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aarch64: Improve strncmp for mutually misaligned inputs

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The mutually misaligned inputs on aarch64 are compared with a simple
byte copy, which is not very efficient.  Enhance the comparison
similar to strcmp by loading a double-word at a time.  The peak
performance improvement (i.e. 4k maxlen comparisons) due to this on
the strncmp microbenchmark is as follows:

falkor: 3.5x (up to 72% time reduction)
cortex-a73: 3.5x (up to 71% time reduction)
cortex-a53: 3.5x (up to 71% time reduction)

All mutually misaligned inputs from 16 bytes maxlen onwards show
upwards of 15% improvement and there is no measurable effect on the
performance of aligned/mutually aligned inputs.

	* sysdeps/aarch64/strncmp.S (count): New macro.
	(strncmp): Store misaligned length in SRC1 in COUNT.
	(mutual_align): Adjust.
	(misaligned8): Load dword at a time when it is safe.
This commit is contained in:
Siddhesh Poyarekar 2018-03-13 23:57:03 +05:30
parent 2cc7bad0ae
commit 7108f1f944
2 changed files with 87 additions and 15 deletions
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7
ChangeLog
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@ -1,3 +1,10 @@
2018-03-13 Siddhesh Poyarekar <siddhesh@sourceware.org>
* sysdeps/aarch64/strncmp.S (count): New macro.
(strncmp): Store misaligned length in SRC1 in COUNT.
(mutual_align): Adjust.
(misaligned8): Load dword at a time when it is safe.
2018-03-12 Zack Weinberg <zackw@panix.com>
[BZ #1190]

95
sysdeps/aarch64/strncmp.S
View File

@ -49,6 +49,7 @@
#define limit_wd x13
#define mask x14
#define endloop x15
#define count mask
ENTRY_ALIGN_AND_PAD (strncmp, 6, 7)
DELOUSE (0)
@ -58,9 +59,9 @@ ENTRY_ALIGN_AND_PAD (strncmp, 6, 7)
eor tmp1, src1, src2
mov zeroones, #REP8_01
tst tmp1, #7
and count, src1, #7
b.ne L(misaligned8)
ands tmp1, src1, #7
b.ne L(mutual_align)
cbnz count, L(mutual_align)
/* Calculate the number of full and partial words -1. */
sub limit_wd, limit, #1 /* limit != 0, so no underflow. */
lsr limit_wd, limit_wd, #3 /* Convert to Dwords. */
@ -165,43 +166,107 @@ L(mutual_align):
bic src1, src1, #7
bic src2, src2, #7
ldr data1, [src1], #8
neg tmp3, tmp1, lsl #3 /* 64 - bits(bytes beyond align). */
neg tmp3, count, lsl #3 /* 64 - bits(bytes beyond align). */
ldr data2, [src2], #8
mov tmp2, #~0
sub limit_wd, limit, #1 /* limit != 0, so no underflow. */
#ifdef __AARCH64EB__
/* Big-endian. Early bytes are at MSB. */
lsl tmp2, tmp2, tmp3 /* Shift (tmp1 & 63). */
lsl tmp2, tmp2, tmp3 /* Shift (count & 63). */
#else
/* Little-endian. Early bytes are at LSB. */
lsr tmp2, tmp2, tmp3 /* Shift (tmp1 & 63). */
lsr tmp2, tmp2, tmp3 /* Shift (count & 63). */
#endif
and tmp3, limit_wd, #7
lsr limit_wd, limit_wd, #3
/* Adjust the limit. Only low 3 bits used, so overflow irrelevant. */
add limit, limit, tmp1
add tmp3, tmp3, tmp1
add limit, limit, count
add tmp3, tmp3, count
orr data1, data1, tmp2
orr data2, data2, tmp2
add limit_wd, limit_wd, tmp3, lsr #3
b L(start_realigned)
L(ret0):
mov result, #0
RET
.p2align 6
/* Don't bother with dwords for up to 16 bytes. */
L(misaligned8):
sub limit, limit, #1
1:
cmp limit, #16
b.hs L(try_misaligned_words)
L(byte_loop):
/* Perhaps we can do better than this. */
ldrb data1w, [src1], #1
ldrb data2w, [src2], #1
subs limit, limit, #1
ccmp data1w, #1, #0, cs /* NZCV = 0b0000. */
ccmp data1w, #1, #0, hi /* NZCV = 0b0000. */
ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
b.eq 1b
b.eq L(byte_loop)
L(done):
sub result, data1, data2
RET
/* Align the SRC1 to a dword by doing a bytewise compare and then do
the dword loop. */
L(try_misaligned_words):
mov limit_wd, limit, lsr #3
cbz count, L(do_misaligned)
neg count, count
and count, count, #7
sub limit, limit, count
mov limit_wd, limit, lsr #3
L(page_end_loop):
ldrb data1w, [src1], #1
ldrb data2w, [src2], #1
cmp data1w, #1
ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
b.ne L(done)
subs count, count, #1
b.hi L(page_end_loop)
L(do_misaligned):
/* Prepare ourselves for the next page crossing. Unlike the aligned
loop, we fetch 1 less dword because we risk crossing bounds on
SRC2. */
mov count, #8
subs limit_wd, limit_wd, #1
b.lo L(done_loop)
L(loop_misaligned):
and tmp2, src2, #0xff8
eor tmp2, tmp2, #0xff8
cbz tmp2, L(page_end_loop)
ldr data1, [src1], #8
ldr data2, [src2], #8
sub tmp1, data1, zeroones
orr tmp2, data1, #REP8_7f
eor diff, data1, data2 /* Non-zero if differences found. */
bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
ccmp diff, #0, #0, eq
b.ne L(not_limit)
subs limit_wd, limit_wd, #1
b.pl L(loop_misaligned)
L(done_loop):
/* We found a difference or a NULL before the limit was reached. */
and limit, limit, #7
cbz limit, L(not_limit)
/* Read the last word. */
sub src1, src1, 8
sub src2, src2, 8
ldr data1, [src1, limit]
ldr data2, [src2, limit]
sub tmp1, data1, zeroones
orr tmp2, data1, #REP8_7f
eor diff, data1, data2 /* Non-zero if differences found. */
bics has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
ccmp diff, #0, #0, eq
b.ne L(not_limit)
L(ret0):
mov result, #0
RET
END (strncmp)
libc_hidden_builtin_def (strncmp)