Date: Sat, 10 Apr 2021 13:58:34 GMT From: Mateusz Guzik <mjg@FreeBSD.org> To: src-committers@FreeBSD.org, dev-commits-src-all@FreeBSD.org, dev-commits-src-branches@FreeBSD.org Subject: git: c16fc9eae3ad - stable/13 - amd64: import asm strlen into libc Message-ID: <202104101358.13ADwY3a041201@gitrepo.freebsd.org>
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The branch stable/13 has been updated by mjg: URL: https://cgit.FreeBSD.org/src/commit/?id=c16fc9eae3adca98f6d12ec4f54e043db1f8902b commit c16fc9eae3adca98f6d12ec4f54e043db1f8902b Author: Mateusz Guzik <mjg@FreeBSD.org> AuthorDate: 2021-02-21 21:20:04 +0000 Commit: Mateusz Guzik <mjg@FreeBSD.org> CommitDate: 2021-04-10 13:54:12 +0000 amd64: import asm strlen into libc Reviewed by: kib Differential Revision: https://reviews.freebsd.org/D28845 (cherry picked from commit 7f06b217c53c3f5e4ac81eb11125adfb71359ac6) --- lib/libc/amd64/string/Makefile.inc | 1 + lib/libc/amd64/string/strlen.S | 81 ++++++++++++++++++++++++++++++++++++++ 2 files changed, 82 insertions(+) diff --git a/lib/libc/amd64/string/Makefile.inc b/lib/libc/amd64/string/Makefile.inc index db88ac723539..cb370bc6be1c 100644 --- a/lib/libc/amd64/string/Makefile.inc +++ b/lib/libc/amd64/string/Makefile.inc @@ -8,4 +8,5 @@ MDSRCS+= \ memset.S \ strcat.S \ strcmp.S \ + strlen.S \ stpcpy.S diff --git a/lib/libc/amd64/string/strlen.S b/lib/libc/amd64/string/strlen.S new file mode 100644 index 000000000000..1d2428e3420e --- /dev/null +++ b/lib/libc/amd64/string/strlen.S @@ -0,0 +1,81 @@ +/* + * Written by Mateusz Guzik <mjg@freebsd.org> + * Public domain. + */ + +#include <machine/asm.h> +__FBSDID("$FreeBSD$"); + +/* + * Note: this routine was written with kernel use in mind (read: no simd), + * it is only present in userspace as a temporary measure until something + * better gets imported. + */ + +#define ALIGN_TEXT .p2align 4,0x90 /* 16-byte alignment, nop filled */ + +/* + * strlen(string) + * %rdi + * + * Uses the ((x - 0x01....01) & ~x & 0x80....80) trick. + * + * 0x01....01 is replaced with 0x0 - 0x01....01 so that it can be added + * with leaq. + * + * For a description see either: + * - "Hacker's Delight" by Henry S. Warren, Jr. + * - "Optimizing subroutines in assembly language: An optimization guide for x86 platforms" + * by Agner Fog + * + * The latter contains a 32-bit variant of the same algorithm coded in assembly for i386. + */ +ENTRY(strlen) + movabsq $0xfefefefefefefeff,%r8 + movabsq $0x8080808080808080,%r9 + + movq %rdi,%r10 + movq %rdi,%rcx + testb $7,%dil + jz 2f + + /* + * Handle misaligned reads: align to 8 and fill + * the spurious bytes. + */ + andq $~7,%rdi + movq (%rdi),%r11 + shlq $3,%rcx + movq $-1,%rdx + shlq %cl,%rdx + notq %rdx + orq %rdx,%r11 + + leaq (%r11,%r8),%rcx + notq %r11 + andq %r11,%rcx + andq %r9,%rcx + jnz 3f + + /* + * Main loop. + */ + ALIGN_TEXT +1: + leaq 8(%rdi),%rdi +2: + movq (%rdi),%r11 + leaq (%r11,%r8),%rcx + notq %r11 + andq %r11,%rcx + andq %r9,%rcx + jz 1b +3: + bsfq %rcx,%rcx + shrq $3,%rcx + leaq (%rcx,%rdi),%rax + subq %r10,%rax + ret +END(strlen) + + .section .note.GNU-stack,"",%progbits
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