Date: Sun, 30 Nov 2025 01:45:52 +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: 650ac47256ab - stable/15 - libc: scalar strlen() in RISC-V assembly Message-ID: <692ba1d0.2a65e.7e1b0d1b@gitrepo.freebsd.org>
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The branch stable/15 has been updated by fuz: URL: https://cgit.FreeBSD.org/src/commit/?id=650ac47256ab933f45a44f92900fffc85189b2b6 commit 650ac47256ab933f45a44f92900fffc85189b2b6 Author: Strahinja Stanišić <strajabot@FreeBSD.org> AuthorDate: 2024-05-17 14:23:48 +0000 Commit: Robert Clausecker <fuz@FreeBSD.org> CommitDate: 2025-11-30 00:43:05 +0000 libc: scalar strlen() in RISC-V assembly Includes a scalar implementation of strlen() for the RISC-V architecture and changes to the corresponding manpage. Performance was benchamarked using before and after: https://github.com/clausecker/strperf os: FreeBSD arch: riscv │ strlen_baseline │ strlen_scalar │ │ sec/op │ sec/op vs base │ Short 541.2µ ± 17% 401.6µ ± 0% -25.78% (p=0.000 n=21+20) Mid 249.6µ ± 3% 191.9µ ± 0% -23.13% (p=0.000 n=21+20) Long 124.6µ ± 0% 110.7µ ± 0% -11.13% (p=0.000 n=21+20) geomean 256.3µ 204.3µ -20.26% │ strlen_baseline │ strlen_scalar │ │ B/s │ B/s vs base │ Short 220.3Mi ± 14% 296.8Mi ± 0% +34.74% (p=0.000 n=21+20) Mid 477.6Mi ± 3% 621.3Mi ± 0% +30.09% (p=0.000 n=21+20) Long 956.9Mi ± 0% 1076.7Mi ± 0% +12.52% (p=0.000 n=21+20) geomean 465.2Mi 583.4Mi +25.40% 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/D45693 (cherry picked from commit e09c1583eddd345bdb79f3db8a91166f6f139207) --- lib/libc/riscv/string/Makefile.inc | 1 + lib/libc/riscv/string/strlen.S | 77 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 78 insertions(+) diff --git a/lib/libc/riscv/string/Makefile.inc b/lib/libc/riscv/string/Makefile.inc index 44aeb65bf1f7..5853ea114277 100644 --- a/lib/libc/riscv/string/Makefile.inc +++ b/lib/libc/riscv/string/Makefile.inc @@ -1,4 +1,5 @@ MDSRCS+= \ memchr.S \ memset.S \ + strlen.S \ strrchr.S diff --git a/lib/libc/riscv/string/strlen.S b/lib/libc/riscv/string/strlen.S new file mode 100644 index 000000000000..3beb160f2e6f --- /dev/null +++ b/lib/libc/riscv/string/strlen.S @@ -0,0 +1,77 @@ +/*- + * SPDX-License-Identifier: BSD-2-Clause + * + * Copyright (c) 2024 Strahinja Stanisic <strajabot@FreeBSD.org> + */ + +#include <machine/asm.h> + +/* + * https://graphics.stanford.edu/~seander/bithacks.html#ZeroInWord + * uses haszero(v) (((v) - 0x01010101UL) & ~(v) & 0x80808080UL) + * which evalutates > 0 when there is zero in v + * + * register a0 - char *s + */ +ENTRY(strlen) + /* + * register a0 - char *str_start + * register a1 - char *str_ptr + * register a2 - char[8] iter + */ + + /* load constants for haszero */ + li t0, 0x0101010101010101 + slli t1, t0, 7 # 0x8080808080808080, avoid li + + /* check alignment of str_start */ + andi a1, a0, ~0b111 + ld a2, (a1) + beq a1, a0, .Lhas_zero + + /* fill bytes before str_start with non-zero */ + slli t2, a0, 3 + addi t3, t2, -64 + neg t3, t3 + srl t3, t0, t3 + or a2, a2, t3 + + /* unrolled iteration of haszero */ + not t2, a2 + sub a2, a2, t0 + and a2, a2, t2 + and a2, a2, t1 + + bnez a2, .Lfind_zero + +.Lloop_has_zero: + ld a2, 8(a1) + addi a1, a1, 8 # move ptr to next 8byte +.Lhas_zero: + not t2, a2 + sub a2, a2, t0 + and a2, a2, t2 + and a2, a2, t1 + + beqz a2, .Lloop_has_zero + +.Lfind_zero: + /* use (iter & -iter) to isolate lowest set bit */ + sub a3, zero, a2 #a3 = -iter + and t1, a2, a3 #t1 = (iter & -iter) + + li t0, 0x0001020304050607 + srli t1, t1, 7 + /* + * lowest set bit is 2^(8*k) + * multiplying by it shifts the idx array in t0 by k bytes to the left + */ + mul t1, t1, t0 + /* highest byte contains idx of first zero */ + srli t1, t1, 56 + + add a1, a1, t1 + sub a0, a1, a0 + ret +END(strlen) +help
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