Date: Sun, 30 Nov 2025 01:45:48 +0000 From: Robert Clausecker <fuz@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-branches@FreeBSD.org Cc: Strahinja =?utf-8?Q?Stani=C5=A1?==?utf-8?Q?i=C4=87?= <strajabot@FreeBSD.org> Subject: git: 514677bd33fb - stable/15 - libc: scalar memchr() in RISC-V assembly Message-ID: <692ba1cc.29374.13d96c2b@gitrepo.freebsd.org>
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The branch stable/15 has been updated by fuz: URL: https://cgit.FreeBSD.org/src/commit/?id=514677bd33fb8f9d99d0f4f51be0efb4bcd86583 commit 514677bd33fb8f9d99d0f4f51be0efb4bcd86583 Author: Strahinja Stanišić <strajabot@FreeBSD.org> AuthorDate: 2024-07-17 11:19:52 +0000 Commit: Robert Clausecker <fuz@FreeBSD.org> CommitDate: 2025-11-30 00:43:05 +0000 libc: scalar memchr() in RISC-V assembly Added an optimized memchr() implementation in RISC-V assembly and updated the relevant manpage. │ memchr_baseline │ memchr_scalar │ │ sec/op │ sec/op vs base │ Short 636.6µ ± 1% 495.9µ ± 1% -22.10% (p=0.000 n=20) Mid 279.7µ ± 1% 224.1µ ± 1% -19.87% (p=0.000 n=20) Long 138.8µ ± 0% 124.9µ ± 0% -10.00% (p=0.000 n=20) geomean 291.3µ 240.3µ -17.48% │ memchr_baseline │ memchr_scalar │ │ B/s │ B/s vs base │ Short 187.3Mi ± 1% 240.4Mi ± 1% +28.37% (p=0.000 n=20) Mid 426.2Mi ± 1% 531.9Mi ± 1% +24.79% (p=0.000 n=20) Long 859.0Mi ± 0% 954.4Mi ± 0% +11.11% (p=0.000 n=20) geomean 409.3Mi 496.0Mi +21.19% MFC after: 1 month MFC to: stable/15 Approved by: mhorne, markj (mentor) Reviewed by: fuz Sponsored by: Google LLC (GSoC 2024) Differential Revision: https://reviews.freebsd.org/D46023 (cherry picked from commit 563efdd3bd5d5f94e356444bb64fd66e13dda5e1) --- lib/libc/riscv/string/Makefile.inc | 1 + lib/libc/riscv/string/memchr.S | 188 +++++++++++++++++++++++++++++++++++++ 2 files changed, 189 insertions(+) diff --git a/lib/libc/riscv/string/Makefile.inc b/lib/libc/riscv/string/Makefile.inc index a9cf8bf52481..cdbc19d286fc 100644 --- a/lib/libc/riscv/string/Makefile.inc +++ b/lib/libc/riscv/string/Makefile.inc @@ -1,2 +1,3 @@ MDSRCS+= \ + memchr.S \ strrchr.S diff --git a/lib/libc/riscv/string/memchr.S b/lib/libc/riscv/string/memchr.S new file mode 100644 index 000000000000..e6e04bfae96a --- /dev/null +++ b/lib/libc/riscv/string/memchr.S @@ -0,0 +1,188 @@ +/*- + * SPDX-License-Identifier: BSD-2-Clause + * + * Copyright (c) 2024 Strahinja Stanisic <strajabot@FreeBSD.org> + */ + +#include <machine/asm.h> + +/* + * a0 - const void *b + * a1 - int c + * a2 - size_t len + */ +ENTRY(memchr) + /* + * a0 - const char *ptr + * a1 - char cccccccc[8] + * a2 - char iter[8] + * a3 - uint8_t *end + * a4 - uint64_t *end_align + * a5 - uint64_t *end_unroll + */ + + beqz a2, .Lno_match + + /* c = (uint8_t) c */ + andi a1, a1, 0xFF + + /* + * t0 = 0x0101010101010101 + * t1 = 0x8080808080808080 + * t2 = b << 3 + * cccccccc = (uint8_t)c * t0 + * end = b + len; + * ptr = b & ~0b111 + */ + add a3, a0, a2 + li t0, 0x01010101 + sltu t2, a0, a3 + slli t1, t0, 32 + neg t2, t2 + or t0, t0, t1 + and a3, a3, t2 + slli t1, t0, 7 + slli t2, a0, 3 + and a0, a0, ~0b111 + mul a1, t0, a1 + + ld a2, (a0) + + /* + * mask_start = REP8_0x01 ^ (REP8_0x01 << t2) + * iter = iter ^ cccccccc + * iter = iter | mask_start + */ + sll t2, t0, t2 + xor a2, a2, a1 + xor t2, t2, t0 + or a2, a2, t2 + + /* has_zero(iter) + * end_align = (end + 7) & ~0b111; + */ + addi a4, a3, 7 + not t2, a2 + sub a2, a2, t0 + and t2, t2, t1 + andi a4, a4, ~0b111 + and a2, a2, t2 + + /* ptr = ptr + 8 */ + addi a0, a0, 8 + + bnez a2, .Lfind_zero + + /* if(ptr == end_align) */ + beq a0, a4, .Lno_match + + /* end_unroll = end_align & ~0b1111 */ + andi a5, a4, ~0b1111 + + /* + * Instead of branching to check if `ptr` is 16-byte aligned: + * - Probe the next 8 bytes for `c` + * - Align `ptr` down to the nearest 16-byte boundary + * + * If `ptr` was already 16-byte aligned, those 8 bytes will be + * checked again inside the unrolled loop. + * + * This removes an unpredictable branch and improves performance. + */ + + ld a2, (a0) + xor a2, a2, a1 + + not t2, a2 + sub a2, a2, t0 + and t2, t2, t1 + and a2, a2, t2 + + addi a0, a0, 8 + + bnez a2, .Lfind_zero + + andi a0, a0, ~0b1111 + + /* while(ptr != end_unroll) */ + beq a0, a5, .Lskip_loop +.Lloop: + ld a2, (a0) + ld t3, 8(a0) + + xor a2, a2, a1 + xor t3, t3, a1 + + not t2, a2 + not t4, t3 + sub a2, a2, t0 + sub t3, t3, t0 + and t2, t2, t1 + and t4, t4, t1 + and a2, a2, t2 + and t3, t3, t4 + + addi a0, a0, 8 + + bnez a2, .Lfind_zero + + /* move into iter for find_zero */ + mv a2, t3 + + addi a0, a0, 8 + + bnez a2, .Lfind_zero + + bne a0, a5, .Lloop +.Lskip_loop: + + /* there might be one 8byte left */ + beq a0, a4, .Lno_match + + ld a2, (a0) + xor a2, a2, a1 + + not t2, a2 + sub a2, a2, t0 + and t2, t2, t1 + and a2, a2, t2 + + addi a0, a0, 8 + + beqz a2, .Lno_match + +.Lfind_zero: + /* + * ptr = ptr - 8 + * t1 = 0x0001020304050607 + * iter = iter & (-iter) + * iter = iter >> 7 + * iter = iter * t1 + * iter = iter >> 56 + */ + li t1, 0x10203000 + neg t0, a2 + slli t1, t1, 4 + and a2, a2, t0 + addi t1, t1, 0x405 + srli a2, a2, 7 + slli t1, t1, 16 + addi a0, a0, -8 + addi t1, t1, 0x607 + mul a2, a2, t1 + srli a2, a2, 56 + + /* left = end - ptr */ + sub t0, a3, a0 + + /* return iter < left ? ptr + iter : NULL */ + sltu t1, a2, t0 + neg t1, t1 + add a0, a0, a2 + and a0, a0, t1 + ret + +.Lno_match: + li a0, 0 + ret +END(memchr)help
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