[glibc] AArch64: Optimize strnlen

[Wilco Dijkstra <wilco@sourceware.org>]

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=ad098893ba3c3344a5f2f6ab1627c47204afdb47

commit ad098893ba3c3344a5f2f6ab1627c47204afdb47
Author: Wilco Dijkstra <wilco.dijkstra@arm.com>
Date:   Wed Jan 11 13:53:05 2023 +0000

    AArch64: Optimize strnlen
    
    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>

Diff:
---
 sysdeps/aarch64/strnlen.S | 39 ++++++++++++++++++---------------------
 1 file changed, 18 insertions(+), 21 deletions(-)

diff --git a/sysdeps/aarch64/strnlen.S b/sysdeps/aarch64/strnlen.S
index 35fd14804d..21112fbf76 100644
--- a/sysdeps/aarch64/strnlen.S
+++ b/sysdeps/aarch64/strnlen.S
@@ -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)