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| author | Wilco Dijkstra <wilco.dijkstra@arm.com> | 2023-01-11 13:52:53 +0000 |
|---|---|---|
| committer | Wilco Dijkstra <wilco.dijkstra@arm.com> | 2023-01-17 15:09:18 +0000 |
| commit | 03c8ce5000198947a4dd7b2c14e5131738fda62b (patch) | |
| tree | fc437473626b27a8a8d6004f533ad82a6ad44fc8 | |
| parent | 349e48c01e85bd96006860084e76d322e6ca02f1 (diff) | |
| download | glibc-03c8ce5000198947a4dd7b2c14e5131738fda62b.tar.gz | |
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>
| -rw-r--r-- | sysdeps/aarch64/strlen.S | 20 |
1 files changed, 12 insertions, 8 deletions
diff --git a/sysdeps/aarch64/strlen.S b/sysdeps/aarch64/strlen.S index b3c92d9dc9..133ef93342 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 |