AuroraMiddleware/glibc
1
0
Fork 0
mirror of https://sourceware.org/git/glibc.git synced 2024-11-25 14:30:06 +00:00
Code Issues Packages Projects Releases Wiki Activity

AArch64: Optimize strlen

Browse Source 
Optimize strlen by unrolling the main loop.  Large strings are 64% faster on
modern CPUs.

Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
This commit is contained in:
Wilco Dijkstra 2023-01-11 13:52:53 +00:00
parent 349e48c01e
commit 03c8ce5000
1 changed files with 12 additions and 8 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

20
sysdeps/aarch64/strlen.S
View File

@ -43,12 +43,9 @@
#define dend d2
/* 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 (STRLEN)
PTR_ARG (0)
@ -68,18 +65,25 @@ ENTRY (STRLEN)
.p2align 5
L(loop):
ldr data, [src, 16]!
ldr data, [src, 16]
cmeq vhas_nul.16b, vdata.16b, 0
umaxp vend.16b, vhas_nul.16b, vhas_nul.16b
fmov synd, dend
cbnz synd, L(loop_end)
ldr data, [src, 32]!
cmeq vhas_nul.16b, vdata.16b, 0
umaxp vend.16b, vhas_nul.16b, vhas_nul.16b
fmov synd, dend
cbz synd, L(loop)
sub src, src, 16
L(loop_end):
shrn vend.8b, vhas_nul.8h, 4 /* 128->64 */
sub result, src, srcin
fmov synd, dend
#ifndef __AARCH64EB__
rbit synd, synd
#endif
add result, result, 16
clz tmp, synd
add result, result, tmp, lsr 2
ret