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aarch64: Improve strcmp unaligned performance 

Replace the simple byte-wise compare in the misaligned case with a
dword compare with page boundary checks in place.  For simplicity I've
chosen a 4K page boundary so that we don't have to query the actual
page size on the system.

This results in up to 3x improvement in performance in the unaligned
case on falkor and about 2.5x improvement on mustang as measured using
bench-strcmp.

	* sysdeps/aarch64/strcmp.S (misaligned8): Compare dword at a
	time whenever possible.
This commit is contained in:
Siddhesh Poyarekar 2017-12-13 18:50:27 +05:30
parent 243b63337c
commit 2bce01ebba
2 changed files with 34 additions and 2 deletions
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5
ChangeLog
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@ -1,3 +1,8 @@
2017-12-13 Siddhesh Poyarekar <siddhesh@sourceware.org>
* sysdeps/aarch64/strcmp.S (misaligned8): Compare dword at a
time whenever possible.
2017-12-12 Carlos O'Donell <carlos@redhat.com>
* elf/Makefile [$(nss-crypt)$(static-nss-crypt) == yesno]

31
sysdeps/aarch64/strcmp.S
View File

@ -72,6 +72,7 @@ L(start_realigned):
cbz syndrome, L(loop_aligned)
/* End of performance-critical section -- one 64B cache line. */
L(end):
#ifndef __AARCH64EB__
rev syndrome, syndrome
rev data1, data1
@ -145,12 +146,38 @@ L(mutual_align):
b L(start_realigned)
L(misaligned8):
/* We can do better than this. */
/* Align SRC1 to 8 bytes and then compare 8 bytes at a time, always
checking to make sure that we don't access beyond page boundary in
SRC2. */
tst src1, #7
b.eq L(loop_misaligned)
L(do_misaligned):
ldrb data1w, [src1], #1
ldrb data2w, [src2], #1
cmp data1w, #1
ccmp data1w, data2w, #0, cs /* NZCV = 0b0000. */
b.eq L(misaligned8)
b.ne L(done)
tst src1, #7
b.ne L(misaligned8)
L(loop_misaligned):
/* Test if we are within the last dword of the end of a 4K page. If
yes then jump back to the misaligned loop to copy a byte at a time. */
and tmp1, src2, #0xff8
eor tmp1, tmp1, #0xff8
cbz tmp1, L(do_misaligned)
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. */
bic has_nul, tmp1, tmp2 /* Non-zero if NUL terminator. */
orr syndrome, diff, has_nul
cbz syndrome, L(loop_misaligned)
b L(end)
L(done):
sub result, data1, data2
RET
END(strcmp)