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* [glibc/release/2.33/master] AArch64: Optimize strlen
@ 2024-04-10 16:37 Wilco Dijkstra
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From: Wilco Dijkstra @ 2024-04-10 16:37 UTC (permalink / raw)
To: glibc-cvs
https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=12784e1d82f6b55b3c7bac887107f06353c1ace0
commit 12784e1d82f6b55b3c7bac887107f06353c1ace0
Author: Wilco Dijkstra <wilco.dijkstra@arm.com>
Date: Wed Jan 11 13:52:53 2023 +0000
AArch64: Optimize strlen
Optimize strlen by unrolling the main loop. Large strings are 64% faster on
modern CPUs.
Reviewed-by: Szabolcs Nagy <szabolcs.nagy@arm.com>
(cherry picked from commit 03c8ce5000198947a4dd7b2c14e5131738fda62b)
Diff:
---
sysdeps/aarch64/strlen.S | 20 ++++++++++++--------
1 file changed, 12 insertions(+), 8 deletions(-)
diff --git a/sysdeps/aarch64/strlen.S b/sysdeps/aarch64/strlen.S
index 5af97de091..7f50073ef1 100644
--- a/sysdeps/aarch64/strlen.S
+++ b/sysdeps/aarch64/strlen.S
@@ -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
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