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aarch64: Optimized memcmp for medium to large sizes

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This improved memcmp provides a fast path for compares up to 16 bytes
and then compares 16 bytes at a time, thus optimizing loads from both
sources.  The glibc memcmp microbenchmark retains performance (with an
error of ~1ns) for smaller compare sizes and reduces up to 31% of
execution time for compares up to 4K on the APM Mustang.  On Qualcomm
Falkor this improves to almost 48%, i.e. it is almost 2x improvement
for sizes of 2K and above.

	* sysdeps/aarch64/memcmp.S: Widen comparison to 16 bytes at a
	time.
This commit is contained in:
Siddhesh Poyarekar 2018-03-06 19:22:39 +05:30
parent adc95fb06a
commit 30a81dae5b
2 changed files with 58 additions and 21 deletions
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3
ChangeLog
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@ -1,5 +1,8 @@
2018-03-06 Siddhesh Poyarekar <siddhesh@sourceware.org>
* sysdeps/aarch64/memcmp.S: Widen comparison to 16 bytes at a
time.
* benchtests/bench-strncmp.c (test_main): Remove 0 length tests.
(do_test_limit): Likewise.

76
sysdeps/aarch64/memcmp.S
View File

@ -34,9 +34,12 @@
/* Internal variables. */
#define data1 x3
#define data1w w3
#define data2 x4
#define data2w w4
#define tmp1 x5
#define data1h x4
#define data2 x5
#define data2w w5
#define data2h x6
#define tmp1 x7
#define tmp2 x8
ENTRY_ALIGN (memcmp, 6)
DELOUSE (0)
@ -46,39 +49,70 @@ ENTRY_ALIGN (memcmp, 6)
subs limit, limit, 8
b.lo L(less8)
/* 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
b.ne L(return)
subs limit, limit, 8
b.gt L(more16)
ldr data1, [src1, limit]
ldr data2, [src2, limit]
b L(return)
L(more16):
ldr data1, [src1], 8
ldr data2, [src2], 8
cmp data1, data2
bne L(return)
/* Jump directly to comparing the last 16 bytes for 32 byte (or less)
strings. */
subs limit, limit, 16
b.ls L(last_bytes)
/* We overlap loads between 0-32 bytes at either side of SRC1 when we
try to align, so limit it only to strings larger than 128 bytes. */
cmp limit, 96
b.ls L(loop8)
/* Align src1 and adjust src2 with bytes not yet done. */
and tmp1, src1, 15
add limit, limit, tmp1
sub src1, src1, tmp1
sub src2, src2, tmp1
subs limit, limit, 8
b.ls L(last_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. */
/* Loop performing 16 bytes per iteration using aligned src1.
Limit is pre-decremented by 16 and must be larger than zero.
Exit if <= 16 bytes left to do or if the data is not equal. */
.p2align 4
L(loop8):
ldr data1, [src1], 8
ldr data2, [src2], 8
subs limit, limit, 8
ccmp data1, data2, 0, hi /* NZCV = 0b0000. */
b.eq L(loop8)
L(loop16):
ldp data1, data1h, [src1], 16
ldp data2, data2h, [src2], 16
subs limit, limit, 16
ccmp data1, data2, 0, hi
ccmp data1h, data2h, 0, eq
b.eq L(loop16)
cmp data1, data2
bne L(return)
mov data1, data1h
mov data2, data2h
cmp data1, data2
bne L(return)
/* Compare last 1-8 bytes using unaligned access. */
/* Compare last 1-16 bytes using unaligned access. */
L(last_bytes):
ldr data1, [src1, limit]
ldr data2, [src2, limit]
add src1, src1, limit
add src2, src2, limit
ldp data1, data1h, [src1]
ldp data2, data2h, [src2]
cmp data1, data2
bne L(return)
mov data1, data1h
mov data2, data2h
cmp data1, data2
/* Compare data bytes and set return value to 0, -1 or 1. */
L(return):