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From: Wilco Dijkstra <wilco@sourceware.org> To: glibc-cvs@sourceware.org Subject: [glibc] AArch64: Optimize strchr Date: Tue, 17 Jan 2023 16:35:51 +0000 (GMT) [thread overview] Message-ID: <20230117163551.9F2473858C60@sourceware.org> (raw) https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=51541a229740801882490177fa178e49264b13fb commit 51541a229740801882490177fa178e49264b13fb Author: Wilco Dijkstra <wilco.dijkstra@arm.com> Date: Wed Jan 11 13:52:08 2023 +0000 AArch64: Optimize strchr 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> Diff: --- sysdeps/aarch64/strchr.S | 52 ++++++++++++++++++++++-------------------------- 1 file changed, 24 insertions(+), 28 deletions(-) diff --git a/sysdeps/aarch64/strchr.S b/sysdeps/aarch64/strchr.S index 900ef15944..14ae1513a7 100644 --- a/sysdeps/aarch64/strchr.S +++ b/sysdeps/aarch64/strchr.S @@ -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
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