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| author | Danila Kutenin <danilak@google.com> | 2022-06-27 16:12:13 +0000 |
|---|---|---|
| committer | Szabolcs Nagy <szabolcs.nagy@arm.com> | 2022-07-06 09:26:20 +0100 |
| commit | 3c9980698988ef64072f1fac339b180f52792faf (patch) | |
| tree | 3c32dabb3fcbfa564647fcedd9be5c7674a30fc2 /sysdeps/aarch64/strlen.S | |
| parent | bd0b58837c7df091046e7531642f379a52e1e157 (diff) | |
| download | glibc-3c9980698988ef64072f1fac339b180f52792faf.tar.xz glibc-3c9980698988ef64072f1fac339b180f52792faf.zip | |
aarch64: Optimize string functions with shrn instruction
We found that string functions were using AND+ADDP
to find the nibble/syndrome mask but there is an easier
opportunity through `SHRN dst.8b, src.8h, 4` (shift
right every 2 bytes by 4 and narrow to 1 byte) and has
same latency on all SIMD ARMv8 targets as ADDP. There
are also possible gaps for memcmp but that's for
another patch.
We see 10-20% savings for small-mid size cases (<=128)
which are primary cases for general workloads.
Diffstat (limited to 'sysdeps/aarch64/strlen.S')
| -rw-r--r-- | sysdeps/aarch64/strlen.S | 25 |
1 files changed, 9 insertions, 16 deletions
diff --git a/sysdeps/aarch64/strlen.S b/sysdeps/aarch64/strlen.S index a2310871c2..3a5d088407 100644 --- a/sysdeps/aarch64/strlen.S +++ b/sysdeps/aarch64/strlen.S @@ -34,35 +34,29 @@ #define src x1 #define synd x2 #define tmp x3 -#define wtmp w3 #define shift x4 #define data q0 #define vdata v0 #define vhas_nul v1 -#define vrepmask v2 -#define vend v3 -#define dend d3 +#define vend v2 +#define dend d2 /* Core algorithm: - For each 16-byte chunk we calculate a 64-bit syndrome value with four bits - per byte. For even bytes, bits 0-3 are set if the relevant byte matched the - requested character or the byte is NUL. Bits 4-7 must be zero. Bits 4-7 are - set likewise for odd bytes 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. */ + 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. */ ENTRY (STRLEN) PTR_ARG (0) bic src, srcin, 15 - mov wtmp, 0xf00f ld1 {vdata.16b}, [src] - dup vrepmask.8h, wtmp cmeq vhas_nul.16b, vdata.16b, 0 lsl shift, srcin, 2 - and vhas_nul.16b, vhas_nul.16b, vrepmask.16b - addp vend.16b, vhas_nul.16b, vhas_nul.16b /* 128->64 */ + shrn vend.8b, vhas_nul.8h, 4 /* 128->64 */ fmov synd, dend lsr synd, synd, shift cbz synd, L(loop) @@ -80,8 +74,7 @@ L(loop): fmov synd, dend cbz synd, L(loop) - and vhas_nul.16b, vhas_nul.16b, vrepmask.16b - addp vend.16b, vhas_nul.16b, vhas_nul.16b /* 128->64 */ + shrn vend.8b, vhas_nul.8h, 4 /* 128->64 */ sub result, src, srcin fmov synd, dend #ifndef __AARCH64EB__ |