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AArch64: Optimize strchr

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Simplify calculation of the mask using shrn.  Unroll the main loop.
Small strings are 20% faster on modern CPUs.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
This commit is contained in:
Wilco Dijkstra 2023-01-11 13:52:08 +00:00
parent 1bbb1a2022
commit 51541a2297
1 changed files with 24 additions and 28 deletions
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52
sysdeps/aarch64/strchr.S
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@ -32,8 +32,7 @@
#define src x2
#define tmp1 x1
#define wtmp2 w3
#define tmp3 x3
#define tmp2 x3
#define vrepchr v0
#define vdata v1
@ -41,39 +40,30 @@
#define vhas_nul v2
#define vhas_chr v3
#define vrepmask v4
#define vrepmask2 v5
#define vend v6
#define dend d6
#define vend v5
#define dend d5
/* Core algorithm.
For each 16-byte chunk we calculate a 64-bit syndrome value with four bits
per byte. For even bytes, bits 0-1 are set if the relevant byte matched the
requested character, bits 2-3 are set if the byte is NUL (or matched), and
bits 4-7 are not used and must be zero if none of bits 0-3 are set). Odd
bytes set bits 4-7 so that adjacent bytes can be merged. Since the bits
in the syndrome reflect the order in which things occur in the original
string, counting trailing zeros identifies exactly which byte matched. */
per byte. Bits 0-1 are set if the relevant byte matched the requested
character, bits 2-3 are set if the byte is NUL or matched. Count trailing
zeroes gives the position of the matching byte if it is a multiple of 4.
If it is not a multiple of 4, there was no match. */
ENTRY (strchr)
PTR_ARG (0)
bic src, srcin, 15
dup vrepchr.16b, chrin
ld1 {vdata.16b}, [src]
mov wtmp2, 0x3003
dup vrepmask.8h, wtmp2
movi vrepmask.16b, 0x33
cmeq vhas_nul.16b, vdata.16b, 0
cmeq vhas_chr.16b, vdata.16b, vrepchr.16b
mov wtmp2, 0xf00f
dup vrepmask2.8h, wtmp2
bit vhas_nul.16b, vhas_chr.16b, vrepmask.16b
and vhas_nul.16b, vhas_nul.16b, vrepmask2.16b
lsl tmp3, srcin, 2
addp vend.16b, vhas_nul.16b, vhas_nul.16b /* 128->64 */
lsl tmp2, srcin, 2
shrn vend.8b, vhas_nul.8h, 4 /* 128->64 */
fmov tmp1, dend
lsr tmp1, tmp1, tmp3
lsr tmp1, tmp1, tmp2
cbz tmp1, L(loop)
rbit tmp1, tmp1
@ -87,28 +77,34 @@ ENTRY (strchr)
.p2align 4
L(loop):
ldr qdata, [src, 16]!
ldr qdata, [src, 16]
cmeq vhas_chr.16b, vdata.16b, vrepchr.16b
cmhs vhas_nul.16b, vhas_chr.16b, vdata.16b
umaxp vend.16b, vhas_nul.16b, vhas_nul.16b
fmov tmp1, dend
cbnz tmp1, L(end)
ldr qdata, [src, 32]!
cmeq vhas_chr.16b, vdata.16b, vrepchr.16b
cmhs vhas_nul.16b, vhas_chr.16b, vdata.16b
umaxp vend.16b, vhas_nul.16b, vhas_nul.16b
fmov tmp1, dend
cbz tmp1, L(loop)
sub src, src, 16
L(end):
#ifdef __AARCH64EB__
bif vhas_nul.16b, vhas_chr.16b, vrepmask.16b
and vhas_nul.16b, vhas_nul.16b, vrepmask2.16b
addp vend.16b, vhas_nul.16b, vhas_nul.16b /* 128->64 */
shrn vend.8b, vhas_nul.8h, 4 /* 128->64 */
fmov tmp1, dend
#else
bit vhas_nul.16b, vhas_chr.16b, vrepmask.16b
and vhas_nul.16b, vhas_nul.16b, vrepmask2.16b
addp vend.16b, vhas_nul.16b, vhas_nul.16b /* 128->64 */
shrn vend.8b, vhas_nul.8h, 4 /* 128->64 */
fmov tmp1, dend
rbit tmp1, tmp1
#endif
add src, src, 16
clz tmp1, tmp1
/* Tmp1 is an even multiple of 2 if the target character was
found first. Otherwise we've found the end of string. */
/* Tmp1 is a multiple of 4 if the target character was found. */
tst tmp1, 2
add result, src, tmp1, lsr 2
csel result, result, xzr, eq