Messages in this thread Patch in this message | | | From | Ammar Faizi <> | Subject | [RFC PATCH v1 2/5] tools/nolibc: x86-64: Use `rep stosb` for `memset()` | Date | Wed, 30 Aug 2023 20:57:23 +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: ``` 00000000000010b1 <memset>: 10b1: 48 89 f0 mov %rsi,%rax 10b4: 48 89 d1 mov %rdx,%rcx 10b7: 48 89 fa mov %rdi,%rdx 10ba: f3 aa rep stos %al,%es:(%rdi) 10bc: 48 89 d0 mov %rdx,%rax 10bf: c3 ret ```
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 c5162170a2ccdff1..42f2674ad1ecdd64 100644 --- a/tools/include/nolibc/arch-x86_64.h +++ b/tools/include/nolibc/arch-x86_64.h @@ -179,6 +179,9 @@ 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" @@ -199,6 +202,16 @@ __asm__ ( "movq %rdx, %rcx\n" "rep movsb\n" "retq\n" + +".section .text.nolibc_memset\n" +".weak memset\n" +"memset:\n" + "movq %rsi, %rax\n" + "movq %rdx, %rcx\n" + "movq %rdi, %rdx\n" + "rep stosb\n" + "movq %rdx, %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 @@ -84,6 +84,7 @@ void *memcpy(void *dst, const void *src, size_t len) } #endif /* #ifndef NOLIBC_ARCH_HAS_MEMCPY */ +#ifndef NOLIBC_ARCH_HAS_MEMSET /* might be ignored by the compiler without -ffreestanding, then found as * missing. */ @@ -99,6 +100,7 @@ void *memset(void *dst, int b, size_t len) } return dst; } +#endif /* #ifndef NOLIBC_ARCH_HAS_MEMSET */ static __attribute__((unused)) char *strchr(const char *s, int c) -- Ammar Faizi
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