diff mbox series

[v5,6/6] elf: Optimize _dl_new_hash in dl-new-hash.h

Message ID 20220509171747.4153703-6-goldstein.w.n@gmail.com
State Superseded
Headers
Series [v5,1/6] elf: Refactor dl_new_hash so it can be tested / benchmarked |

Checks

Context Check Description
dj/TryBot-apply_patch success Patch applied to master at the time it was sent
dj/TryBot-32bit fail Patch series failed to build

Commit Message

Noah Goldstein May 9, 2022, 5:17 p.m. UTC 
  Unroll slightly and enforce good instruction scheduling. This improves
performance on out-of-order machines. Note the unrolling allows
for pipelined multiplies which helps a bit, but most of the gain
is from enforcing better instruction scheduling for more ILP.
Unrolling further started to induce slowdowns for sizes [0, 4]
but can help the loop so if larger sizes are the target further
unrolling can be beneficial.

Results for _dl_new_hash
Benchmarked on Tigerlake: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz

Time as Geometric Mean of N=25 runs
Geometric of all benchmark New / Old: 0.791
  type, length, New Time, Old Time, New Time / Old Time
 fixed,      0,    0.641,    0.658,               0.974
 fixed,      1,    1.888,    1.883,               1.003
 fixed,      2,    2.712,    2.833,               0.957
 fixed,      3,    3.314,    3.739,               0.886
 fixed,      4,    4.316,    4.866,               0.887
 fixed,      5,     5.16,    5.966,               0.865
 fixed,      6,    5.986,    7.241,               0.827
 fixed,      7,    7.264,    8.435,               0.861
 fixed,      8,    8.052,    9.846,               0.818
 fixed,      9,    9.369,   11.316,               0.828
 fixed,     10,   10.256,   12.925,               0.794
 fixed,     11,   12.191,   14.546,               0.838
 fixed,     12,   12.667,    15.92,               0.796
 fixed,     13,   14.442,   17.465,               0.827
 fixed,     14,   14.808,   18.981,                0.78
 fixed,     15,   16.244,   20.565,                0.79
 fixed,     16,   17.166,   22.044,               0.779
 fixed,     32,   35.447,   50.558,               0.701
 fixed,     64,   86.479,  134.529,               0.643
 fixed,    128,  155.453,  287.527,               0.541
 fixed,    256,   302.57,   593.64,                0.51
random,      2,   11.168,    10.61,               1.053
random,      4,   13.308,    13.53,               0.984
random,      8,   16.579,   19.437,               0.853
random,     16,   21.292,   24.776,               0.859
random,     32,    30.56,   35.906,               0.851
random,     64,   49.249,   68.577,               0.718
random,    128,   81.845,  140.664,               0.582
random,    256,  152.517,  292.204,               0.522

Co-authored-by: Alexander Monakov <amonakov@ispras.ru>
---
 elf/dl-new-hash.h | 50 ++++++++++++++++++++++++++++++++++++++++++-----
 1 file changed, 45 insertions(+), 5 deletions(-)

Comments

Adhemerval Zanella May 10, 2022, 11:58 a.m. UTC | #1 
On 09/05/2022 14:17, Noah Goldstein via Libc-alpha wrote:
> Unroll slightly and enforce good instruction scheduling. This improves
> performance on out-of-order machines. Note the unrolling allows
> for pipelined multiplies which helps a bit, but most of the gain
> is from enforcing better instruction scheduling for more ILP.
> Unrolling further started to induce slowdowns for sizes [0, 4]
> but can help the loop so if larger sizes are the target further
> unrolling can be beneficial.
> 
> Results for _dl_new_hash
> Benchmarked on Tigerlake: 11th Gen Intel(R) Core(TM) i7-1165G7 @ 2.80GHz
> 
> Time as Geometric Mean of N=25 runs
> Geometric of all benchmark New / Old: 0.791
>   type, length, New Time, Old Time, New Time / Old Time
>  fixed,      0,    0.641,    0.658,               0.974
>  fixed,      1,    1.888,    1.883,               1.003
>  fixed,      2,    2.712,    2.833,               0.957
>  fixed,      3,    3.314,    3.739,               0.886
>  fixed,      4,    4.316,    4.866,               0.887
>  fixed,      5,     5.16,    5.966,               0.865
>  fixed,      6,    5.986,    7.241,               0.827
>  fixed,      7,    7.264,    8.435,               0.861
>  fixed,      8,    8.052,    9.846,               0.818
>  fixed,      9,    9.369,   11.316,               0.828
>  fixed,     10,   10.256,   12.925,               0.794
>  fixed,     11,   12.191,   14.546,               0.838
>  fixed,     12,   12.667,    15.92,               0.796
>  fixed,     13,   14.442,   17.465,               0.827
>  fixed,     14,   14.808,   18.981,                0.78
>  fixed,     15,   16.244,   20.565,                0.79
>  fixed,     16,   17.166,   22.044,               0.779
>  fixed,     32,   35.447,   50.558,               0.701
>  fixed,     64,   86.479,  134.529,               0.643
>  fixed,    128,  155.453,  287.527,               0.541
>  fixed,    256,   302.57,   593.64,                0.51
> random,      2,   11.168,    10.61,               1.053
> random,      4,   13.308,    13.53,               0.984
> random,      8,   16.579,   19.437,               0.853
> random,     16,   21.292,   24.776,               0.859
> random,     32,    30.56,   35.906,               0.851
> random,     64,   49.249,   68.577,               0.718
> random,    128,   81.845,  140.664,               0.582
> random,    256,  152.517,  292.204,               0.522
> 
> Co-authored-by: Alexander Monakov <amonakov@ispras.ru>

Buildbot failed to build it [1]:

make[2]: Entering directory '/glibc/elf'
gcc -m32 dl-lookup.c -c -std=gnu11 -fgnu89-inline  -g -O2 -Wall -Wwrite-strings -Wundef -Werror -fmerge-all-constants -frounding-math -fno-stack-protector -fno-common -Wstrict-prototypes -Wold-style-definition -fmath-errno    -fPIC  -fno-stack-protector -DSTACK_PROTECTOR_LEVEL=0 -Wa,-mtune=i686  -mno-sse -mno-mmx -mfpmath=387    -fexceptions -fasynchronous-unwind-tables  -ftls-model=initial-exec      -I../include -I/build/elf  -I/build  -I../sysdeps/unix/sysv/linux/i386/i686  -I../sysdeps/i386/i686/nptl  -I../sysdeps/unix/sysv/linux/i386  -I../sysdeps/unix/sysv/linux/x86/include -I../sysdeps/unix/sysv/linux/x86  -I../sysdeps/x86/nptl  -I../sysdeps/i386/nptl  -I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux  -I../sysdeps/nptl  -I../sysdeps/pthread  -I../sysdeps/gnu  -I../sysdeps/unix/inet  -I../sysdeps/unix/sysv  -I../sysdeps/unix/i386  -I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/i386/i686/fpu/multiarch  -I../sysdeps/i386/i686/fpu  -I../sysdeps/i386/i686/multiarch  -I../sysdeps/i386/i686  -I../sysdeps/i386/fpu  -I../sysdeps/x86/fpu  -I../sysdeps/i386  -I../sysdeps/x86/include -I../sysdeps/x86  -I../sysdeps/wordsize-32  -I../sysdeps/ieee754/float128  -I../sysdeps/ieee754/ldbl-96/include -I../sysdeps/ieee754/ldbl-96  -I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32  -I../sysdeps/ieee754  -I../sysdeps/generic  -I.. -I../libio -I.  -D_LIBC_REENTRANT -include /build/libc-modules.h -DMODULE_NAME=rtld -include ../include/libc-symbols.h  -DPIC -DSHARED     -DTOP_NAMESPACE=glibc -o /build/elf/dl-lookup.os -MD -MP -MF /build/elf/dl-lookup.os.dt -MT /build/elf/dl-lookup.os
In file included from dl-lookup.c:27:
./dl-new-hash.h: In function '_dl_new_hash':
./dl-new-hash.h:30:28: error: pointer targets in initialization of 'const unsigned char *' from 'const char *' differ in signedness [-Werror=pointer-sign]
   30 |   const unsigned char *s = signed_s;
      |                            ^~~~~~~~
cc1: all warnings being treated as errors
make[2]: *** [../o-iterator.mk:9: /build/elf/dl-lookup.os] Error 1
gcc -m32 dl-lookup.c -c -std=gnu11 -fgnu89-inline  -g -O2 -Wall -Wwrite-strings -Wundef -Werror -fmerge-all-constants -frounding-math -fno-stack-protector -fno-common -Wstrict-prototypes -Wold-style-definition -fmath-errno    -fpie  -fno-stack-protector -DSTACK_PROTECTOR_LEVEL=0 -Wa,-mtune=i686 -fexceptions -fasynchronous-unwind-tables  -ftls-model=initial-exec      -I../include -I/build/elf  -I/build  -I../sysdeps/unix/sysv/linux/i386/i686  -I../sysdeps/i386/i686/nptl  -I../sysdeps/unix/sysv/linux/i386  -I../sysdeps/unix/sysv/linux/x86/include -I../sysdeps/unix/sysv/linux/x86  -I../sysdeps/x86/nptl  -I../sysdeps/i386/nptl  -I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux  -I../sysdeps/nptl  -I../sysdeps/pthread  -I../sysdeps/gnu  -I../sysdeps/unix/inet  -I../sysdeps/unix/sysv  -I../sysdeps/unix/i386  -I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/i386/i686/fpu/multiarch  -I../sysdeps/i386/i686/fpu  -I../sysdeps/i386/i686/multiarch  -I../sysdeps/i386/i686  -I../sysdeps/i386/fpu  -I../sysdeps/x86/fpu  -I../sysdeps/i386  -I../sysdeps/x86/include -I../sysdeps/x86  -I../sysdeps/wordsize-32  -I../sysdeps/ieee754/float128  -I../sysdeps/ieee754/ldbl-96/include -I../sysdeps/ieee754/ldbl-96  -I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32  -I../sysdeps/ieee754  -I../sysdeps/generic  -I.. -I../libio -I.  -D_LIBC_REENTRANT -include /build/libc-modules.h -DMODULE_NAME=libc -include ../include/libc-symbols.h  -DPIC     -DTOP_NAMESPACE=glibc -o /build/elf/dl-lookup.o -MD -MP -MF /build/elf/dl-lookup.o.dt -MT /build/elf/dl-lookup.o
In file included from dl-lookup.c:27:
./dl-new-hash.h: In function '_dl_new_hash':
./dl-new-hash.h:30:28: error: pointer targets in initialization of 'const unsigned char *' from 'const char *' differ in signedness [-Werror=pointer-sign]
   30 |   const unsigned char *s = signed_s;
      |                            ^~~~~~~~

[1] https://www.delorie.com/trybots/32bit/9123/make.tail.txt
diff mbox series

Patch

diff --git a/elf/dl-new-hash.h b/elf/dl-new-hash.h
index 40d88c81f9..70891d374c 100644
--- a/elf/dl-new-hash.h
+++ b/elf/dl-new-hash.h
@@ -20,15 +20,55 @@ 
 #define _DL_NEW_HASH_H 1
 
 #include <stdint.h>
+/* For __glibc_unlikely.  */
+#include <sys/cdefs.h>
 
 static inline uint32_t
 __attribute__ ((unused))
-_dl_new_hash (const char *s)
+_dl_new_hash (const char *signed_s)
 {
-  uint32_t h = 5381;
-  for (unsigned char c = *s; c != '\0'; c = *++s)
-    h = h * 33 + c;
-  return h;
+  const unsigned char *s = signed_s;
+  unsigned int h = 5381;
+  unsigned int c0, c1;
+  for (;;)
+    {
+      c0 = (unsigned int) *s;
+      /* Unlikely length zero string so evens will be slightly less
+	 common.  */
+      if (__glibc_unlikely (c0 == 0))
+	{
+	  return h;
+	}
+
+      c1 = (unsigned int) *(s + 1);
+      if (c1 == 0)
+	{
+	  c0 += h;
+	  /* Ideal instruction scheduling is:
+	     c0 += h;
+	     h *= 32;
+	     h += c0;
+	     The asm statement ensures the compiler can't mess that up.  */
+	  asm("" : "+r"(h) : "r"(c0));
+	  h = h * 32 + c0;
+      return h;
+	}
+
+      /* Ideal instruction scheduling is:
+	 c1 += c0;
+	 h *= 33 * 33;
+	 c0 *= 32;
+	 c1 += c0;
+	 h  += c1;
+	 The asm statements ensures the compiler can't mess that up.  */
+      c1 += c0;
+      asm("" : "+r"(c1), "+r"(c0));
+      h *= 33 * 33;
+      c1 += c0 * 32;
+      asm("" : "+r"(c1));
+      h += c1;
+      s += 2;
+    }
 }