[AArch64] Optimized memcmp
Commit Message
This is an optimized memcmp for AArch64. This is a complete rewrite
using a different algorithm. The previous version split into cases
where both inputs were aligned, the inputs were mutually aligned and
unaligned using a byte loop. The new version combines all these cases,
while small inputs of less than 8 bytes are handled separately.
This allows the main code to be sped up using unaligned loads since
there are now at least 8 bytes to be compared. After the first 8 bytes,
align the first input. This ensures each iteration does at most one
unaligned access and mutually aligned inputs behave as aligned.
After the main loop, process the last 8 bytes using unaligned accesses.
This improves performance of (mutually) aligned cases by 25% and
unaligned by >500% (yes >6 times faster) on large inputs.
ChangeLog:
2017-07-07 Wilco Dijkstra <wdijkstr@arm.com>
* sysdeps/aarch64/memcmp.S (memcmp):
Rewrite of optimized memcmp.
GLIBC benchtests/bench-memcmp.c performance comparison for Cortex-A53:
Length 1, alignment 1/ 1: 153%
Length 1, alignment 1/ 1: 119%
Length 1, alignment 1/ 1: 154%
Length 2, alignment 2/ 2: 121%
Length 2, alignment 2/ 2: 140%
Length 2, alignment 2/ 2: 121%
Length 3, alignment 3/ 3: 105%
Length 3, alignment 3/ 3: 105%
Length 3, alignment 3/ 3: 105%
Length 4, alignment 4/ 4: 155%
Length 4, alignment 4/ 4: 154%
Length 4, alignment 4/ 4: 161%
Length 5, alignment 5/ 5: 173%
Length 5, alignment 5/ 5: 173%
Length 5, alignment 5/ 5: 173%
Length 6, alignment 6/ 6: 145%
Length 6, alignment 6/ 6: 145%
Length 6, alignment 6/ 6: 145%
Length 7, alignment 7/ 7: 125%
Length 7, alignment 7/ 7: 125%
Length 7, alignment 7/ 7: 125%
Length 8, alignment 8/ 8: 111%
Length 8, alignment 8/ 8: 130%
Length 8, alignment 8/ 8: 124%
Length 9, alignment 9/ 9: 160%
Length 9, alignment 9/ 9: 160%
Length 9, alignment 9/ 9: 150%
Length 10, alignment 10/10: 170%
Length 10, alignment 10/10: 137%
Length 10, alignment 10/10: 150%
Length 11, alignment 11/11: 160%
Length 11, alignment 11/11: 160%
Length 11, alignment 11/11: 160%
Length 12, alignment 12/12: 146%
Length 12, alignment 12/12: 168%
Length 12, alignment 12/12: 156%
Length 13, alignment 13/13: 167%
Length 13, alignment 13/13: 167%
Length 13, alignment 13/13: 173%
Length 14, alignment 14/14: 167%
Length 14, alignment 14/14: 168%
Length 14, alignment 14/14: 168%
Length 15, alignment 15/15: 168%
Length 15, alignment 15/15: 173%
Length 15, alignment 15/15: 173%
Length 1, alignment 0/ 0: 134%
Length 1, alignment 0/ 0: 127%
Length 1, alignment 0/ 0: 119%
Length 2, alignment 0/ 0: 94%
Length 2, alignment 0/ 0: 94%
Length 2, alignment 0/ 0: 106%
Length 3, alignment 0/ 0: 82%
Length 3, alignment 0/ 0: 87%
Length 3, alignment 0/ 0: 82%
Length 4, alignment 0/ 0: 115%
Length 4, alignment 0/ 0: 115%
Length 4, alignment 0/ 0: 122%
Length 5, alignment 0/ 0: 127%
Length 5, alignment 0/ 0: 119%
Length 5, alignment 0/ 0: 127%
Length 6, alignment 0/ 0: 103%
Length 6, alignment 0/ 0: 100%
Length 6, alignment 0/ 0: 100%
Length 7, alignment 0/ 0: 82%
Length 7, alignment 0/ 0: 91%
Length 7, alignment 0/ 0: 87%
Length 8, alignment 0/ 0: 111%
Length 8, alignment 0/ 0: 124%
Length 8, alignment 0/ 0: 124%
Length 9, alignment 0/ 0: 136%
Length 9, alignment 0/ 0: 136%
Length 9, alignment 0/ 0: 136%
Length 10, alignment 0/ 0: 136%
Length 10, alignment 0/ 0: 135%
Length 10, alignment 0/ 0: 136%
Length 11, alignment 0/ 0: 136%
Length 11, alignment 0/ 0: 136%
Length 11, alignment 0/ 0: 135%
Length 12, alignment 0/ 0: 136%
Length 12, alignment 0/ 0: 136%
Length 12, alignment 0/ 0: 136%
Length 13, alignment 0/ 0: 135%
Length 13, alignment 0/ 0: 136%
Length 13, alignment 0/ 0: 136%
Length 14, alignment 0/ 0: 136%
Length 14, alignment 0/ 0: 136%
Length 14, alignment 0/ 0: 136%
Length 15, alignment 0/ 0: 136%
Length 15, alignment 0/ 0: 136%
Length 15, alignment 0/ 0: 136%
Length 4, alignment 0/ 0: 115%
Length 4, alignment 0/ 0: 115%
Length 4, alignment 0/ 0: 115%
Length 32, alignment 0/ 0: 127%
Length 32, alignment 7/ 2: 395%
Length 32, alignment 0/ 0: 127%
Length 32, alignment 0/ 0: 127%
Length 8, alignment 0/ 0: 111%
Length 8, alignment 0/ 0: 124%
Length 8, alignment 0/ 0: 124%
Length 64, alignment 0/ 0: 128%
Length 64, alignment 6/ 4: 475%
Length 64, alignment 0/ 0: 131%
Length 64, alignment 0/ 0: 134%
Length 16, alignment 0/ 0: 128%
Length 16, alignment 0/ 0: 119%
Length 16, alignment 0/ 0: 128%
Length 128, alignment 0/ 0: 129%
Length 128, alignment 5/ 6: 475%
Length 128, alignment 0/ 0: 130%
Length 128, alignment 0/ 0: 129%
Length 32, alignment 0/ 0: 126%
Length 32, alignment 0/ 0: 126%
Length 32, alignment 0/ 0: 126%
Length 256, alignment 0/ 0: 127%
Length 256, alignment 4/ 8: 545%
Length 256, alignment 0/ 0: 126%
Length 256, alignment 0/ 0: 128%
Length 64, alignment 0/ 0: 171%
Length 64, alignment 0/ 0: 171%
Length 64, alignment 0/ 0: 174%
Length 512, alignment 0/ 0: 126%
Length 512, alignment 3/10: 585%
Length 512, alignment 0/ 0: 126%
Length 512, alignment 0/ 0: 127%
Length 128, alignment 0/ 0: 129%
Length 128, alignment 0/ 0: 128%
Length 128, alignment 0/ 0: 129%
Length 1024, alignment 0/ 0: 125%
Length 1024, alignment 2/12: 611%
Length 1024, alignment 0/ 0: 126%
Length 1024, alignment 0/ 0: 126%
Length 256, alignment 0/ 0: 128%
Length 256, alignment 0/ 0: 127%
Length 256, alignment 0/ 0: 128%
Length 2048, alignment 0/ 0: 125%
Length 2048, alignment 1/14: 625%
Length 2048, alignment 0/ 0: 125%
Length 2048, alignment 0/ 0: 125%
Length 512, alignment 0/ 0: 126%
Length 512, alignment 0/ 0: 127%
Length 512, alignment 0/ 0: 127%
Length 4096, alignment 0/ 0: 125%
Length 4096, alignment 0/16: 125%
Length 4096, alignment 0/ 0: 125%
Length 4096, alignment 0/ 0: 125%
Length 1024, alignment 0/ 0: 126%
Length 1024, alignment 0/ 0: 126%
Length 1024, alignment 0/ 0: 126%
Length 8192, alignment 0/ 0: 125%
Length 8192, alignment 63/18: 636%
Length 8192, alignment 0/ 0: 125%
Length 8192, alignment 0/ 0: 125%
Length 16, alignment 1/ 2: 317%
Length 16, alignment 1/ 2: 317%
Length 16, alignment 1/ 2: 317%
Length 32, alignment 2/ 4: 395%
Length 32, alignment 2/ 4: 395%
Length 32, alignment 2/ 4: 398%
Length 64, alignment 3/ 6: 475%
Length 64, alignment 3/ 6: 475%
Length 64, alignment 3/ 6: 477%
Length 128, alignment 4/ 8: 479%
Length 128, alignment 4/ 8: 479%
Length 128, alignment 4/ 8: 479%
Length 256, alignment 5/10: 543%
Length 256, alignment 5/10: 539%
Length 256, alignment 5/10: 543%
Length 512, alignment 6/12: 585%
Length 512, alignment 6/12: 585%
Length 512, alignment 6/12: 585%
Length 1024, alignment 7/14: 611%
Length 1024, alignment 7/14: 611%
Length 1024, alignment 7/14: 611%
---
@@ -22,132 +22,98 @@
/* Assumptions:
*
- * ARMv8-a, AArch64
+ * ARMv8-a, AArch64, unaligned accesses.
*/
/* Parameters and result. */
#define src1 x0
#define src2 x1
#define limit x2
-#define result x0
+#define result w0
/* Internal variables. */
#define data1 x3
#define data1w w3
#define data2 x4
#define data2w w4
-#define has_nul x5
-#define diff x6
-#define endloop x7
-#define tmp1 x8
-#define tmp2 x9
-#define tmp3 x10
-#define pos x11
-#define limit_wd x12
-#define mask x13
+#define tmp1 x5
ENTRY_ALIGN (memcmp, 6)
DELOUSE (0)
DELOUSE (1)
DELOUSE (2)
- cbz limit, L(ret0)
- eor tmp1, src1, src2
- tst tmp1, #7
- b.ne L(misaligned8)
- ands tmp1, src1, #7
- b.ne L(mutual_align)
- add limit_wd, limit, #7
- lsr limit_wd, limit_wd, #3
- /* Start of performance-critical section -- one 64B cache line. */
-L(loop_aligned):
- ldr data1, [src1], #8
- ldr data2, [src2], #8
-L(start_realigned):
- subs limit_wd, limit_wd, #1
- eor diff, data1, data2 /* Non-zero if differences found. */
- csinv endloop, diff, xzr, ne /* Last Dword or differences. */
- cbz endloop, L(loop_aligned)
- /* End of performance-critical section -- one 64B cache line. */
-
- /* Not reached the limit, must have found a diff. */
- cbnz limit_wd, L(not_limit)
-
- /* Limit % 8 == 0 => all bytes significant. */
- ands limit, limit, #7
- b.eq L(not_limit)
-
- lsl limit, limit, #3 /* Bits -> bytes. */
- mov mask, #~0
-#ifdef __AARCH64EB__
- lsr mask, mask, limit
-#else
- lsl mask, mask, limit
-#endif
- bic data1, data1, mask
- bic data2, data2, mask
-
- orr diff, diff, mask
-L(not_limit):
-#ifndef __AARCH64EB__
- rev diff, diff
+ subs limit, limit, 8
+ b.lo .Lless8
+
+ /* Limit >= 8, so check first 8 bytes using unaligned loads. */
+ ldr data1, [src1], 8
+ ldr data2, [src2], 8
+ and tmp1, src1, 7
+ add limit, limit, tmp1
+ cmp data1, data2
+ bne .Lreturn
+
+ /* Align src1 and adjust src2 with bytes not yet done. */
+ sub src1, src1, tmp1
+ sub src2, src2, tmp1
+
+ subs limit, limit, 8
+ b.ls .Llast_bytes
+
+ /* Loop performing 8 bytes per iteration using aligned src1.
+ Limit is pre-decremented by 8 and must be larger than zero.
+ Exit if <= 8 bytes left to do or if the data is not equal. */
+ .p2align 4
+.Lloop8:
+ ldr data1, [src1], 8
+ ldr data2, [src2], 8
+ subs limit, limit, 8
+ ccmp data1, data2, 0, hi /* NZCV = 0b0000. */
+ b.eq .Lloop8
+
+ cmp data1, data2
+ bne .Lreturn
+
+ /* Compare last 1-8 bytes using unaligned access. */
+.Llast_bytes:
+ ldr data1, [src1, limit]
+ ldr data2, [src2, limit]
+
+ /* Compare data bytes and set return value to 0, -1 or 1. */
+.Lreturn:
+#ifndef __AARCH64EB__
rev data1, data1
rev data2, data2
#endif
- /* The MS-non-zero bit of DIFF marks either the first bit
- that is different, or the end of the significant data.
- Shifting left now will bring the critical information into the
- top bits. */
- clz pos, diff
- lsl data1, data1, pos
- lsl data2, data2, pos
- /* But we need to zero-extend (char is unsigned) the value and then
- perform a signed 32-bit subtraction. */
- lsr data1, data1, #56
- sub result, data1, data2, lsr #56
- RET
-
-L(mutual_align):
- /* Sources are mutually aligned, but are not currently at an
- alignment boundary. Round down the addresses and then mask off
- the bytes that precede the start point. */
- bic src1, src1, #7
- bic src2, src2, #7
- add limit, limit, tmp1 /* Adjust the limit for the extra. */
- lsl tmp1, tmp1, #3 /* Bytes beyond alignment -> bits. */
- ldr data1, [src1], #8
- neg tmp1, tmp1 /* Bits to alignment -64. */
- ldr data2, [src2], #8
- mov tmp2, #~0
-#ifdef __AARCH64EB__
- /* Big-endian. Early bytes are at MSB. */
- lsl tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */
-#else
- /* Little-endian. Early bytes are at LSB. */
- lsr tmp2, tmp2, tmp1 /* Shift (tmp1 & 63). */
-#endif
- add limit_wd, limit, #7
- orr data1, data1, tmp2
- orr data2, data2, tmp2
- lsr limit_wd, limit_wd, #3
- b L(start_realigned)
-
-L(ret0):
- mov result, #0
- RET
-
- .p2align 6
-L(misaligned8):
- sub limit, limit, #1
-1:
- /* Perhaps we can do better than this. */
- ldrb data1w, [src1], #1
- ldrb data2w, [src2], #1
- subs limit, limit, #1
- ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
- b.eq 1b
- sub result, data1, data2
- RET
+ cmp data1, data2
+.Lret_eq:
+ cset result, ne
+ cneg result, result, lo
+ ret
+
+ .p2align 4
+ /* Compare up to 8 bytes. Limit is [-8..-1]. */
+.Lless8:
+ adds limit, limit, 4
+ b.lo .Lless4
+ ldr data1w, [src1], 4
+ ldr data2w, [src2], 4
+ cmp data1w, data2w
+ b.ne .Lreturn
+ sub limit, limit, 4
+.Lless4:
+ adds limit, limit, 4
+ beq .Lret_eq
+.Lbyte_loop:
+ ldrb data1w, [src1], 1
+ ldrb data2w, [src2], 1
+ subs limit, limit, 1
+ ccmp data1w, data2w, 0, ne /* NZCV = 0b0000. */
+ b.eq .Lbyte_loop
+ sub result, data1w, data2w
+ ret
+
END (memcmp)
#undef bcmp
weak_alias (memcmp, bcmp)