>> -
>>> -       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
>>> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
>>> -
>>>         VMOVU   (VEC_SIZE * 3)(%rsi, %rdx), %YMM4
>>> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM3, %YMM4
>>> -       VPCMP   $4, %YMM4, %YMM0, %k1
>>> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM1, %YMM4
>>> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
>>> +       VPTEST  %YMM4, %YMM4, %k1
>>>         kmovd   %k1, %ecx
>>> -       /* Restore s1 pointer to rdi.  */
>>> -       movq    %rdx, %rdi
>>>         testl   %ecx, %ecx
>>> -       jnz     L(8x_return_vec_0_1_2_3)
>>> +       jnz     L(8x_end_return_vec_0_1_2_3)
>>>         /* NB: eax must be zero to reach here.  */
>>>         ret
>>>
>>>         /* Only entry is from L(more_8x_vec).  */
>>> -       .p2align 4
>>> +       .p2align 4,, 10
>>>  L(8x_last_2x_vec):
>>>         VPCMP   $4, (VEC_SIZE * 2)(%rdx), %YMM3, %k1
>>>         kmovd   %k1, %eax
>>> @@ -355,7 +349,31 @@ L(8x_last_1x_vec):
>>>         jnz     L(8x_return_vec_3)
>>>         ret
>>>
>>> -       .p2align 4
>>> +       /* Not ideally aligned (at offset +9 bytes in fetch block) but
>>> +          not aligning keeps it in the same cache line as
>>> +          L(8x_last_1x/2x_vec) so likely worth it. As well, saves code
>>> +          size.  */
>>> +       .p2align 4,, 4
>>> +L(8x_return_vec_2):
>>> +       subq    $VEC_SIZE, %rdx
>>> +L(8x_return_vec_3):
>>> +       bsfl    %eax, %eax
>>> +# ifdef USE_AS_WMEMCMP
>>> +       leaq    (%rdx, %rax, CHAR_SIZE), %rax
>>> +       movl    (VEC_SIZE * 3)(%rax), %ecx
>>> +       xorl    %edx, %edx
>>> +       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
>>> +       setg    %dl
>>> +       leal    -1(%rdx, %rdx), %eax
>>> +# else
>>> +       addq    %rdx, %rax
>>> +       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
>>> +       movzbl  (VEC_SIZE * 3)(%rax), %eax
>>> +       subl    %ecx, %eax
>>> +# endif
>>> +       ret
>>> +
>>> +       .p2align 4,, 10
>>>  L(last_2x_vec):
>>>         /* Check second to last VEC.  */
>>>         VMOVU   -(VEC_SIZE * 2)(%rsi, %rdx, CHAR_SIZE), %YMM1
>>> @@ -374,26 +392,49 @@ L(last_1x_vec):
>>>         jnz     L(return_vec_0_end)
>>>         ret
>>>
>>> -       .p2align 4
>>> -L(8x_return_vec_2):
>>> -       subq    $VEC_SIZE, %rdx
>>> -L(8x_return_vec_3):
>>> -       tzcntl  %eax, %eax
>>> +       .p2align 4,, 10
>>> +L(return_vec_1_end):
>>> +       /* Use bsf to save code size. This is necessary to have
>>> +          L(one_or_less) fit in aligning bytes between.  */
>>> +       bsfl    %eax, %eax
>>> +       addl    %edx, %eax
>>>  # ifdef USE_AS_WMEMCMP
>>> -       leaq    (%rdx, %rax, CHAR_SIZE), %rax
>>> -       movl    (VEC_SIZE * 3)(%rax), %ecx
>>> +       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>>>         xorl    %edx, %edx
>>> -       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
>>> +       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
>>>         setg    %dl
>>>         leal    -1(%rdx, %rdx), %eax
>>>  # else
>>> -       addq    %rdx, %rax
>>> -       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
>>> -       movzbl  (VEC_SIZE * 3)(%rax), %eax
>>> +       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
>>> +       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
>>>         subl    %ecx, %eax
>>>  # endif
>>>         ret
>>>
>>> +       /* NB: L(one_or_less) fits in alignment padding between
>>> +          L(return_vec_1_end) and L(return_vec_0_end).  */
>>> +# ifdef USE_AS_WMEMCMP
>>> +L(one_or_less):
>>> +       jb      L(zero)
>>> +       movl    (%rdi), %ecx
>>> +       xorl    %edx, %edx
>>> +       cmpl    (%rsi), %ecx
>>> +       je      L(zero)
>>> +       setg    %dl
>>> +       leal    -1(%rdx, %rdx), %eax
>>> +       ret
>>> +# else
>>> +L(one_or_less):
>>> +       jb      L(zero)
>>> +       movzbl  (%rsi), %ecx
>>> +       movzbl  (%rdi), %eax
>>> +       subl    %ecx, %eax
>>> +       ret
>>> +# endif
>>> +L(zero):
>>> +       xorl    %eax, %eax
>>> +       ret
>>> +
>>>         .p2align 4
>>>  L(return_vec_0_end):
>>>         tzcntl  %eax, %eax
>>> @@ -412,23 +453,56 @@ L(return_vec_0_end):
>>>         ret
>>>
>>>         .p2align 4
>>> -L(return_vec_1_end):
>>> +L(less_vec):
>>> +       /* Check if one or less CHAR. This is necessary for size == 0
>>> +          but is also faster for size == CHAR_SIZE.  */
>>> +       cmpl    $1, %edx
>>> +       jbe     L(one_or_less)
>>> +
>>> +       /* Check if loading one VEC from either s1 or s2 could cause a
>>> +          page cross. This can have false positives but is by far the
>>> +          fastest method.  */
>>> +       movl    %edi, %eax
>>> +       orl     %esi, %eax
>>> +       andl    $(PAGE_SIZE - 1), %eax
>>> +       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
>>> +       jg      L(page_cross_less_vec)
>>> +
>>> +       /* No page cross possible.  */
>>> +       VMOVU   (%rsi), %YMM2
>>> +       VPCMP   $4, (%rdi), %YMM2, %k1
>>> +       kmovd   %k1, %eax
>>> +       /* Check if any matches where in bounds. Intentionally not
>>> +          storing result in eax to limit dependency chain if it goes to
>>> +          L(return_vec_0_lv).  */
>>> +       bzhil   %edx, %eax, %edx
>>> +       jnz     L(return_vec_0_lv)
>>> +       xorl    %eax, %eax
>>> +       ret
>>> +
>>> +       /* Essentially duplicate of L(return_vec_0). Ends up not costing
>>> +          any code as shrinks L(less_vec) by allowing 2-byte encoding of
>>> +          the jump and ends up fitting in aligning bytes. As well fits
>>> on
>>> +          same cache line as L(less_vec) so also saves a line from
>>> having
>>> +          to be fetched on cold calls to memcmp.  */
>>> +       .p2align 4,, 4
>>> +L(return_vec_0_lv):
>>>         tzcntl  %eax, %eax
>>> -       addl    %edx, %eax
>>>  # ifdef USE_AS_WMEMCMP
>>> -       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>>> +       movl    (%rdi, %rax, CHAR_SIZE), %ecx
>>>         xorl    %edx, %edx
>>> -       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
>>> +       cmpl    (%rsi, %rax, CHAR_SIZE), %ecx
>>> +       /* NB: no partial register stall here because xorl zero idiom
>>> +          above.  */
>>>         setg    %dl
>>>         leal    -1(%rdx, %rdx), %eax
>>>  # else
>>> -       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
>>> -       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
>>> +       movzbl  (%rsi, %rax), %ecx
>>> +       movzbl  (%rdi, %rax), %eax
>>>         subl    %ecx, %eax
>>>  # endif
>>>         ret
>>>
>>> -
>>>         .p2align 4
>>>  L(page_cross_less_vec):
>>>         /* if USE_AS_WMEMCMP it can only be 0, 4, 8, 12, 16, 20, 24, 28
>>> @@ -439,108 +513,84 @@ L(page_cross_less_vec):
>>>         cmpl    $8, %edx
>>>         jae     L(between_8_15)
>>>         cmpl    $4, %edx
>>> -       jae     L(between_4_7)
>>> -L(between_2_3):
>>> -       /* Load as big endian to avoid branches.  */
>>> -       movzwl  (%rdi), %eax
>>> -       movzwl  (%rsi), %ecx
>>> -       shll    $8, %eax
>>> -       shll    $8, %ecx
>>> -       bswap   %eax
>>> -       bswap   %ecx
>>> -       movzbl  -1(%rdi, %rdx), %edi
>>> -       movzbl  -1(%rsi, %rdx), %esi
>>> -       orl     %edi, %eax
>>> -       orl     %esi, %ecx
>>> -       /* Subtraction is okay because the upper 8 bits are zero.  */
>>> -       subl    %ecx, %eax
>>> -       ret
>>> -       .p2align 4
>>> -L(one_or_less):
>>> -       jb      L(zero)
>>> -       movzbl  (%rsi), %ecx
>>> -       movzbl  (%rdi), %eax
>>> -       subl    %ecx, %eax
>>> +       jb      L(between_2_3)
>>> +
>>> +       /* Load as big endian with overlapping movbe to avoid branches.
>>> +        */
>>> +       movbe   (%rdi), %eax
>>> +       movbe   (%rsi), %ecx
>>> +       shlq    $32, %rax
>>> +       shlq    $32, %rcx
>>> +       movbe   -4(%rdi, %rdx), %edi
>>> +       movbe   -4(%rsi, %rdx), %esi
>>> +       orq     %rdi, %rax
>>> +       orq     %rsi, %rcx
>>> +       subq    %rcx, %rax
>>> +       /* edx is guranteed to be positive int32 in range [4, 7].  */
>>> +       cmovne  %edx, %eax
>>> +       /* ecx is -1 if rcx > rax. Otherwise 0.  */
>>> +       sbbl    %ecx, %ecx
>>> +       /* If rcx > rax, then ecx is 0 and eax is positive. If rcx ==
>>> +          rax then eax and ecx are zero. If rax < rax then ecx is -1 so
>>> +          eax doesn't matter.  */
>>> +       orl     %ecx, %eax
>>>         ret
>>>
>>> -       .p2align 4
>>> +       .p2align 4,, 8
>>>  L(between_8_15):
>>>  # endif
>>>         /* If USE_AS_WMEMCMP fall through into 8-15 byte case.  */
>>> -       vmovq   (%rdi), %XMM1
>>> -       vmovq   (%rsi), %XMM2
>>> -       VPCMP   $4, %XMM1, %XMM2, %k1
>>> +       vmovq   (%rdi), %xmm1
>>> +       vmovq   (%rsi), %xmm2
>>> +       VPCMP   $4, %xmm1, %xmm2, %k1
>>>         kmovd   %k1, %eax
>>>         testl   %eax, %eax
>>> -       jnz     L(return_vec_0)
>>> +       jnz     L(return_vec_0_lv)
>>>         /* Use overlapping loads to avoid branches.  */
>>> -       leaq    -8(%rdi, %rdx, CHAR_SIZE), %rdi
>>> -       leaq    -8(%rsi, %rdx, CHAR_SIZE), %rsi
>>> -       vmovq   (%rdi), %XMM1
>>> -       vmovq   (%rsi), %XMM2
>>> -       VPCMP   $4, %XMM1, %XMM2, %k1
>>> +       vmovq   -8(%rdi, %rdx, CHAR_SIZE), %xmm1
>>> +       vmovq   -8(%rsi, %rdx, CHAR_SIZE), %xmm2
>>> +       VPCMP   $4, %xmm1, %xmm2, %k1
>>> +       addl    $(CHAR_PER_VEC - (8 / CHAR_SIZE)), %edx
>>>         kmovd   %k1, %eax
>>>         testl   %eax, %eax
>>> -       jnz     L(return_vec_0)
>>> -       ret
>>> -
>>> -       .p2align 4
>>> -L(zero):
>>> -       xorl    %eax, %eax
>>> +       jnz     L(return_vec_0_end)
>>>         ret
>>>
>>> -       .p2align 4
>>> +       .p2align 4,, 8
>>>  L(between_16_31):
>>>         /* From 16 to 31 bytes.  No branch when size == 16.  */
>>> -       VMOVU   (%rsi), %XMM2
>>> -       VPCMP   $4, (%rdi), %XMM2, %k1
>>> +
>>> +       /* Use movups to save code size.  */
>>> +       movups  (%rsi), %xmm2
>>> +       VPCMP   $4, (%rdi), %xmm2, %k1
>>>         kmovd   %k1, %eax
>>>         testl   %eax, %eax
>>> -       jnz     L(return_vec_0)
>>> -
>>> +       jnz     L(return_vec_0_lv)
>>>         /* Use overlapping loads to avoid branches.  */
>>> -
>>> -       VMOVU   -16(%rsi, %rdx, CHAR_SIZE), %XMM2
>>> -       leaq    -16(%rdi, %rdx, CHAR_SIZE), %rdi
>>> -       leaq    -16(%rsi, %rdx, CHAR_SIZE), %rsi
>>> -       VPCMP   $4, (%rdi), %XMM2, %k1
>>> +       movups  -16(%rsi, %rdx, CHAR_SIZE), %xmm2
>>> +       VPCMP   $4, -16(%rdi, %rdx, CHAR_SIZE), %xmm2, %k1
>>> +       addl    $(CHAR_PER_VEC - (16 / CHAR_SIZE)), %edx
>>>         kmovd   %k1, %eax
>>>         testl   %eax, %eax
>>> -       jnz     L(return_vec_0)
>>> -       ret
>>> -
>>> -# ifdef USE_AS_WMEMCMP
>>> -       .p2align 4
>>> -L(one_or_less):
>>> -       jb      L(zero)
>>> -       movl    (%rdi), %ecx
>>> -       xorl    %edx, %edx
>>> -       cmpl    (%rsi), %ecx
>>> -       je      L(zero)
>>> -       setg    %dl
>>> -       leal    -1(%rdx, %rdx), %eax
>>> +       jnz     L(return_vec_0_end)
>>>         ret
>>> -# else
>>>
>>> -       .p2align 4
>>> -L(between_4_7):
>>> -       /* Load as big endian with overlapping movbe to avoid branches.
>>> -        */
>>> -       movbe   (%rdi), %eax
>>> -       movbe   (%rsi), %ecx
>>> -       shlq    $32, %rax
>>> -       shlq    $32, %rcx
>>> -       movbe   -4(%rdi, %rdx), %edi
>>> -       movbe   -4(%rsi, %rdx), %esi
>>> -       orq     %rdi, %rax
>>> -       orq     %rsi, %rcx
>>> -       subq    %rcx, %rax
>>> -       jz      L(zero_4_7)
>>> -       sbbl    %eax, %eax
>>> -       orl     $1, %eax
>>> -L(zero_4_7):
>>> +# ifndef USE_AS_WMEMCMP
>>> +L(between_2_3):
>>> +       /* Load as big endian to avoid branches.  */
>>> +       movzwl  (%rdi), %eax
>>> +       movzwl  (%rsi), %ecx
>>> +       shll    $8, %eax
>>> +       shll    $8, %ecx
>>> +       bswap   %eax
>>> +       bswap   %ecx
>>> +       movzbl  -1(%rdi, %rdx), %edi
>>> +       movzbl  -1(%rsi, %rdx), %esi
>>> +       orl     %edi, %eax
>>> +       orl     %esi, %ecx
>>> +       /* Subtraction is okay because the upper 8 bits are zero.  */
>>> +       subl    %ecx, %eax
>>>         ret
>>>  # endif
>>> -
>>>  END (MEMCMP)
>>>  #endif
>>> --
>>> 2.25.1
>>>
>>> ping
>
ping
  

On Tue, Sep 21, 2021 at 5:51 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
>
> No bug.
>
> The frontend optimizations are to:
> 1. Reorganize logically connected basic blocks so they are either in
>    the same cache line or adjacent cache lines.
> 2. Avoid cases when basic blocks unnecissarily cross cache lines.
> 3. Try and 32 byte align any basic blocks possible without sacrificing
>    code size. Smaller / Less hot basic blocks are used for this.
>
> Overall code size shrunk by 168 bytes. This should make up for any
> extra costs due to aligning to 64 bytes.
>
> In general performance before deviated a great deal dependending on
> whether entry alignment % 64 was 0, 16, 32, or 48. These changes
> essentially make it so that the current implementation is at least
> equal to the best alignment of the original for any arguments.
>
> The only additional optimization is in the page cross case. Branch on
> equals case was removed from the size == [4, 7] case. As well the [4,
> 7] and [2, 3] case where swapped as [4, 7] is likely a more hot
> argument size.
>
> test-memcmp and test-wmemcmp are both passing.
> ---
> Performance results attached in reply.
>
> Benchmark CPU: https://ark.intel.com/content/www/us/en/ark/products/208921/intel-core-i7-1165g7-processor-12m-cache-up-to-4-70-ghz-with-ipu.html
>
> Results are from geometric mean of N = 20 runs.
>
> Time for new implemention shown.
>
> Score(N) is the New Time / Old Time with entry alignment == N
>
> In general there is one performance degregation for size range [2, 3]
> in the page cross case. This is expected as the new code intentionally
> gives the fallthrough path to the hotter size range [4, 7].
>
>
> For the micro benchmarks there is no outright performance improvement
> in the sense that for each benchmark configuration there is at least
> one entry alignment where the old version had roughly the same
> performance. But this should provide a great deal more performance
> stability.
>
> The hope, however, is that with the code size shrink and
> reorganization of basic blocks for better icache behavior. Concerns
> such as those brought up in this paper:
>
> https://storage.googleapis.com/pub-tools-public-publication-data/pdf/e52f61fd2c51e8962305120548581efacbc06ffc.pdf
>
> should be better addressed.
>
>  sysdeps/x86_64/multiarch/memcmp-evex-movbe.S | 434 +++++++++++--------
>  1 file changed, 242 insertions(+), 192 deletions(-)
>
> diff --git a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> index 654dc7ac8c..2761b54f2e 100644
> --- a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> +++ b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> @@ -34,7 +34,24 @@
>        area.
>     7. Use 2 vector compares when size is 2 * CHAR_PER_VEC or less.
>     8. Use 4 vector compares when size is 4 * CHAR_PER_VEC or less.
> -   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.  */
> +   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.
> +
> +When possible the implementation tries to optimize for frontend in the
> +following ways:
> +Throughput:
> +    1. All code sections that fit are able to run optimally out of the
> +       LSD.
> +    2. All code sections that fit are able to run optimally out of the
> +       DSB
> +    3. Basic blocks are contained in minimum number of fetch blocks
> +       necessary.
> +
> +Latency:
> +    1. Logically connected basic blocks are put in the same
> +       cache-line.
> +    2. Logically connected basic blocks that do not fit in the same
> +       cache-line are put in adjacent lines. This can get beneficial
> +       L2 spatial prefetching and L1 next-line prefetching.  */
>
>  # include <sysdep.h>
>
> @@ -47,9 +64,11 @@
>  # ifdef USE_AS_WMEMCMP
>  #  define CHAR_SIZE    4
>  #  define VPCMP        vpcmpd
> +#  define VPTEST       vptestmd
>  # else
>  #  define CHAR_SIZE    1
>  #  define VPCMP        vpcmpub
> +#  define VPTEST       vptestmb
>  # endif
>
>  # define VEC_SIZE      32
> @@ -75,7 +94,9 @@
>  */
>
>         .section .text.evex,"ax",@progbits
> -ENTRY (MEMCMP)
> +/* Cache align memcmp entry. This allows for much more thorough
> +   frontend optimization.  */
> +ENTRY_P2ALIGN (MEMCMP, 6)
>  # ifdef __ILP32__
>         /* Clear the upper 32 bits.  */
>         movl    %edx, %edx
> @@ -89,7 +110,7 @@ ENTRY (MEMCMP)
>         VPCMP   $4, (%rdi), %YMM1, %k1
>         kmovd   %k1, %eax
>         /* NB: eax must be destination register if going to
> -          L(return_vec_[0,2]). For L(return_vec_3 destination register
> +          L(return_vec_[0,2]). For L(return_vec_3) destination register
>            must be ecx.  */
>         testl   %eax, %eax
>         jnz     L(return_vec_0)
> @@ -121,10 +142,6 @@ ENTRY (MEMCMP)
>         testl   %ecx, %ecx
>         jnz     L(return_vec_3)
>
> -       /* Zero YMM0. 4x VEC reduction is done with vpxor + vtern so
> -          compare with zero to get a mask is needed.  */
> -       vpxorq  %XMM0, %XMM0, %XMM0
> -
>         /* Go to 4x VEC loop.  */
>         cmpq    $(CHAR_PER_VEC * 8), %rdx
>         ja      L(more_8x_vec)
> @@ -148,47 +165,61 @@ ENTRY (MEMCMP)
>
>         VMOVU   (VEC_SIZE * 2)(%rsi), %YMM3
>         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> -       /* Or together YMM1, YMM2, and YMM3 into YMM3.  */
> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
>
>         VMOVU   (VEC_SIZE * 3)(%rsi), %YMM4
>         /* Ternary logic to xor (VEC_SIZE * 3)(%rdi) with YMM4 while
> -          oring with YMM3. Result is stored in YMM4.  */
> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
> -       /* Compare YMM4 with 0. If any 1s s1 and s2 don't match.  */
> -       VPCMP   $4, %YMM4, %YMM0, %k1
> +          oring with YMM1. Result is stored in YMM4.  */
> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
> +
> +       /* Or together YMM2, YMM3, and YMM4 into YMM4.  */
> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> +
> +       /* Test YMM4 against itself. Store any CHAR mismatches in k1.
> +        */
> +       VPTEST  %YMM4, %YMM4, %k1
> +       /* k1 must go to ecx for L(return_vec_0_1_2_3).  */
>         kmovd   %k1, %ecx
>         testl   %ecx, %ecx
>         jnz     L(return_vec_0_1_2_3)
>         /* NB: eax must be zero to reach here.  */
>         ret
>
> -       /* NB: aligning 32 here allows for the rest of the jump targets
> -          to be tuned for 32 byte alignment. Most important this ensures
> -          the L(more_8x_vec) loop is 32 byte aligned.  */
> -       .p2align 5
> -L(less_vec):
> -       /* Check if one or less CHAR. This is necessary for size = 0 but
> -          is also faster for size = CHAR_SIZE.  */
> -       cmpl    $1, %edx
> -       jbe     L(one_or_less)
> +       .p2align 4
> +L(8x_end_return_vec_0_1_2_3):
> +       movq    %rdx, %rdi
> +L(8x_return_vec_0_1_2_3):
> +       addq    %rdi, %rsi
> +L(return_vec_0_1_2_3):
> +       VPTEST  %YMM1, %YMM1, %k0
> +       kmovd   %k0, %eax
> +       testl   %eax, %eax
> +       jnz     L(return_vec_0)
>
> -       /* Check if loading one VEC from either s1 or s2 could cause a
> -          page cross. This can have false positives but is by far the
> -          fastest method.  */
> -       movl    %edi, %eax
> -       orl     %esi, %eax
> -       andl    $(PAGE_SIZE - 1), %eax
> -       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
> -       jg      L(page_cross_less_vec)
> +       VPTEST  %YMM2, %YMM2, %k0
> +       kmovd   %k0, %eax
> +       testl   %eax, %eax
> +       jnz     L(return_vec_1)
>
> -       /* No page cross possible.  */
> -       VMOVU   (%rsi), %YMM2
> -       VPCMP   $4, (%rdi), %YMM2, %k1
> -       kmovd   %k1, %eax
> -       /* Create mask in ecx for potentially in bound matches.  */
> -       bzhil   %edx, %eax, %eax
> -       jnz     L(return_vec_0)
> +       VPTEST  %YMM3, %YMM3, %k0
> +       kmovd   %k0, %eax
> +       testl   %eax, %eax
> +       jnz     L(return_vec_2)
> +L(return_vec_3):
> +       /* bsf saves 1 byte from tzcnt. This keep L(return_vec_3) in one
> +          fetch block and the entire L(*return_vec_0_1_2_3) in 1 cache
> +          line.  */
> +       bsfl    %ecx, %ecx
> +# ifdef USE_AS_WMEMCMP
> +       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
> +       xorl    %edx, %edx
> +       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
> +       setg    %dl
> +       leal    -1(%rdx, %rdx), %eax
> +# else
> +       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
> +       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
> +       subl    %ecx, %eax
> +# endif
>         ret
>
>         .p2align 4
> @@ -209,10 +240,11 @@ L(return_vec_0):
>  # endif
>         ret
>
> -       /* NB: No p2align necessary. Alignment  % 16 is naturally 1
> -          which is good enough for a target not in a loop.  */
> +       .p2align 4
>  L(return_vec_1):
> -       tzcntl  %eax, %eax
> +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_1) in one
> +          fetch block.  */
> +       bsfl    %eax, %eax
>  # ifdef USE_AS_WMEMCMP
>         movl    VEC_SIZE(%rdi, %rax, CHAR_SIZE), %ecx
>         xorl    %edx, %edx
> @@ -226,10 +258,11 @@ L(return_vec_1):
>  # endif
>         ret
>
> -       /* NB: No p2align necessary. Alignment  % 16 is naturally 2
> -          which is good enough for a target not in a loop.  */
> +       .p2align 4,, 10
>  L(return_vec_2):
> -       tzcntl  %eax, %eax
> +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_2) in one
> +          fetch block.  */
> +       bsfl    %eax, %eax
>  # ifdef USE_AS_WMEMCMP
>         movl    (VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>         xorl    %edx, %edx
> @@ -243,40 +276,6 @@ L(return_vec_2):
>  # endif
>         ret
>
> -       .p2align 4
> -L(8x_return_vec_0_1_2_3):
> -       /* Returning from L(more_8x_vec) requires restoring rsi.  */
> -       addq    %rdi, %rsi
> -L(return_vec_0_1_2_3):
> -       VPCMP   $4, %YMM1, %YMM0, %k0
> -       kmovd   %k0, %eax
> -       testl   %eax, %eax
> -       jnz     L(return_vec_0)
> -
> -       VPCMP   $4, %YMM2, %YMM0, %k0
> -       kmovd   %k0, %eax
> -       testl   %eax, %eax
> -       jnz     L(return_vec_1)
> -
> -       VPCMP   $4, %YMM3, %YMM0, %k0
> -       kmovd   %k0, %eax
> -       testl   %eax, %eax
> -       jnz     L(return_vec_2)
> -L(return_vec_3):
> -       tzcntl  %ecx, %ecx
> -# ifdef USE_AS_WMEMCMP
> -       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
> -       xorl    %edx, %edx
> -       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
> -       setg    %dl
> -       leal    -1(%rdx, %rdx), %eax
> -# else
> -       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
> -       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
> -       subl    %ecx, %eax
> -# endif
> -       ret
> -
>         .p2align 4
>  L(more_8x_vec):
>         /* Set end of s1 in rdx.  */
> @@ -288,21 +287,19 @@ L(more_8x_vec):
>         andq    $-VEC_SIZE, %rdi
>         /* Adjust because first 4x vec where check already.  */
>         subq    $-(VEC_SIZE * 4), %rdi
> +
>         .p2align 4
>  L(loop_4x_vec):
>         VMOVU   (%rsi, %rdi), %YMM1
>         vpxorq  (%rdi), %YMM1, %YMM1
> -
>         VMOVU   VEC_SIZE(%rsi, %rdi), %YMM2
>         vpxorq  VEC_SIZE(%rdi), %YMM2, %YMM2
> -
>         VMOVU   (VEC_SIZE * 2)(%rsi, %rdi), %YMM3
>         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> -
>         VMOVU   (VEC_SIZE * 3)(%rsi, %rdi), %YMM4
> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
> -       VPCMP   $4, %YMM4, %YMM0, %k1
> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> +       VPTEST  %YMM4, %YMM4, %k1
>         kmovd   %k1, %ecx
>         testl   %ecx, %ecx
>         jnz     L(8x_return_vec_0_1_2_3)
> @@ -319,28 +316,25 @@ L(loop_4x_vec):
>         cmpl    $(VEC_SIZE * 2), %edi
>         jae     L(8x_last_2x_vec)
>
> +       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
> +
>         VMOVU   (%rsi, %rdx), %YMM1
>         vpxorq  (%rdx), %YMM1, %YMM1
>
>         VMOVU   VEC_SIZE(%rsi, %rdx), %YMM2
>         vpxorq  VEC_SIZE(%rdx), %YMM2, %YMM2
> -
> -       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
> -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> -
>         VMOVU   (VEC_SIZE * 3)(%rsi, %rdx), %YMM4
> -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM3, %YMM4
> -       VPCMP   $4, %YMM4, %YMM0, %k1
> +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM1, %YMM4
> +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> +       VPTEST  %YMM4, %YMM4, %k1
>         kmovd   %k1, %ecx
> -       /* Restore s1 pointer to rdi.  */
> -       movq    %rdx, %rdi
>         testl   %ecx, %ecx
> -       jnz     L(8x_return_vec_0_1_2_3)
> +       jnz     L(8x_end_return_vec_0_1_2_3)
>         /* NB: eax must be zero to reach here.  */
>         ret
>
>         /* Only entry is from L(more_8x_vec).  */
> -       .p2align 4
> +       .p2align 4,, 10
>  L(8x_last_2x_vec):
>         VPCMP   $4, (VEC_SIZE * 2)(%rdx), %YMM3, %k1
>         kmovd   %k1, %eax
> @@ -355,7 +349,31 @@ L(8x_last_1x_vec):
>         jnz     L(8x_return_vec_3)
>         ret
>
> -       .p2align 4
> +       /* Not ideally aligned (at offset +9 bytes in fetch block) but
> +          not aligning keeps it in the same cache line as
> +          L(8x_last_1x/2x_vec) so likely worth it. As well, saves code
> +          size.  */
> +       .p2align 4,, 4
> +L(8x_return_vec_2):
> +       subq    $VEC_SIZE, %rdx
> +L(8x_return_vec_3):
> +       bsfl    %eax, %eax
> +# ifdef USE_AS_WMEMCMP
> +       leaq    (%rdx, %rax, CHAR_SIZE), %rax
> +       movl    (VEC_SIZE * 3)(%rax), %ecx
> +       xorl    %edx, %edx
> +       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
> +       setg    %dl
> +       leal    -1(%rdx, %rdx), %eax
> +# else
> +       addq    %rdx, %rax
> +       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
> +       movzbl  (VEC_SIZE * 3)(%rax), %eax
> +       subl    %ecx, %eax
> +# endif
> +       ret
> +
> +       .p2align 4,, 10
>  L(last_2x_vec):
>         /* Check second to last VEC.  */
>         VMOVU   -(VEC_SIZE * 2)(%rsi, %rdx, CHAR_SIZE), %YMM1
> @@ -374,26 +392,49 @@ L(last_1x_vec):
>         jnz     L(return_vec_0_end)
>         ret
>
> -       .p2align 4
> -L(8x_return_vec_2):
> -       subq    $VEC_SIZE, %rdx
> -L(8x_return_vec_3):
> -       tzcntl  %eax, %eax
> +       .p2align 4,, 10
> +L(return_vec_1_end):
> +       /* Use bsf to save code size. This is necessary to have
> +          L(one_or_less) fit in aligning bytes between.  */
> +       bsfl    %eax, %eax
> +       addl    %edx, %eax
>  # ifdef USE_AS_WMEMCMP
> -       leaq    (%rdx, %rax, CHAR_SIZE), %rax
> -       movl    (VEC_SIZE * 3)(%rax), %ecx
> +       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
>         xorl    %edx, %edx
> -       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
> +       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
>         setg    %dl
>         leal    -1(%rdx, %rdx), %eax
>  # else
> -       addq    %rdx, %rax
> -       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
> -       movzbl  (VEC_SIZE * 3)(%rax), %eax
> +       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
> +       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
>         subl    %ecx, %eax
>  # endif
>         ret
>
> +       /* NB: L(one_or_less) fits in alignment padding between
> +          L(return_vec_1_end) and L(return_vec_0_end).  */
> +# ifdef USE_AS_WMEMCMP
> +L(one_or_less):
> +       jb      L(zero)
> +       movl    (%rdi), %ecx
> +       xorl    %edx, %edx
> +       cmpl    (%rsi), %ecx
> +       je      L(zero)
> +       setg    %dl
> +       leal    -1(%rdx, %rdx), %eax
> +       ret
> +# else
> +L(one_or_less):
> +       jb      L(zero)
> +       movzbl  (%rsi), %ecx
> +       movzbl  (%rdi), %eax
> +       subl    %ecx, %eax
> +       ret
> +# endif
> +L(zero):
> +       xorl    %eax, %eax
> +       ret
> +
>         .p2align 4
>  L(return_vec_0_end):
>         tzcntl  %eax, %eax
> @@ -412,23 +453,56 @@ L(return_vec_0_end):
>         ret
>
>         .p2align 4
> -L(return_vec_1_end):
> +L(less_vec):
> +       /* Check if one or less CHAR. This is necessary for size == 0
> +          but is also faster for size == CHAR_SIZE.  */
> +       cmpl    $1, %edx
> +       jbe     L(one_or_less)
> +
> +       /* Check if loading one VEC from either s1 or s2 could cause a
> +          page cross. This can have false positives but is by far the
> +          fastest method.  */
> +       movl    %edi, %eax
> +       orl     %esi, %eax
> +       andl    $(PAGE_SIZE - 1), %eax
> +       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
> +       jg      L(page_cross_less_vec)
> +
> +       /* No page cross possible.  */
> +       VMOVU   (%rsi), %YMM2
> +       VPCMP   $4, (%rdi), %YMM2, %k1
> +       kmovd   %k1, %eax
> +       /* Check if any matches where in bounds. Intentionally not
> +          storing result in eax to limit dependency chain if it goes to
> +          L(return_vec_0_lv).  */
> +       bzhil   %edx, %eax, %edx
> +       jnz     L(return_vec_0_lv)
> +       xorl    %eax, %eax
> +       ret
> +
> +       /* Essentially duplicate of L(return_vec_0). Ends up not costing
> +          any code as shrinks L(less_vec) by allowing 2-byte encoding of
> +          the jump and ends up fitting in aligning bytes. As well fits on
> +          same cache line as L(less_vec) so also saves a line from having
> +          to be fetched on cold calls to memcmp.  */
> +       .p2align 4,, 4
> +L(return_vec_0_lv):
>         tzcntl  %eax, %eax
> -       addl    %edx, %eax
>  # ifdef USE_AS_WMEMCMP
> -       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
> +       movl    (%rdi, %rax, CHAR_SIZE), %ecx
>         xorl    %edx, %edx
> -       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
> +       cmpl    (%rsi, %rax, CHAR_SIZE), %ecx
> +       /* NB: no partial register stall here because xorl zero idiom
> +          above.  */
>         setg    %dl
>         leal    -1(%rdx, %rdx), %eax
>  # else
> -       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
> -       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
> +       movzbl  (%rsi, %rax), %ecx
> +       movzbl  (%rdi, %rax), %eax
>         subl    %ecx, %eax
>  # endif
>         ret
>
> -
>         .p2align 4
>  L(page_cross_less_vec):
>         /* if USE_AS_WMEMCMP it can only be 0, 4, 8, 12, 16, 20, 24, 28
> @@ -439,108 +513,84 @@ L(page_cross_less_vec):
>         cmpl    $8, %edx
>         jae     L(between_8_15)
>         cmpl    $4, %edx
> -       jae     L(between_4_7)
> -L(between_2_3):
> -       /* Load as big endian to avoid branches.  */
> -       movzwl  (%rdi), %eax
> -       movzwl  (%rsi), %ecx
> -       shll    $8, %eax
> -       shll    $8, %ecx
> -       bswap   %eax
> -       bswap   %ecx
> -       movzbl  -1(%rdi, %rdx), %edi
> -       movzbl  -1(%rsi, %rdx), %esi
> -       orl     %edi, %eax
> -       orl     %esi, %ecx
> -       /* Subtraction is okay because the upper 8 bits are zero.  */
> -       subl    %ecx, %eax
> -       ret
> -       .p2align 4
> -L(one_or_less):
> -       jb      L(zero)
> -       movzbl  (%rsi), %ecx
> -       movzbl  (%rdi), %eax
> -       subl    %ecx, %eax
> +       jb      L(between_2_3)
> +
> +       /* Load as big endian with overlapping movbe to avoid branches.
> +        */
> +       movbe   (%rdi), %eax
> +       movbe   (%rsi), %ecx
> +       shlq    $32, %rax
> +       shlq    $32, %rcx
> +       movbe   -4(%rdi, %rdx), %edi
> +       movbe   -4(%rsi, %rdx), %esi
> +       orq     %rdi, %rax
> +       orq     %rsi, %rcx
> +       subq    %rcx, %rax
> +       /* edx is guranteed to be positive int32 in range [4, 7].  */
> +       cmovne  %edx, %eax
> +       /* ecx is -1 if rcx > rax. Otherwise 0.  */
> +       sbbl    %ecx, %ecx
> +       /* If rcx > rax, then ecx is 0 and eax is positive. If rcx ==
> +          rax then eax and ecx are zero. If rax < rax then ecx is -1 so
> +          eax doesn't matter.  */
> +       orl     %ecx, %eax
>         ret
>
> -       .p2align 4
> +       .p2align 4,, 8
>  L(between_8_15):
>  # endif
>         /* If USE_AS_WMEMCMP fall through into 8-15 byte case.  */
> -       vmovq   (%rdi), %XMM1
> -       vmovq   (%rsi), %XMM2
> -       VPCMP   $4, %XMM1, %XMM2, %k1
> +       vmovq   (%rdi), %xmm1
> +       vmovq   (%rsi), %xmm2
> +       VPCMP   $4, %xmm1, %xmm2, %k1
>         kmovd   %k1, %eax
>         testl   %eax, %eax
> -       jnz     L(return_vec_0)
> +       jnz     L(return_vec_0_lv)
>         /* Use overlapping loads to avoid branches.  */
> -       leaq    -8(%rdi, %rdx, CHAR_SIZE), %rdi
> -       leaq    -8(%rsi, %rdx, CHAR_SIZE), %rsi
> -       vmovq   (%rdi), %XMM1
> -       vmovq   (%rsi), %XMM2
> -       VPCMP   $4, %XMM1, %XMM2, %k1
> +       vmovq   -8(%rdi, %rdx, CHAR_SIZE), %xmm1
> +       vmovq   -8(%rsi, %rdx, CHAR_SIZE), %xmm2
> +       VPCMP   $4, %xmm1, %xmm2, %k1
> +       addl    $(CHAR_PER_VEC - (8 / CHAR_SIZE)), %edx
>         kmovd   %k1, %eax
>         testl   %eax, %eax
> -       jnz     L(return_vec_0)
> -       ret
> -
> -       .p2align 4
> -L(zero):
> -       xorl    %eax, %eax
> +       jnz     L(return_vec_0_end)
>         ret
>
> -       .p2align 4
> +       .p2align 4,, 8
>  L(between_16_31):
>         /* From 16 to 31 bytes.  No branch when size == 16.  */
> -       VMOVU   (%rsi), %XMM2
> -       VPCMP   $4, (%rdi), %XMM2, %k1
> +
> +       /* Use movups to save code size.  */
> +       movups  (%rsi), %xmm2
> +       VPCMP   $4, (%rdi), %xmm2, %k1
>         kmovd   %k1, %eax
>         testl   %eax, %eax
> -       jnz     L(return_vec_0)
> -
> +       jnz     L(return_vec_0_lv)
>         /* Use overlapping loads to avoid branches.  */
> -
> -       VMOVU   -16(%rsi, %rdx, CHAR_SIZE), %XMM2
> -       leaq    -16(%rdi, %rdx, CHAR_SIZE), %rdi
> -       leaq    -16(%rsi, %rdx, CHAR_SIZE), %rsi
> -       VPCMP   $4, (%rdi), %XMM2, %k1
> +       movups  -16(%rsi, %rdx, CHAR_SIZE), %xmm2
> +       VPCMP   $4, -16(%rdi, %rdx, CHAR_SIZE), %xmm2, %k1
> +       addl    $(CHAR_PER_VEC - (16 / CHAR_SIZE)), %edx
>         kmovd   %k1, %eax
>         testl   %eax, %eax
> -       jnz     L(return_vec_0)
> -       ret
> -
> -# ifdef USE_AS_WMEMCMP
> -       .p2align 4
> -L(one_or_less):
> -       jb      L(zero)
> -       movl    (%rdi), %ecx
> -       xorl    %edx, %edx
> -       cmpl    (%rsi), %ecx
> -       je      L(zero)
> -       setg    %dl
> -       leal    -1(%rdx, %rdx), %eax
> +       jnz     L(return_vec_0_end)
>         ret
> -# else
>
> -       .p2align 4
> -L(between_4_7):
> -       /* Load as big endian with overlapping movbe to avoid branches.
> -        */
> -       movbe   (%rdi), %eax
> -       movbe   (%rsi), %ecx
> -       shlq    $32, %rax
> -       shlq    $32, %rcx
> -       movbe   -4(%rdi, %rdx), %edi
> -       movbe   -4(%rsi, %rdx), %esi
> -       orq     %rdi, %rax
> -       orq     %rsi, %rcx
> -       subq    %rcx, %rax
> -       jz      L(zero_4_7)
> -       sbbl    %eax, %eax
> -       orl     $1, %eax
> -L(zero_4_7):
> +# ifndef USE_AS_WMEMCMP
> +L(between_2_3):
> +       /* Load as big endian to avoid branches.  */
> +       movzwl  (%rdi), %eax
> +       movzwl  (%rsi), %ecx
> +       shll    $8, %eax
> +       shll    $8, %ecx
> +       bswap   %eax
> +       bswap   %ecx
> +       movzbl  -1(%rdi, %rdx), %edi
> +       movzbl  -1(%rsi, %rdx), %esi
> +       orl     %edi, %eax
> +       orl     %esi, %ecx
> +       /* Subtraction is okay because the upper 8 bits are zero.  */
> +       subl    %ecx, %eax
>         ret
>  # endif
> -
>  END (MEMCMP)
>  #endif
> --
> 2.25.1
>

LGTM.

Reviewed-by: H.J. Lu <hjl.tools@gmail.com>

Thanks.
  

On Tue, Oct 12, 2021 at 10:25 AM H.J. Lu <hjl.tools@gmail.com> wrote:
>
> On Tue, Sep 21, 2021 at 5:51 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> >
> > No bug.
> >
> > The frontend optimizations are to:
> > 1. Reorganize logically connected basic blocks so they are either in
> >    the same cache line or adjacent cache lines.
> > 2. Avoid cases when basic blocks unnecissarily cross cache lines.
> > 3. Try and 32 byte align any basic blocks possible without sacrificing
> >    code size. Smaller / Less hot basic blocks are used for this.
> >
> > Overall code size shrunk by 168 bytes. This should make up for any
> > extra costs due to aligning to 64 bytes.
> >
> > In general performance before deviated a great deal dependending on
> > whether entry alignment % 64 was 0, 16, 32, or 48. These changes
> > essentially make it so that the current implementation is at least
> > equal to the best alignment of the original for any arguments.
> >
> > The only additional optimization is in the page cross case. Branch on
> > equals case was removed from the size == [4, 7] case. As well the [4,
> > 7] and [2, 3] case where swapped as [4, 7] is likely a more hot
> > argument size.
> >
> > test-memcmp and test-wmemcmp are both passing.
> > ---
> > Performance results attached in reply.
> >
> > Benchmark CPU: https://ark.intel.com/content/www/us/en/ark/products/208921/intel-core-i7-1165g7-processor-12m-cache-up-to-4-70-ghz-with-ipu.html
> >
> > Results are from geometric mean of N = 20 runs.
> >
> > Time for new implemention shown.
> >
> > Score(N) is the New Time / Old Time with entry alignment == N
> >
> > In general there is one performance degregation for size range [2, 3]
> > in the page cross case. This is expected as the new code intentionally
> > gives the fallthrough path to the hotter size range [4, 7].
> >
> >
> > For the micro benchmarks there is no outright performance improvement
> > in the sense that for each benchmark configuration there is at least
> > one entry alignment where the old version had roughly the same
> > performance. But this should provide a great deal more performance
> > stability.
> >
> > The hope, however, is that with the code size shrink and
> > reorganization of basic blocks for better icache behavior. Concerns
> > such as those brought up in this paper:
> >
> > https://storage.googleapis.com/pub-tools-public-publication-data/pdf/e52f61fd2c51e8962305120548581efacbc06ffc.pdf
> >
> > should be better addressed.
> >
> >  sysdeps/x86_64/multiarch/memcmp-evex-movbe.S | 434 +++++++++++--------
> >  1 file changed, 242 insertions(+), 192 deletions(-)
> >
> > diff --git a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > index 654dc7ac8c..2761b54f2e 100644
> > --- a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > +++ b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > @@ -34,7 +34,24 @@
> >        area.
> >     7. Use 2 vector compares when size is 2 * CHAR_PER_VEC or less.
> >     8. Use 4 vector compares when size is 4 * CHAR_PER_VEC or less.
> > -   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.  */
> > +   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.
> > +
> > +When possible the implementation tries to optimize for frontend in the
> > +following ways:
> > +Throughput:
> > +    1. All code sections that fit are able to run optimally out of the
> > +       LSD.
> > +    2. All code sections that fit are able to run optimally out of the
> > +       DSB
> > +    3. Basic blocks are contained in minimum number of fetch blocks
> > +       necessary.
> > +
> > +Latency:
> > +    1. Logically connected basic blocks are put in the same
> > +       cache-line.
> > +    2. Logically connected basic blocks that do not fit in the same
> > +       cache-line are put in adjacent lines. This can get beneficial
> > +       L2 spatial prefetching and L1 next-line prefetching.  */
> >
> >  # include <sysdep.h>
> >
> > @@ -47,9 +64,11 @@
> >  # ifdef USE_AS_WMEMCMP
> >  #  define CHAR_SIZE    4
> >  #  define VPCMP        vpcmpd
> > +#  define VPTEST       vptestmd
> >  # else
> >  #  define CHAR_SIZE    1
> >  #  define VPCMP        vpcmpub
> > +#  define VPTEST       vptestmb
> >  # endif
> >
> >  # define VEC_SIZE      32
> > @@ -75,7 +94,9 @@
> >  */
> >
> >         .section .text.evex,"ax",@progbits
> > -ENTRY (MEMCMP)
> > +/* Cache align memcmp entry. This allows for much more thorough
> > +   frontend optimization.  */
> > +ENTRY_P2ALIGN (MEMCMP, 6)
> >  # ifdef __ILP32__
> >         /* Clear the upper 32 bits.  */
> >         movl    %edx, %edx
> > @@ -89,7 +110,7 @@ ENTRY (MEMCMP)
> >         VPCMP   $4, (%rdi), %YMM1, %k1
> >         kmovd   %k1, %eax
> >         /* NB: eax must be destination register if going to
> > -          L(return_vec_[0,2]). For L(return_vec_3 destination register
> > +          L(return_vec_[0,2]). For L(return_vec_3) destination register
> >            must be ecx.  */
> >         testl   %eax, %eax
> >         jnz     L(return_vec_0)
> > @@ -121,10 +142,6 @@ ENTRY (MEMCMP)
> >         testl   %ecx, %ecx
> >         jnz     L(return_vec_3)
> >
> > -       /* Zero YMM0. 4x VEC reduction is done with vpxor + vtern so
> > -          compare with zero to get a mask is needed.  */
> > -       vpxorq  %XMM0, %XMM0, %XMM0
> > -
> >         /* Go to 4x VEC loop.  */
> >         cmpq    $(CHAR_PER_VEC * 8), %rdx
> >         ja      L(more_8x_vec)
> > @@ -148,47 +165,61 @@ ENTRY (MEMCMP)
> >
> >         VMOVU   (VEC_SIZE * 2)(%rsi), %YMM3
> >         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> > -       /* Or together YMM1, YMM2, and YMM3 into YMM3.  */
> > -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> >
> >         VMOVU   (VEC_SIZE * 3)(%rsi), %YMM4
> >         /* Ternary logic to xor (VEC_SIZE * 3)(%rdi) with YMM4 while
> > -          oring with YMM3. Result is stored in YMM4.  */
> > -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
> > -       /* Compare YMM4 with 0. If any 1s s1 and s2 don't match.  */
> > -       VPCMP   $4, %YMM4, %YMM0, %k1
> > +          oring with YMM1. Result is stored in YMM4.  */
> > +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
> > +
> > +       /* Or together YMM2, YMM3, and YMM4 into YMM4.  */
> > +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> > +
> > +       /* Test YMM4 against itself. Store any CHAR mismatches in k1.
> > +        */
> > +       VPTEST  %YMM4, %YMM4, %k1
> > +       /* k1 must go to ecx for L(return_vec_0_1_2_3).  */
> >         kmovd   %k1, %ecx
> >         testl   %ecx, %ecx
> >         jnz     L(return_vec_0_1_2_3)
> >         /* NB: eax must be zero to reach here.  */
> >         ret
> >
> > -       /* NB: aligning 32 here allows for the rest of the jump targets
> > -          to be tuned for 32 byte alignment. Most important this ensures
> > -          the L(more_8x_vec) loop is 32 byte aligned.  */
> > -       .p2align 5
> > -L(less_vec):
> > -       /* Check if one or less CHAR. This is necessary for size = 0 but
> > -          is also faster for size = CHAR_SIZE.  */
> > -       cmpl    $1, %edx
> > -       jbe     L(one_or_less)
> > +       .p2align 4
> > +L(8x_end_return_vec_0_1_2_3):
> > +       movq    %rdx, %rdi
> > +L(8x_return_vec_0_1_2_3):
> > +       addq    %rdi, %rsi
> > +L(return_vec_0_1_2_3):
> > +       VPTEST  %YMM1, %YMM1, %k0
> > +       kmovd   %k0, %eax
> > +       testl   %eax, %eax
> > +       jnz     L(return_vec_0)
> >
> > -       /* Check if loading one VEC from either s1 or s2 could cause a
> > -          page cross. This can have false positives but is by far the
> > -          fastest method.  */
> > -       movl    %edi, %eax
> > -       orl     %esi, %eax
> > -       andl    $(PAGE_SIZE - 1), %eax
> > -       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
> > -       jg      L(page_cross_less_vec)
> > +       VPTEST  %YMM2, %YMM2, %k0
> > +       kmovd   %k0, %eax
> > +       testl   %eax, %eax
> > +       jnz     L(return_vec_1)
> >
> > -       /* No page cross possible.  */
> > -       VMOVU   (%rsi), %YMM2
> > -       VPCMP   $4, (%rdi), %YMM2, %k1
> > -       kmovd   %k1, %eax
> > -       /* Create mask in ecx for potentially in bound matches.  */
> > -       bzhil   %edx, %eax, %eax
> > -       jnz     L(return_vec_0)
> > +       VPTEST  %YMM3, %YMM3, %k0
> > +       kmovd   %k0, %eax
> > +       testl   %eax, %eax
> > +       jnz     L(return_vec_2)
> > +L(return_vec_3):
> > +       /* bsf saves 1 byte from tzcnt. This keep L(return_vec_3) in one
> > +          fetch block and the entire L(*return_vec_0_1_2_3) in 1 cache
> > +          line.  */
> > +       bsfl    %ecx, %ecx
> > +# ifdef USE_AS_WMEMCMP
> > +       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
> > +       xorl    %edx, %edx
> > +       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
> > +       setg    %dl
> > +       leal    -1(%rdx, %rdx), %eax
> > +# else
> > +       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
> > +       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
> > +       subl    %ecx, %eax
> > +# endif
> >         ret
> >
> >         .p2align 4
> > @@ -209,10 +240,11 @@ L(return_vec_0):
> >  # endif
> >         ret
> >
> > -       /* NB: No p2align necessary. Alignment  % 16 is naturally 1
> > -          which is good enough for a target not in a loop.  */
> > +       .p2align 4
> >  L(return_vec_1):
> > -       tzcntl  %eax, %eax
> > +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_1) in one
> > +          fetch block.  */
> > +       bsfl    %eax, %eax
> >  # ifdef USE_AS_WMEMCMP
> >         movl    VEC_SIZE(%rdi, %rax, CHAR_SIZE), %ecx
> >         xorl    %edx, %edx
> > @@ -226,10 +258,11 @@ L(return_vec_1):
> >  # endif
> >         ret
> >
> > -       /* NB: No p2align necessary. Alignment  % 16 is naturally 2
> > -          which is good enough for a target not in a loop.  */
> > +       .p2align 4,, 10
> >  L(return_vec_2):
> > -       tzcntl  %eax, %eax
> > +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_2) in one
> > +          fetch block.  */
> > +       bsfl    %eax, %eax
> >  # ifdef USE_AS_WMEMCMP
> >         movl    (VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
> >         xorl    %edx, %edx
> > @@ -243,40 +276,6 @@ L(return_vec_2):
> >  # endif
> >         ret
> >
> > -       .p2align 4
> > -L(8x_return_vec_0_1_2_3):
> > -       /* Returning from L(more_8x_vec) requires restoring rsi.  */
> > -       addq    %rdi, %rsi
> > -L(return_vec_0_1_2_3):
> > -       VPCMP   $4, %YMM1, %YMM0, %k0
> > -       kmovd   %k0, %eax
> > -       testl   %eax, %eax
> > -       jnz     L(return_vec_0)
> > -
> > -       VPCMP   $4, %YMM2, %YMM0, %k0
> > -       kmovd   %k0, %eax
> > -       testl   %eax, %eax
> > -       jnz     L(return_vec_1)
> > -
> > -       VPCMP   $4, %YMM3, %YMM0, %k0
> > -       kmovd   %k0, %eax
> > -       testl   %eax, %eax
> > -       jnz     L(return_vec_2)
> > -L(return_vec_3):
> > -       tzcntl  %ecx, %ecx
> > -# ifdef USE_AS_WMEMCMP
> > -       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
> > -       xorl    %edx, %edx
> > -       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
> > -       setg    %dl
> > -       leal    -1(%rdx, %rdx), %eax
> > -# else
> > -       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
> > -       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
> > -       subl    %ecx, %eax
> > -# endif
> > -       ret
> > -
> >         .p2align 4
> >  L(more_8x_vec):
> >         /* Set end of s1 in rdx.  */
> > @@ -288,21 +287,19 @@ L(more_8x_vec):
> >         andq    $-VEC_SIZE, %rdi
> >         /* Adjust because first 4x vec where check already.  */
> >         subq    $-(VEC_SIZE * 4), %rdi
> > +
> >         .p2align 4
> >  L(loop_4x_vec):
> >         VMOVU   (%rsi, %rdi), %YMM1
> >         vpxorq  (%rdi), %YMM1, %YMM1
> > -
> >         VMOVU   VEC_SIZE(%rsi, %rdi), %YMM2
> >         vpxorq  VEC_SIZE(%rdi), %YMM2, %YMM2
> > -
> >         VMOVU   (VEC_SIZE * 2)(%rsi, %rdi), %YMM3
> >         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> > -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> > -
> >         VMOVU   (VEC_SIZE * 3)(%rsi, %rdi), %YMM4
> > -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
> > -       VPCMP   $4, %YMM4, %YMM0, %k1
> > +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
> > +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> > +       VPTEST  %YMM4, %YMM4, %k1
> >         kmovd   %k1, %ecx
> >         testl   %ecx, %ecx
> >         jnz     L(8x_return_vec_0_1_2_3)
> > @@ -319,28 +316,25 @@ L(loop_4x_vec):
> >         cmpl    $(VEC_SIZE * 2), %edi
> >         jae     L(8x_last_2x_vec)
> >
> > +       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
> > +
> >         VMOVU   (%rsi, %rdx), %YMM1
> >         vpxorq  (%rdx), %YMM1, %YMM1
> >
> >         VMOVU   VEC_SIZE(%rsi, %rdx), %YMM2
> >         vpxorq  VEC_SIZE(%rdx), %YMM2, %YMM2
> > -
> > -       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
> > -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> > -
> >         VMOVU   (VEC_SIZE * 3)(%rsi, %rdx), %YMM4
> > -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM3, %YMM4
> > -       VPCMP   $4, %YMM4, %YMM0, %k1
> > +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM1, %YMM4
> > +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> > +       VPTEST  %YMM4, %YMM4, %k1
> >         kmovd   %k1, %ecx
> > -       /* Restore s1 pointer to rdi.  */
> > -       movq    %rdx, %rdi
> >         testl   %ecx, %ecx
> > -       jnz     L(8x_return_vec_0_1_2_3)
> > +       jnz     L(8x_end_return_vec_0_1_2_3)
> >         /* NB: eax must be zero to reach here.  */
> >         ret
> >
> >         /* Only entry is from L(more_8x_vec).  */
> > -       .p2align 4
> > +       .p2align 4,, 10
> >  L(8x_last_2x_vec):
> >         VPCMP   $4, (VEC_SIZE * 2)(%rdx), %YMM3, %k1
> >         kmovd   %k1, %eax
> > @@ -355,7 +349,31 @@ L(8x_last_1x_vec):
> >         jnz     L(8x_return_vec_3)
> >         ret
> >
> > -       .p2align 4
> > +       /* Not ideally aligned (at offset +9 bytes in fetch block) but
> > +          not aligning keeps it in the same cache line as
> > +          L(8x_last_1x/2x_vec) so likely worth it. As well, saves code
> > +          size.  */
> > +       .p2align 4,, 4
> > +L(8x_return_vec_2):
> > +       subq    $VEC_SIZE, %rdx
> > +L(8x_return_vec_3):
> > +       bsfl    %eax, %eax
> > +# ifdef USE_AS_WMEMCMP
> > +       leaq    (%rdx, %rax, CHAR_SIZE), %rax
> > +       movl    (VEC_SIZE * 3)(%rax), %ecx
> > +       xorl    %edx, %edx
> > +       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
> > +       setg    %dl
> > +       leal    -1(%rdx, %rdx), %eax
> > +# else
> > +       addq    %rdx, %rax
> > +       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
> > +       movzbl  (VEC_SIZE * 3)(%rax), %eax
> > +       subl    %ecx, %eax
> > +# endif
> > +       ret
> > +
> > +       .p2align 4,, 10
> >  L(last_2x_vec):
> >         /* Check second to last VEC.  */
> >         VMOVU   -(VEC_SIZE * 2)(%rsi, %rdx, CHAR_SIZE), %YMM1
> > @@ -374,26 +392,49 @@ L(last_1x_vec):
> >         jnz     L(return_vec_0_end)
> >         ret
> >
> > -       .p2align 4
> > -L(8x_return_vec_2):
> > -       subq    $VEC_SIZE, %rdx
> > -L(8x_return_vec_3):
> > -       tzcntl  %eax, %eax
> > +       .p2align 4,, 10
> > +L(return_vec_1_end):
> > +       /* Use bsf to save code size. This is necessary to have
> > +          L(one_or_less) fit in aligning bytes between.  */
> > +       bsfl    %eax, %eax
> > +       addl    %edx, %eax
> >  # ifdef USE_AS_WMEMCMP
> > -       leaq    (%rdx, %rax, CHAR_SIZE), %rax
> > -       movl    (VEC_SIZE * 3)(%rax), %ecx
> > +       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
> >         xorl    %edx, %edx
> > -       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
> > +       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
> >         setg    %dl
> >         leal    -1(%rdx, %rdx), %eax
> >  # else
> > -       addq    %rdx, %rax
> > -       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
> > -       movzbl  (VEC_SIZE * 3)(%rax), %eax
> > +       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
> > +       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
> >         subl    %ecx, %eax
> >  # endif
> >         ret
> >
> > +       /* NB: L(one_or_less) fits in alignment padding between
> > +          L(return_vec_1_end) and L(return_vec_0_end).  */
> > +# ifdef USE_AS_WMEMCMP
> > +L(one_or_less):
> > +       jb      L(zero)
> > +       movl    (%rdi), %ecx
> > +       xorl    %edx, %edx
> > +       cmpl    (%rsi), %ecx
> > +       je      L(zero)
> > +       setg    %dl
> > +       leal    -1(%rdx, %rdx), %eax
> > +       ret
> > +# else
> > +L(one_or_less):
> > +       jb      L(zero)
> > +       movzbl  (%rsi), %ecx
> > +       movzbl  (%rdi), %eax
> > +       subl    %ecx, %eax
> > +       ret
> > +# endif
> > +L(zero):
> > +       xorl    %eax, %eax
> > +       ret
> > +
> >         .p2align 4
> >  L(return_vec_0_end):
> >         tzcntl  %eax, %eax
> > @@ -412,23 +453,56 @@ L(return_vec_0_end):
> >         ret
> >
> >         .p2align 4
> > -L(return_vec_1_end):
> > +L(less_vec):
> > +       /* Check if one or less CHAR. This is necessary for size == 0
> > +          but is also faster for size == CHAR_SIZE.  */
> > +       cmpl    $1, %edx
> > +       jbe     L(one_or_less)
> > +
> > +       /* Check if loading one VEC from either s1 or s2 could cause a
> > +          page cross. This can have false positives but is by far the
> > +          fastest method.  */
> > +       movl    %edi, %eax
> > +       orl     %esi, %eax
> > +       andl    $(PAGE_SIZE - 1), %eax
> > +       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
> > +       jg      L(page_cross_less_vec)
> > +
> > +       /* No page cross possible.  */
> > +       VMOVU   (%rsi), %YMM2
> > +       VPCMP   $4, (%rdi), %YMM2, %k1
> > +       kmovd   %k1, %eax
> > +       /* Check if any matches where in bounds. Intentionally not
> > +          storing result in eax to limit dependency chain if it goes to
> > +          L(return_vec_0_lv).  */
> > +       bzhil   %edx, %eax, %edx
> > +       jnz     L(return_vec_0_lv)
> > +       xorl    %eax, %eax
> > +       ret
> > +
> > +       /* Essentially duplicate of L(return_vec_0). Ends up not costing
> > +          any code as shrinks L(less_vec) by allowing 2-byte encoding of
> > +          the jump and ends up fitting in aligning bytes. As well fits on
> > +          same cache line as L(less_vec) so also saves a line from having
> > +          to be fetched on cold calls to memcmp.  */
> > +       .p2align 4,, 4
> > +L(return_vec_0_lv):
> >         tzcntl  %eax, %eax
> > -       addl    %edx, %eax
> >  # ifdef USE_AS_WMEMCMP
> > -       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
> > +       movl    (%rdi, %rax, CHAR_SIZE), %ecx
> >         xorl    %edx, %edx
> > -       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
> > +       cmpl    (%rsi, %rax, CHAR_SIZE), %ecx
> > +       /* NB: no partial register stall here because xorl zero idiom
> > +          above.  */
> >         setg    %dl
> >         leal    -1(%rdx, %rdx), %eax
> >  # else
> > -       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
> > -       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
> > +       movzbl  (%rsi, %rax), %ecx
> > +       movzbl  (%rdi, %rax), %eax
> >         subl    %ecx, %eax
> >  # endif
> >         ret
> >
> > -
> >         .p2align 4
> >  L(page_cross_less_vec):
> >         /* if USE_AS_WMEMCMP it can only be 0, 4, 8, 12, 16, 20, 24, 28
> > @@ -439,108 +513,84 @@ L(page_cross_less_vec):
> >         cmpl    $8, %edx
> >         jae     L(between_8_15)
> >         cmpl    $4, %edx
> > -       jae     L(between_4_7)
> > -L(between_2_3):
> > -       /* Load as big endian to avoid branches.  */
> > -       movzwl  (%rdi), %eax
> > -       movzwl  (%rsi), %ecx
> > -       shll    $8, %eax
> > -       shll    $8, %ecx
> > -       bswap   %eax
> > -       bswap   %ecx
> > -       movzbl  -1(%rdi, %rdx), %edi
> > -       movzbl  -1(%rsi, %rdx), %esi
> > -       orl     %edi, %eax
> > -       orl     %esi, %ecx
> > -       /* Subtraction is okay because the upper 8 bits are zero.  */
> > -       subl    %ecx, %eax
> > -       ret
> > -       .p2align 4
> > -L(one_or_less):
> > -       jb      L(zero)
> > -       movzbl  (%rsi), %ecx
> > -       movzbl  (%rdi), %eax
> > -       subl    %ecx, %eax
> > +       jb      L(between_2_3)
> > +
> > +       /* Load as big endian with overlapping movbe to avoid branches.
> > +        */
> > +       movbe   (%rdi), %eax
> > +       movbe   (%rsi), %ecx
> > +       shlq    $32, %rax
> > +       shlq    $32, %rcx
> > +       movbe   -4(%rdi, %rdx), %edi
> > +       movbe   -4(%rsi, %rdx), %esi
> > +       orq     %rdi, %rax
> > +       orq     %rsi, %rcx
> > +       subq    %rcx, %rax
> > +       /* edx is guranteed to be positive int32 in range [4, 7].  */
> > +       cmovne  %edx, %eax
> > +       /* ecx is -1 if rcx > rax. Otherwise 0.  */
> > +       sbbl    %ecx, %ecx
> > +       /* If rcx > rax, then ecx is 0 and eax is positive. If rcx ==
> > +          rax then eax and ecx are zero. If rax < rax then ecx is -1 so
> > +          eax doesn't matter.  */
> > +       orl     %ecx, %eax
> >         ret
> >
> > -       .p2align 4
> > +       .p2align 4,, 8
> >  L(between_8_15):
> >  # endif
> >         /* If USE_AS_WMEMCMP fall through into 8-15 byte case.  */
> > -       vmovq   (%rdi), %XMM1
> > -       vmovq   (%rsi), %XMM2
> > -       VPCMP   $4, %XMM1, %XMM2, %k1
> > +       vmovq   (%rdi), %xmm1
> > +       vmovq   (%rsi), %xmm2
> > +       VPCMP   $4, %xmm1, %xmm2, %k1
> >         kmovd   %k1, %eax
> >         testl   %eax, %eax
> > -       jnz     L(return_vec_0)
> > +       jnz     L(return_vec_0_lv)
> >         /* Use overlapping loads to avoid branches.  */
> > -       leaq    -8(%rdi, %rdx, CHAR_SIZE), %rdi
> > -       leaq    -8(%rsi, %rdx, CHAR_SIZE), %rsi
> > -       vmovq   (%rdi), %XMM1
> > -       vmovq   (%rsi), %XMM2
> > -       VPCMP   $4, %XMM1, %XMM2, %k1
> > +       vmovq   -8(%rdi, %rdx, CHAR_SIZE), %xmm1
> > +       vmovq   -8(%rsi, %rdx, CHAR_SIZE), %xmm2
> > +       VPCMP   $4, %xmm1, %xmm2, %k1
> > +       addl    $(CHAR_PER_VEC - (8 / CHAR_SIZE)), %edx
> >         kmovd   %k1, %eax
> >         testl   %eax, %eax
> > -       jnz     L(return_vec_0)
> > -       ret
> > -
> > -       .p2align 4
> > -L(zero):
> > -       xorl    %eax, %eax
> > +       jnz     L(return_vec_0_end)
> >         ret
> >
> > -       .p2align 4
> > +       .p2align 4,, 8
> >  L(between_16_31):
> >         /* From 16 to 31 bytes.  No branch when size == 16.  */
> > -       VMOVU   (%rsi), %XMM2
> > -       VPCMP   $4, (%rdi), %XMM2, %k1
> > +
> > +       /* Use movups to save code size.  */
> > +       movups  (%rsi), %xmm2
> > +       VPCMP   $4, (%rdi), %xmm2, %k1
> >         kmovd   %k1, %eax
> >         testl   %eax, %eax
> > -       jnz     L(return_vec_0)
> > -
> > +       jnz     L(return_vec_0_lv)
> >         /* Use overlapping loads to avoid branches.  */
> > -
> > -       VMOVU   -16(%rsi, %rdx, CHAR_SIZE), %XMM2
> > -       leaq    -16(%rdi, %rdx, CHAR_SIZE), %rdi
> > -       leaq    -16(%rsi, %rdx, CHAR_SIZE), %rsi
> > -       VPCMP   $4, (%rdi), %XMM2, %k1
> > +       movups  -16(%rsi, %rdx, CHAR_SIZE), %xmm2
> > +       VPCMP   $4, -16(%rdi, %rdx, CHAR_SIZE), %xmm2, %k1
> > +       addl    $(CHAR_PER_VEC - (16 / CHAR_SIZE)), %edx
> >         kmovd   %k1, %eax
> >         testl   %eax, %eax
> > -       jnz     L(return_vec_0)
> > -       ret
> > -
> > -# ifdef USE_AS_WMEMCMP
> > -       .p2align 4
> > -L(one_or_less):
> > -       jb      L(zero)
> > -       movl    (%rdi), %ecx
> > -       xorl    %edx, %edx
> > -       cmpl    (%rsi), %ecx
> > -       je      L(zero)
> > -       setg    %dl
> > -       leal    -1(%rdx, %rdx), %eax
> > +       jnz     L(return_vec_0_end)
> >         ret
> > -# else
> >
> > -       .p2align 4
> > -L(between_4_7):
> > -       /* Load as big endian with overlapping movbe to avoid branches.
> > -        */
> > -       movbe   (%rdi), %eax
> > -       movbe   (%rsi), %ecx
> > -       shlq    $32, %rax
> > -       shlq    $32, %rcx
> > -       movbe   -4(%rdi, %rdx), %edi
> > -       movbe   -4(%rsi, %rdx), %esi
> > -       orq     %rdi, %rax
> > -       orq     %rsi, %rcx
> > -       subq    %rcx, %rax
> > -       jz      L(zero_4_7)
> > -       sbbl    %eax, %eax
> > -       orl     $1, %eax
> > -L(zero_4_7):
> > +# ifndef USE_AS_WMEMCMP
> > +L(between_2_3):
> > +       /* Load as big endian to avoid branches.  */
> > +       movzwl  (%rdi), %eax
> > +       movzwl  (%rsi), %ecx
> > +       shll    $8, %eax
> > +       shll    $8, %ecx
> > +       bswap   %eax
> > +       bswap   %ecx
> > +       movzbl  -1(%rdi, %rdx), %edi
> > +       movzbl  -1(%rsi, %rdx), %esi
> > +       orl     %edi, %eax
> > +       orl     %esi, %ecx
> > +       /* Subtraction is okay because the upper 8 bits are zero.  */
> > +       subl    %ecx, %eax
> >         ret
> >  # endif
> > -
> >  END (MEMCMP)
> >  #endif
> > --
> > 2.25.1
> >
>
> LGTM.
>
> Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
>
> Thanks.
>
> --
> H.J.

Thanks! Pushed.
  

On Tue, Oct 12, 2021 at 10:44 AM Noah Goldstein via Libc-alpha
<libc-alpha@sourceware.org> wrote:
>
> On Tue, Oct 12, 2021 at 10:25 AM H.J. Lu <hjl.tools@gmail.com> wrote:
> >
> > On Tue, Sep 21, 2021 at 5:51 PM Noah Goldstein <goldstein.w.n@gmail.com> wrote:
> > >
> > > No bug.
> > >
> > > The frontend optimizations are to:
> > > 1. Reorganize logically connected basic blocks so they are either in
> > >    the same cache line or adjacent cache lines.
> > > 2. Avoid cases when basic blocks unnecissarily cross cache lines.
> > > 3. Try and 32 byte align any basic blocks possible without sacrificing
> > >    code size. Smaller / Less hot basic blocks are used for this.
> > >
> > > Overall code size shrunk by 168 bytes. This should make up for any
> > > extra costs due to aligning to 64 bytes.
> > >
> > > In general performance before deviated a great deal dependending on
> > > whether entry alignment % 64 was 0, 16, 32, or 48. These changes
> > > essentially make it so that the current implementation is at least
> > > equal to the best alignment of the original for any arguments.
> > >
> > > The only additional optimization is in the page cross case. Branch on
> > > equals case was removed from the size == [4, 7] case. As well the [4,
> > > 7] and [2, 3] case where swapped as [4, 7] is likely a more hot
> > > argument size.
> > >
> > > test-memcmp and test-wmemcmp are both passing.
> > > ---
> > > Performance results attached in reply.
> > >
> > > Benchmark CPU: https://ark.intel.com/content/www/us/en/ark/products/208921/intel-core-i7-1165g7-processor-12m-cache-up-to-4-70-ghz-with-ipu.html
> > >
> > > Results are from geometric mean of N = 20 runs.
> > >
> > > Time for new implemention shown.
> > >
> > > Score(N) is the New Time / Old Time with entry alignment == N
> > >
> > > In general there is one performance degregation for size range [2, 3]
> > > in the page cross case. This is expected as the new code intentionally
> > > gives the fallthrough path to the hotter size range [4, 7].
> > >
> > >
> > > For the micro benchmarks there is no outright performance improvement
> > > in the sense that for each benchmark configuration there is at least
> > > one entry alignment where the old version had roughly the same
> > > performance. But this should provide a great deal more performance
> > > stability.
> > >
> > > The hope, however, is that with the code size shrink and
> > > reorganization of basic blocks for better icache behavior. Concerns
> > > such as those brought up in this paper:
> > >
> > > https://storage.googleapis.com/pub-tools-public-publication-data/pdf/e52f61fd2c51e8962305120548581efacbc06ffc.pdf
> > >
> > > should be better addressed.
> > >
> > >  sysdeps/x86_64/multiarch/memcmp-evex-movbe.S | 434 +++++++++++--------
> > >  1 file changed, 242 insertions(+), 192 deletions(-)
> > >
> > > diff --git a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > > index 654dc7ac8c..2761b54f2e 100644
> > > --- a/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > > +++ b/sysdeps/x86_64/multiarch/memcmp-evex-movbe.S
> > > @@ -34,7 +34,24 @@
> > >        area.
> > >     7. Use 2 vector compares when size is 2 * CHAR_PER_VEC or less.
> > >     8. Use 4 vector compares when size is 4 * CHAR_PER_VEC or less.
> > > -   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.  */
> > > +   9. Use 8 vector compares when size is 8 * CHAR_PER_VEC or less.
> > > +
> > > +When possible the implementation tries to optimize for frontend in the
> > > +following ways:
> > > +Throughput:
> > > +    1. All code sections that fit are able to run optimally out of the
> > > +       LSD.
> > > +    2. All code sections that fit are able to run optimally out of the
> > > +       DSB
> > > +    3. Basic blocks are contained in minimum number of fetch blocks
> > > +       necessary.
> > > +
> > > +Latency:
> > > +    1. Logically connected basic blocks are put in the same
> > > +       cache-line.
> > > +    2. Logically connected basic blocks that do not fit in the same
> > > +       cache-line are put in adjacent lines. This can get beneficial
> > > +       L2 spatial prefetching and L1 next-line prefetching.  */
> > >
> > >  # include <sysdep.h>
> > >
> > > @@ -47,9 +64,11 @@
> > >  # ifdef USE_AS_WMEMCMP
> > >  #  define CHAR_SIZE    4
> > >  #  define VPCMP        vpcmpd
> > > +#  define VPTEST       vptestmd
> > >  # else
> > >  #  define CHAR_SIZE    1
> > >  #  define VPCMP        vpcmpub
> > > +#  define VPTEST       vptestmb
> > >  # endif
> > >
> > >  # define VEC_SIZE      32
> > > @@ -75,7 +94,9 @@
> > >  */
> > >
> > >         .section .text.evex,"ax",@progbits
> > > -ENTRY (MEMCMP)
> > > +/* Cache align memcmp entry. This allows for much more thorough
> > > +   frontend optimization.  */
> > > +ENTRY_P2ALIGN (MEMCMP, 6)
> > >  # ifdef __ILP32__
> > >         /* Clear the upper 32 bits.  */
> > >         movl    %edx, %edx
> > > @@ -89,7 +110,7 @@ ENTRY (MEMCMP)
> > >         VPCMP   $4, (%rdi), %YMM1, %k1
> > >         kmovd   %k1, %eax
> > >         /* NB: eax must be destination register if going to
> > > -          L(return_vec_[0,2]). For L(return_vec_3 destination register
> > > +          L(return_vec_[0,2]). For L(return_vec_3) destination register
> > >            must be ecx.  */
> > >         testl   %eax, %eax
> > >         jnz     L(return_vec_0)
> > > @@ -121,10 +142,6 @@ ENTRY (MEMCMP)
> > >         testl   %ecx, %ecx
> > >         jnz     L(return_vec_3)
> > >
> > > -       /* Zero YMM0. 4x VEC reduction is done with vpxor + vtern so
> > > -          compare with zero to get a mask is needed.  */
> > > -       vpxorq  %XMM0, %XMM0, %XMM0
> > > -
> > >         /* Go to 4x VEC loop.  */
> > >         cmpq    $(CHAR_PER_VEC * 8), %rdx
> > >         ja      L(more_8x_vec)
> > > @@ -148,47 +165,61 @@ ENTRY (MEMCMP)
> > >
> > >         VMOVU   (VEC_SIZE * 2)(%rsi), %YMM3
> > >         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> > > -       /* Or together YMM1, YMM2, and YMM3 into YMM3.  */
> > > -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> > >
> > >         VMOVU   (VEC_SIZE * 3)(%rsi), %YMM4
> > >         /* Ternary logic to xor (VEC_SIZE * 3)(%rdi) with YMM4 while
> > > -          oring with YMM3. Result is stored in YMM4.  */
> > > -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
> > > -       /* Compare YMM4 with 0. If any 1s s1 and s2 don't match.  */
> > > -       VPCMP   $4, %YMM4, %YMM0, %k1
> > > +          oring with YMM1. Result is stored in YMM4.  */
> > > +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
> > > +
> > > +       /* Or together YMM2, YMM3, and YMM4 into YMM4.  */
> > > +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> > > +
> > > +       /* Test YMM4 against itself. Store any CHAR mismatches in k1.
> > > +        */
> > > +       VPTEST  %YMM4, %YMM4, %k1
> > > +       /* k1 must go to ecx for L(return_vec_0_1_2_3).  */
> > >         kmovd   %k1, %ecx
> > >         testl   %ecx, %ecx
> > >         jnz     L(return_vec_0_1_2_3)
> > >         /* NB: eax must be zero to reach here.  */
> > >         ret
> > >
> > > -       /* NB: aligning 32 here allows for the rest of the jump targets
> > > -          to be tuned for 32 byte alignment. Most important this ensures
> > > -          the L(more_8x_vec) loop is 32 byte aligned.  */
> > > -       .p2align 5
> > > -L(less_vec):
> > > -       /* Check if one or less CHAR. This is necessary for size = 0 but
> > > -          is also faster for size = CHAR_SIZE.  */
> > > -       cmpl    $1, %edx
> > > -       jbe     L(one_or_less)
> > > +       .p2align 4
> > > +L(8x_end_return_vec_0_1_2_3):
> > > +       movq    %rdx, %rdi
> > > +L(8x_return_vec_0_1_2_3):
> > > +       addq    %rdi, %rsi
> > > +L(return_vec_0_1_2_3):
> > > +       VPTEST  %YMM1, %YMM1, %k0
> > > +       kmovd   %k0, %eax
> > > +       testl   %eax, %eax
> > > +       jnz     L(return_vec_0)
> > >
> > > -       /* Check if loading one VEC from either s1 or s2 could cause a
> > > -          page cross. This can have false positives but is by far the
> > > -          fastest method.  */
> > > -       movl    %edi, %eax
> > > -       orl     %esi, %eax
> > > -       andl    $(PAGE_SIZE - 1), %eax
> > > -       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
> > > -       jg      L(page_cross_less_vec)
> > > +       VPTEST  %YMM2, %YMM2, %k0
> > > +       kmovd   %k0, %eax
> > > +       testl   %eax, %eax
> > > +       jnz     L(return_vec_1)
> > >
> > > -       /* No page cross possible.  */
> > > -       VMOVU   (%rsi), %YMM2
> > > -       VPCMP   $4, (%rdi), %YMM2, %k1
> > > -       kmovd   %k1, %eax
> > > -       /* Create mask in ecx for potentially in bound matches.  */
> > > -       bzhil   %edx, %eax, %eax
> > > -       jnz     L(return_vec_0)
> > > +       VPTEST  %YMM3, %YMM3, %k0
> > > +       kmovd   %k0, %eax
> > > +       testl   %eax, %eax
> > > +       jnz     L(return_vec_2)
> > > +L(return_vec_3):
> > > +       /* bsf saves 1 byte from tzcnt. This keep L(return_vec_3) in one
> > > +          fetch block and the entire L(*return_vec_0_1_2_3) in 1 cache
> > > +          line.  */
> > > +       bsfl    %ecx, %ecx
> > > +# ifdef USE_AS_WMEMCMP
> > > +       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
> > > +       xorl    %edx, %edx
> > > +       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
> > > +       setg    %dl
> > > +       leal    -1(%rdx, %rdx), %eax
> > > +# else
> > > +       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
> > > +       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
> > > +       subl    %ecx, %eax
> > > +# endif
> > >         ret
> > >
> > >         .p2align 4
> > > @@ -209,10 +240,11 @@ L(return_vec_0):
> > >  # endif
> > >         ret
> > >
> > > -       /* NB: No p2align necessary. Alignment  % 16 is naturally 1
> > > -          which is good enough for a target not in a loop.  */
> > > +       .p2align 4
> > >  L(return_vec_1):
> > > -       tzcntl  %eax, %eax
> > > +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_1) in one
> > > +          fetch block.  */
> > > +       bsfl    %eax, %eax
> > >  # ifdef USE_AS_WMEMCMP
> > >         movl    VEC_SIZE(%rdi, %rax, CHAR_SIZE), %ecx
> > >         xorl    %edx, %edx
> > > @@ -226,10 +258,11 @@ L(return_vec_1):
> > >  # endif
> > >         ret
> > >
> > > -       /* NB: No p2align necessary. Alignment  % 16 is naturally 2
> > > -          which is good enough for a target not in a loop.  */
> > > +       .p2align 4,, 10
> > >  L(return_vec_2):
> > > -       tzcntl  %eax, %eax
> > > +       /* bsf saves 1 byte over tzcnt and keeps L(return_vec_2) in one
> > > +          fetch block.  */
> > > +       bsfl    %eax, %eax
> > >  # ifdef USE_AS_WMEMCMP
> > >         movl    (VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
> > >         xorl    %edx, %edx
> > > @@ -243,40 +276,6 @@ L(return_vec_2):
> > >  # endif
> > >         ret
> > >
> > > -       .p2align 4
> > > -L(8x_return_vec_0_1_2_3):
> > > -       /* Returning from L(more_8x_vec) requires restoring rsi.  */
> > > -       addq    %rdi, %rsi
> > > -L(return_vec_0_1_2_3):
> > > -       VPCMP   $4, %YMM1, %YMM0, %k0
> > > -       kmovd   %k0, %eax
> > > -       testl   %eax, %eax
> > > -       jnz     L(return_vec_0)
> > > -
> > > -       VPCMP   $4, %YMM2, %YMM0, %k0
> > > -       kmovd   %k0, %eax
> > > -       testl   %eax, %eax
> > > -       jnz     L(return_vec_1)
> > > -
> > > -       VPCMP   $4, %YMM3, %YMM0, %k0
> > > -       kmovd   %k0, %eax
> > > -       testl   %eax, %eax
> > > -       jnz     L(return_vec_2)
> > > -L(return_vec_3):
> > > -       tzcntl  %ecx, %ecx
> > > -# ifdef USE_AS_WMEMCMP
> > > -       movl    (VEC_SIZE * 3)(%rdi, %rcx, CHAR_SIZE), %eax
> > > -       xorl    %edx, %edx
> > > -       cmpl    (VEC_SIZE * 3)(%rsi, %rcx, CHAR_SIZE), %eax
> > > -       setg    %dl
> > > -       leal    -1(%rdx, %rdx), %eax
> > > -# else
> > > -       movzbl  (VEC_SIZE * 3)(%rdi, %rcx), %eax
> > > -       movzbl  (VEC_SIZE * 3)(%rsi, %rcx), %ecx
> > > -       subl    %ecx, %eax
> > > -# endif
> > > -       ret
> > > -
> > >         .p2align 4
> > >  L(more_8x_vec):
> > >         /* Set end of s1 in rdx.  */
> > > @@ -288,21 +287,19 @@ L(more_8x_vec):
> > >         andq    $-VEC_SIZE, %rdi
> > >         /* Adjust because first 4x vec where check already.  */
> > >         subq    $-(VEC_SIZE * 4), %rdi
> > > +
> > >         .p2align 4
> > >  L(loop_4x_vec):
> > >         VMOVU   (%rsi, %rdi), %YMM1
> > >         vpxorq  (%rdi), %YMM1, %YMM1
> > > -
> > >         VMOVU   VEC_SIZE(%rsi, %rdi), %YMM2
> > >         vpxorq  VEC_SIZE(%rdi), %YMM2, %YMM2
> > > -
> > >         VMOVU   (VEC_SIZE * 2)(%rsi, %rdi), %YMM3
> > >         vpxorq  (VEC_SIZE * 2)(%rdi), %YMM3, %YMM3
> > > -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> > > -
> > >         VMOVU   (VEC_SIZE * 3)(%rsi, %rdi), %YMM4
> > > -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM3, %YMM4
> > > -       VPCMP   $4, %YMM4, %YMM0, %k1
> > > +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdi), %YMM1, %YMM4
> > > +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> > > +       VPTEST  %YMM4, %YMM4, %k1
> > >         kmovd   %k1, %ecx
> > >         testl   %ecx, %ecx
> > >         jnz     L(8x_return_vec_0_1_2_3)
> > > @@ -319,28 +316,25 @@ L(loop_4x_vec):
> > >         cmpl    $(VEC_SIZE * 2), %edi
> > >         jae     L(8x_last_2x_vec)
> > >
> > > +       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
> > > +
> > >         VMOVU   (%rsi, %rdx), %YMM1
> > >         vpxorq  (%rdx), %YMM1, %YMM1
> > >
> > >         VMOVU   VEC_SIZE(%rsi, %rdx), %YMM2
> > >         vpxorq  VEC_SIZE(%rdx), %YMM2, %YMM2
> > > -
> > > -       vpxorq  (VEC_SIZE * 2)(%rdx), %YMM3, %YMM3
> > > -       vpternlogd $0xfe, %YMM1, %YMM2, %YMM3
> > > -
> > >         VMOVU   (VEC_SIZE * 3)(%rsi, %rdx), %YMM4
> > > -       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM3, %YMM4
> > > -       VPCMP   $4, %YMM4, %YMM0, %k1
> > > +       vpternlogd $0xde, (VEC_SIZE * 3)(%rdx), %YMM1, %YMM4
> > > +       vpternlogd $0xfe, %YMM2, %YMM3, %YMM4
> > > +       VPTEST  %YMM4, %YMM4, %k1
> > >         kmovd   %k1, %ecx
> > > -       /* Restore s1 pointer to rdi.  */
> > > -       movq    %rdx, %rdi
> > >         testl   %ecx, %ecx
> > > -       jnz     L(8x_return_vec_0_1_2_3)
> > > +       jnz     L(8x_end_return_vec_0_1_2_3)
> > >         /* NB: eax must be zero to reach here.  */
> > >         ret
> > >
> > >         /* Only entry is from L(more_8x_vec).  */
> > > -       .p2align 4
> > > +       .p2align 4,, 10
> > >  L(8x_last_2x_vec):
> > >         VPCMP   $4, (VEC_SIZE * 2)(%rdx), %YMM3, %k1
> > >         kmovd   %k1, %eax
> > > @@ -355,7 +349,31 @@ L(8x_last_1x_vec):
> > >         jnz     L(8x_return_vec_3)
> > >         ret
> > >
> > > -       .p2align 4
> > > +       /* Not ideally aligned (at offset +9 bytes in fetch block) but
> > > +          not aligning keeps it in the same cache line as
> > > +          L(8x_last_1x/2x_vec) so likely worth it. As well, saves code
> > > +          size.  */
> > > +       .p2align 4,, 4
> > > +L(8x_return_vec_2):
> > > +       subq    $VEC_SIZE, %rdx
> > > +L(8x_return_vec_3):
> > > +       bsfl    %eax, %eax
> > > +# ifdef USE_AS_WMEMCMP
> > > +       leaq    (%rdx, %rax, CHAR_SIZE), %rax
> > > +       movl    (VEC_SIZE * 3)(%rax), %ecx
> > > +       xorl    %edx, %edx
> > > +       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
> > > +       setg    %dl
> > > +       leal    -1(%rdx, %rdx), %eax
> > > +# else
> > > +       addq    %rdx, %rax
> > > +       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
> > > +       movzbl  (VEC_SIZE * 3)(%rax), %eax
> > > +       subl    %ecx, %eax
> > > +# endif
> > > +       ret
> > > +
> > > +       .p2align 4,, 10
> > >  L(last_2x_vec):
> > >         /* Check second to last VEC.  */
> > >         VMOVU   -(VEC_SIZE * 2)(%rsi, %rdx, CHAR_SIZE), %YMM1
> > > @@ -374,26 +392,49 @@ L(last_1x_vec):
> > >         jnz     L(return_vec_0_end)
> > >         ret
> > >
> > > -       .p2align 4
> > > -L(8x_return_vec_2):
> > > -       subq    $VEC_SIZE, %rdx
> > > -L(8x_return_vec_3):
> > > -       tzcntl  %eax, %eax
> > > +       .p2align 4,, 10
> > > +L(return_vec_1_end):
> > > +       /* Use bsf to save code size. This is necessary to have
> > > +          L(one_or_less) fit in aligning bytes between.  */
> > > +       bsfl    %eax, %eax
> > > +       addl    %edx, %eax
> > >  # ifdef USE_AS_WMEMCMP
> > > -       leaq    (%rdx, %rax, CHAR_SIZE), %rax
> > > -       movl    (VEC_SIZE * 3)(%rax), %ecx
> > > +       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
> > >         xorl    %edx, %edx
> > > -       cmpl    (VEC_SIZE * 3)(%rsi, %rax), %ecx
> > > +       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
> > >         setg    %dl
> > >         leal    -1(%rdx, %rdx), %eax
> > >  # else
> > > -       addq    %rdx, %rax
> > > -       movzbl  (VEC_SIZE * 3)(%rsi, %rax), %ecx
> > > -       movzbl  (VEC_SIZE * 3)(%rax), %eax
> > > +       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
> > > +       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
> > >         subl    %ecx, %eax
> > >  # endif
> > >         ret
> > >
> > > +       /* NB: L(one_or_less) fits in alignment padding between
> > > +          L(return_vec_1_end) and L(return_vec_0_end).  */
> > > +# ifdef USE_AS_WMEMCMP
> > > +L(one_or_less):
> > > +       jb      L(zero)
> > > +       movl    (%rdi), %ecx
> > > +       xorl    %edx, %edx
> > > +       cmpl    (%rsi), %ecx
> > > +       je      L(zero)
> > > +       setg    %dl
> > > +       leal    -1(%rdx, %rdx), %eax
> > > +       ret
> > > +# else
> > > +L(one_or_less):
> > > +       jb      L(zero)
> > > +       movzbl  (%rsi), %ecx
> > > +       movzbl  (%rdi), %eax
> > > +       subl    %ecx, %eax
> > > +       ret
> > > +# endif
> > > +L(zero):
> > > +       xorl    %eax, %eax
> > > +       ret
> > > +
> > >         .p2align 4
> > >  L(return_vec_0_end):
> > >         tzcntl  %eax, %eax
> > > @@ -412,23 +453,56 @@ L(return_vec_0_end):
> > >         ret
> > >
> > >         .p2align 4
> > > -L(return_vec_1_end):
> > > +L(less_vec):
> > > +       /* Check if one or less CHAR. This is necessary for size == 0
> > > +          but is also faster for size == CHAR_SIZE.  */
> > > +       cmpl    $1, %edx
> > > +       jbe     L(one_or_less)
> > > +
> > > +       /* Check if loading one VEC from either s1 or s2 could cause a
> > > +          page cross. This can have false positives but is by far the
> > > +          fastest method.  */
> > > +       movl    %edi, %eax
> > > +       orl     %esi, %eax
> > > +       andl    $(PAGE_SIZE - 1), %eax
> > > +       cmpl    $(PAGE_SIZE - VEC_SIZE), %eax
> > > +       jg      L(page_cross_less_vec)
> > > +
> > > +       /* No page cross possible.  */
> > > +       VMOVU   (%rsi), %YMM2
> > > +       VPCMP   $4, (%rdi), %YMM2, %k1
> > > +       kmovd   %k1, %eax
> > > +       /* Check if any matches where in bounds. Intentionally not
> > > +          storing result in eax to limit dependency chain if it goes to
> > > +          L(return_vec_0_lv).  */
> > > +       bzhil   %edx, %eax, %edx
> > > +       jnz     L(return_vec_0_lv)
> > > +       xorl    %eax, %eax
> > > +       ret
> > > +
> > > +       /* Essentially duplicate of L(return_vec_0). Ends up not costing
> > > +          any code as shrinks L(less_vec) by allowing 2-byte encoding of
> > > +          the jump and ends up fitting in aligning bytes. As well fits on
> > > +          same cache line as L(less_vec) so also saves a line from having
> > > +          to be fetched on cold calls to memcmp.  */
> > > +       .p2align 4,, 4
> > > +L(return_vec_0_lv):
> > >         tzcntl  %eax, %eax
> > > -       addl    %edx, %eax
> > >  # ifdef USE_AS_WMEMCMP
> > > -       movl    -(VEC_SIZE * 2)(%rdi, %rax, CHAR_SIZE), %ecx
> > > +       movl    (%rdi, %rax, CHAR_SIZE), %ecx
> > >         xorl    %edx, %edx
> > > -       cmpl    -(VEC_SIZE * 2)(%rsi, %rax, CHAR_SIZE), %ecx
> > > +       cmpl    (%rsi, %rax, CHAR_SIZE), %ecx
> > > +       /* NB: no partial register stall here because xorl zero idiom
> > > +          above.  */
> > >         setg    %dl
> > >         leal    -1(%rdx, %rdx), %eax
> > >  # else
> > > -       movzbl  -(VEC_SIZE * 2)(%rsi, %rax), %ecx
> > > -       movzbl  -(VEC_SIZE * 2)(%rdi, %rax), %eax
> > > +       movzbl  (%rsi, %rax), %ecx
> > > +       movzbl  (%rdi, %rax), %eax
> > >         subl    %ecx, %eax
> > >  # endif
> > >         ret
> > >
> > > -
> > >         .p2align 4
> > >  L(page_cross_less_vec):
> > >         /* if USE_AS_WMEMCMP it can only be 0, 4, 8, 12, 16, 20, 24, 28
> > > @@ -439,108 +513,84 @@ L(page_cross_less_vec):
> > >         cmpl    $8, %edx
> > >         jae     L(between_8_15)
> > >         cmpl    $4, %edx
> > > -       jae     L(between_4_7)
> > > -L(between_2_3):
> > > -       /* Load as big endian to avoid branches.  */
> > > -       movzwl  (%rdi), %eax
> > > -       movzwl  (%rsi), %ecx
> > > -       shll    $8, %eax
> > > -       shll    $8, %ecx
> > > -       bswap   %eax
> > > -       bswap   %ecx
> > > -       movzbl  -1(%rdi, %rdx), %edi
> > > -       movzbl  -1(%rsi, %rdx), %esi
> > > -       orl     %edi, %eax
> > > -       orl     %esi, %ecx
> > > -       /* Subtraction is okay because the upper 8 bits are zero.  */
> > > -       subl    %ecx, %eax
> > > -       ret
> > > -       .p2align 4
> > > -L(one_or_less):
> > > -       jb      L(zero)
> > > -       movzbl  (%rsi), %ecx
> > > -       movzbl  (%rdi), %eax
> > > -       subl    %ecx, %eax
> > > +       jb      L(between_2_3)
> > > +
> > > +       /* Load as big endian with overlapping movbe to avoid branches.
> > > +        */
> > > +       movbe   (%rdi), %eax
> > > +       movbe   (%rsi), %ecx
> > > +       shlq    $32, %rax
> > > +       shlq    $32, %rcx
> > > +       movbe   -4(%rdi, %rdx), %edi
> > > +       movbe   -4(%rsi, %rdx), %esi
> > > +       orq     %rdi, %rax
> > > +       orq     %rsi, %rcx
> > > +       subq    %rcx, %rax
> > > +       /* edx is guranteed to be positive int32 in range [4, 7].  */
> > > +       cmovne  %edx, %eax
> > > +       /* ecx is -1 if rcx > rax. Otherwise 0.  */
> > > +       sbbl    %ecx, %ecx
> > > +       /* If rcx > rax, then ecx is 0 and eax is positive. If rcx ==
> > > +          rax then eax and ecx are zero. If rax < rax then ecx is -1 so
> > > +          eax doesn't matter.  */
> > > +       orl     %ecx, %eax
> > >         ret
> > >
> > > -       .p2align 4
> > > +       .p2align 4,, 8
> > >  L(between_8_15):
> > >  # endif
> > >         /* If USE_AS_WMEMCMP fall through into 8-15 byte case.  */
> > > -       vmovq   (%rdi), %XMM1
> > > -       vmovq   (%rsi), %XMM2
> > > -       VPCMP   $4, %XMM1, %XMM2, %k1
> > > +       vmovq   (%rdi), %xmm1
> > > +       vmovq   (%rsi), %xmm2
> > > +       VPCMP   $4, %xmm1, %xmm2, %k1
> > >         kmovd   %k1, %eax
> > >         testl   %eax, %eax
> > > -       jnz     L(return_vec_0)
> > > +       jnz     L(return_vec_0_lv)
> > >         /* Use overlapping loads to avoid branches.  */
> > > -       leaq    -8(%rdi, %rdx, CHAR_SIZE), %rdi
> > > -       leaq    -8(%rsi, %rdx, CHAR_SIZE), %rsi
> > > -       vmovq   (%rdi), %XMM1
> > > -       vmovq   (%rsi), %XMM2
> > > -       VPCMP   $4, %XMM1, %XMM2, %k1
> > > +       vmovq   -8(%rdi, %rdx, CHAR_SIZE), %xmm1
> > > +       vmovq   -8(%rsi, %rdx, CHAR_SIZE), %xmm2
> > > +       VPCMP   $4, %xmm1, %xmm2, %k1
> > > +       addl    $(CHAR_PER_VEC - (8 / CHAR_SIZE)), %edx
> > >         kmovd   %k1, %eax
> > >         testl   %eax, %eax
> > > -       jnz     L(return_vec_0)
> > > -       ret
> > > -
> > > -       .p2align 4
> > > -L(zero):
> > > -       xorl    %eax, %eax
> > > +       jnz     L(return_vec_0_end)
> > >         ret
> > >
> > > -       .p2align 4
> > > +       .p2align 4,, 8
> > >  L(between_16_31):
> > >         /* From 16 to 31 bytes.  No branch when size == 16.  */
> > > -       VMOVU   (%rsi), %XMM2
> > > -       VPCMP   $4, (%rdi), %XMM2, %k1
> > > +
> > > +       /* Use movups to save code size.  */
> > > +       movups  (%rsi), %xmm2
> > > +       VPCMP   $4, (%rdi), %xmm2, %k1
> > >         kmovd   %k1, %eax
> > >         testl   %eax, %eax
> > > -       jnz     L(return_vec_0)
> > > -
> > > +       jnz     L(return_vec_0_lv)
> > >         /* Use overlapping loads to avoid branches.  */
> > > -
> > > -       VMOVU   -16(%rsi, %rdx, CHAR_SIZE), %XMM2
> > > -       leaq    -16(%rdi, %rdx, CHAR_SIZE), %rdi
> > > -       leaq    -16(%rsi, %rdx, CHAR_SIZE), %rsi
> > > -       VPCMP   $4, (%rdi), %XMM2, %k1
> > > +       movups  -16(%rsi, %rdx, CHAR_SIZE), %xmm2
> > > +       VPCMP   $4, -16(%rdi, %rdx, CHAR_SIZE), %xmm2, %k1
> > > +       addl    $(CHAR_PER_VEC - (16 / CHAR_SIZE)), %edx
> > >         kmovd   %k1, %eax
> > >         testl   %eax, %eax
> > > -       jnz     L(return_vec_0)
> > > -       ret
> > > -
> > > -# ifdef USE_AS_WMEMCMP
> > > -       .p2align 4
> > > -L(one_or_less):
> > > -       jb      L(zero)
> > > -       movl    (%rdi), %ecx
> > > -       xorl    %edx, %edx
> > > -       cmpl    (%rsi), %ecx
> > > -       je      L(zero)
> > > -       setg    %dl
> > > -       leal    -1(%rdx, %rdx), %eax
> > > +       jnz     L(return_vec_0_end)
> > >         ret
> > > -# else
> > >
> > > -       .p2align 4
> > > -L(between_4_7):
> > > -       /* Load as big endian with overlapping movbe to avoid branches.
> > > -        */
> > > -       movbe   (%rdi), %eax
> > > -       movbe   (%rsi), %ecx
> > > -       shlq    $32, %rax
> > > -       shlq    $32, %rcx
> > > -       movbe   -4(%rdi, %rdx), %edi
> > > -       movbe   -4(%rsi, %rdx), %esi
> > > -       orq     %rdi, %rax
> > > -       orq     %rsi, %rcx
> > > -       subq    %rcx, %rax
> > > -       jz      L(zero_4_7)
> > > -       sbbl    %eax, %eax
> > > -       orl     $1, %eax
> > > -L(zero_4_7):
> > > +# ifndef USE_AS_WMEMCMP
> > > +L(between_2_3):
> > > +       /* Load as big endian to avoid branches.  */
> > > +       movzwl  (%rdi), %eax
> > > +       movzwl  (%rsi), %ecx
> > > +       shll    $8, %eax
> > > +       shll    $8, %ecx
> > > +       bswap   %eax
> > > +       bswap   %ecx
> > > +       movzbl  -1(%rdi, %rdx), %edi
> > > +       movzbl  -1(%rsi, %rdx), %esi
> > > +       orl     %edi, %eax
> > > +       orl     %esi, %ecx
> > > +       /* Subtraction is okay because the upper 8 bits are zero.  */
> > > +       subl    %ecx, %eax
> > >         ret
> > >  # endif
> > > -
> > >  END (MEMCMP)
> > >  #endif
> > > --
> > > 2.25.1
> > >
> >
> > LGTM.
> >
> > Reviewed-by: H.J. Lu <hjl.tools@gmail.com>
> >
> > Thanks.
> >
> > --
> > H.J.
>
> Thanks! Pushed.

I would like to backport this patch to release branches.
Any comments or objections?

--Sunil