PlaneteCasio
/
libc
1
0
Fork 0
Code Issues 6 Pull Requests Releases Wiki Activity

strcmp.S: Improve performance for misaligned strings 

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 in glibc.
This commit is contained in:
Siddhesh Poyarekar 2018-06-29 18:08:22 +05:30 committed by Corinna Vinschen
parent 2d9f35c2cc
commit d02cc7a09d
1 changed files with 40 additions and 11 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

51
newlib/libc/machine/aarch64/strcmp.S
View File

@ -1,4 +1,4 @@
/* Copyright (c) 2012-2013, Linaro Limited
/* Copyright (c) 2012-2018, Linaro Limited
All rights reserved.
Redistribution and use in source and binary forms, with or without
@ -41,6 +41,8 @@
\f:
.endm
#define L(label) .L ## label
#define REP8_01 0x0101010101010101
#define REP8_7f 0x7f7f7f7f7f7f7f7f
#define REP8_80 0x8080808080808080
@ -69,24 +71,25 @@ def_fn strcmp p2align=6
eor tmp1, src1, src2
mov zeroones, #REP8_01
tst tmp1, #7
b.ne .Lmisaligned8
b.ne L(misaligned8)
ands tmp1, src1, #7
b.ne .Lmutual_align
b.ne L(mutual_align)
/* NUL detection works on the principle that (X - 1) & (~X) & 0x80
(=> (X - 1) & ~(X | 0x7f)) is non-zero iff a byte is zero, and
can be done in parallel across the entire word. */
.Lloop_aligned:
L(loop_aligned):
ldr data1, [src1], #8
ldr data2, [src2], #8
.Lstart_realigned:
L(start_realigned):
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, .Lloop_aligned
cbz syndrome, L(loop_aligned)
/* End of performance-critical section -- one 64B cache line. */
L(end):
#ifndef __AARCH64EB__
rev syndrome, syndrome
rev data1, data1
@ -137,7 +140,7 @@ def_fn strcmp p2align=6
ret
#endif
.Lmutual_align:
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 preceed the start point. */
@ -157,15 +160,41 @@ def_fn strcmp p2align=6
#endif
orr data1, data1, tmp2
orr data2, data2, tmp2
b .Lstart_realigned
b L(start_realigned)
.Lmisaligned8:
/* We can do better than this. */
L(misaligned8):
/* 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 .Lmisaligned8
b.ne L(done)
tst src1, #7
b.ne L(do_misaligned)
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
.size strcmp, .-strcmp