Messages in this thread Patch in this message | | | From | Ammar Faizi <> | Subject | [RFC PATCH v2 2/4] tools/nolibc: x86-64: Use `rep stosb` for `memset()` | Date | Sat, 2 Sep 2023 12:50:43 +0700 |
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Simplify memset() on the x86-64 arch.
The x86-64 arch has a 'rep stosb' instruction, which can perform memset() using only a single instruction, given:
%al = value (just like the second argument of memset()) %rdi = destination %rcx = length
Before this patch: ``` 00000000000010c9 <memset>: 10c9: 48 89 f8 mov %rdi,%rax 10cc: 48 85 d2 test %rdx,%rdx 10cf: 74 0e je 10df <memset+0x16> 10d1: 31 c9 xor %ecx,%ecx 10d3: 40 88 34 08 mov %sil,(%rax,%rcx,1) 10d7: 48 ff c1 inc %rcx 10da: 48 39 ca cmp %rcx,%rdx 10dd: 75 f4 jne 10d3 <memset+0xa> 10df: c3 ret ```
After this patch: ``` 0000000000001511 <memset>: 1511: 96 xchg %eax,%esi 1512: 48 89 d1 mov %rdx,%rcx 1515: 57 push %rdi 1516: f3 aa rep stos %al,%es:(%rdi) 1518: 58 pop %rax 1519: c3 ret ```
v2 code size shrink: - Use pushq %rdi / popq %rax (Alviro). - Use xchg %eax, %esi (Willy).
Link: https://lore.kernel.org/lkml/ZO9e6h2jjVIMpBJP@1wt.eu Suggested-by: Alviro Iskandar Setiawan <alviro.iskandar@gnuweeb.org> Suggested-by: Willy Tarreau <w@1wt.eu> Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org> --- tools/include/nolibc/arch-x86_64.h | 13 +++++++++++++ tools/include/nolibc/string.h | 2 ++ 2 files changed, 15 insertions(+)
diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h index 297ffa364b2312eb..ed01b130e25c6740 100644 --- a/tools/include/nolibc/arch-x86_64.h +++ b/tools/include/nolibc/arch-x86_64.h @@ -162,50 +162,63 @@ * */ void __attribute__((weak, noreturn, optimize("Os", "omit-frame-pointer"))) __no_stack_protector _start(void) { __asm__ volatile ( "xor %ebp, %ebp\n" /* zero the stack frame */ "mov %rsp, %rdi\n" /* save stack pointer to %rdi, as arg1 of _start_c */ "and $-16, %rsp\n" /* %rsp must be 16-byte aligned before call */ "call _start_c\n" /* transfer to c runtime */ "hlt\n" /* ensure it does not return */ ); __builtin_unreachable(); } #define NOLIBC_ARCH_HAS_MEMMOVE void *memmove(void *dst, const void *src, size_t len); #define NOLIBC_ARCH_HAS_MEMCPY void *memcpy(void *dst, const void *src, size_t len); +#define NOLIBC_ARCH_HAS_MEMSET +void *memset(void *dst, int c, size_t len); + __asm__ ( ".section .text.nolibc_memmove\n" ".weak memmove\n" "memmove:\n" "movq %rdx, %rcx\n" "movq %rdi, %rdx\n" "movq %rdi, %rax\n" "subq %rsi, %rdx\n" "cmpq %rcx, %rdx\n" "jnb .Lforward_copy\n" "leaq -1(%rdi, %rcx, 1), %rdi\n" "leaq -1(%rsi, %rcx, 1), %rsi\n" "std\n" "rep movsb\n" "cld\n" "retq\n" ".Lforward_copy:\n" "rep movsb\n" "retq\n" ".section .text.nolibc_memcpy\n" ".weak memcpy\n" "memcpy:\n" "movq %rdi, %rax\n" "movq %rdx, %rcx\n" "rep movsb\n" "retq\n" + +".section .text.nolibc_memset\n" +".weak memset\n" +"memset:\n" + "xchg %eax, %esi\n" + "movq %rdx, %rcx\n" + "pushq %rdi\n" + "rep stosb\n" + "popq %rax\n" + "retq\n" ); #endif /* _NOLIBC_ARCH_X86_64_H */ diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h index 6eca267ec6fa7177..1bad6121ef8c4ab5 100644 --- a/tools/include/nolibc/string.h +++ b/tools/include/nolibc/string.h @@ -67,55 +67,57 @@ void *memmove(void *dst, const void *src, size_t len) } while (len) { pos += dir; ((char *)dst)[pos] = ((const char *)src)[pos]; len--; } return dst; } #endif /* #ifndef NOLIBC_ARCH_HAS_MEMMOVE */ #ifndef NOLIBC_ARCH_HAS_MEMCPY /* must be exported, as it's used by libgcc on ARM */ __attribute__((weak,unused,section(".text.nolibc_memcpy"))) void *memcpy(void *dst, const void *src, size_t len) { return _nolibc_memcpy_up(dst, src, len); } #endif /* #ifndef NOLIBC_ARCH_HAS_MEMCPY */ +#ifndef NOLIBC_ARCH_HAS_MEMSET /* might be ignored by the compiler without -ffreestanding, then found as * missing. */ __attribute__((weak,unused,section(".text.nolibc_memset"))) void *memset(void *dst, int b, size_t len) { char *p = dst; while (len--) { /* prevent gcc from recognizing memset() here */ __asm__ volatile(""); *(p++) = b; } return dst; } +#endif /* #ifndef NOLIBC_ARCH_HAS_MEMSET */ static __attribute__((unused)) char *strchr(const char *s, int c) { while (*s) { if (*s == (char)c) return (char *)s; s++; } return NULL; } static __attribute__((unused)) int strcmp(const char *a, const char *b) { unsigned int c; int diff; while (!(diff = (unsigned char)*a++ - (c = (unsigned char)*b++)) && c) ; -- Ammar Faizi
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