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AArch64: Optimize strnlen

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Optimize strnlen using the shrn instruction and improve the main loop.
Small strings are around 10% faster, large strings are 40% faster on
modern CPUs.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
(cherry picked from commit ad098893ba3c3344a5f2f6ab1627c47204afdb47)
This commit is contained in:
Wilco Dijkstra 2023-01-11 13:53:05 +00:00
parent e059e458b8
commit 83cb4f5026
1 changed files with 18 additions and 21 deletions
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39
sysdeps/aarch64/strnlen.S
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@ -44,19 +44,16 @@
/*
Core algorithm:
For each 16-byte chunk we calculate a 64-bit nibble mask value with four bits
per byte. We take 4 bits of every comparison byte with shift right and narrow
by 4 instruction. Since the bits in the nibble mask reflect the order in
which things occur in the original string, counting trailing zeros identifies
exactly which byte matched. */
Process the string in 16-byte aligned chunks. Compute a 64-bit mask with
four bits per byte using the shrn instruction. A count trailing zeros then
identifies the first zero byte. */
ENTRY (__strnlen)
PTR_ARG (0)
SIZE_ARG (1)
bic src, srcin, 15
cbz cntin, L(nomatch)
ld1 {vdata.16b}, [src], 16
ld1 {vdata.16b}, [src]
cmeq vhas_chr.16b, vdata.16b, 0
lsl shift, srcin, 2
shrn vend.8b, vhas_chr.8h, 4 /* 128->64 */
@ -71,36 +68,40 @@ L(finish):
csel result, cntin, result, ls
ret
L(nomatch):
mov result, cntin
ret
L(start_loop):
sub tmp, src, srcin
add tmp, tmp, 17
subs cntrem, cntin, tmp
b.ls L(nomatch)
b.lo L(nomatch)
/* Make sure that it won't overread by a 16-byte chunk */
add tmp, cntrem, 15
tbnz tmp, 4, L(loop32_2)
tbz cntrem, 4, L(loop32_2)
sub src, src, 16
.p2align 5
L(loop32):
ldr qdata, [src], 16
ldr qdata, [src, 32]!
cmeq vhas_chr.16b, vdata.16b, 0
umaxp vend.16b, vhas_chr.16b, vhas_chr.16b /* 128->64 */
fmov synd, dend
cbnz synd, L(end)
L(loop32_2):
ldr qdata, [src], 16
ldr qdata, [src, 16]
subs cntrem, cntrem, 32
cmeq vhas_chr.16b, vdata.16b, 0
b.ls L(end)
b.lo L(end_2)
umaxp vend.16b, vhas_chr.16b, vhas_chr.16b /* 128->64 */
fmov synd, dend
cbz synd, L(loop32)
L(end_2):
add src, src, 16
L(end):
shrn vend.8b, vhas_chr.8h, 4 /* 128->64 */
sub src, src, 16
mov synd, vend.d[0]
sub result, src, srcin
fmov synd, dend
#ifndef __AARCH64EB__
rbit synd, synd
#endif
@ -110,10 +111,6 @@ L(end):
csel result, cntin, result, ls
ret
L(nomatch):
mov result, cntin
ret
END (__strnlen)
libc_hidden_def (__strnlen)
weak_alias (__strnlen, strnlen)