Messages in this thread Patch in this message | | | From | Ammar Faizi <> | Subject | [RFC PATCH v3 1/4] tools/nolibc: x86-64: Use `rep movsb` for `memcpy()` and `memmove()` | Date | Sat, 2 Sep 2023 20:35:02 +0700 |
| |
Simplify memcpy() and memmove() on the x86-64 arch.
The x86-64 arch has a 'rep movsb' instruction, which can perform memcpy() using only a single instruction, given:
%rdi = destination %rsi = source %rcx = length
Additionally, it can also handle the overlapping case by setting DF=1 (backward copy), which can be used as the memmove() implementation.
Before this patch: ``` 00000000000010ab <memmove>: 10ab: 48 89 f8 mov %rdi,%rax 10ae: 31 c9 xor %ecx,%ecx 10b0: 48 39 f7 cmp %rsi,%rdi 10b3: 48 83 d1 ff adc $0xffffffffffffffff,%rcx 10b7: 48 85 d2 test %rdx,%rdx 10ba: 74 25 je 10e1 <memmove+0x36> 10bc: 48 83 c9 01 or $0x1,%rcx 10c0: 48 39 f0 cmp %rsi,%rax 10c3: 48 c7 c7 ff ff ff ff mov $0xffffffffffffffff,%rdi 10ca: 48 0f 43 fa cmovae %rdx,%rdi 10ce: 48 01 cf add %rcx,%rdi 10d1: 44 8a 04 3e mov (%rsi,%rdi,1),%r8b 10d5: 44 88 04 38 mov %r8b,(%rax,%rdi,1) 10d9: 48 01 cf add %rcx,%rdi 10dc: 48 ff ca dec %rdx 10df: 75 f0 jne 10d1 <memmove+0x26> 10e1: c3 ret
00000000000010e2 <memcpy>: 10e2: 48 89 f8 mov %rdi,%rax 10e5: 48 85 d2 test %rdx,%rdx 10e8: 74 12 je 10fc <memcpy+0x1a> 10ea: 31 c9 xor %ecx,%ecx 10ec: 40 8a 3c 0e mov (%rsi,%rcx,1),%dil 10f0: 40 88 3c 08 mov %dil,(%rax,%rcx,1) 10f4: 48 ff c1 inc %rcx 10f7: 48 39 ca cmp %rcx,%rdx 10fa: 75 f0 jne 10ec <memcpy+0xa> 10fc: c3 ret ```
After this patch: ``` // memmove is an alias for memcpy 000000000040133b <memcpy>: 40133b: 48 89 d1 mov %rdx,%rcx 40133e: 48 89 f8 mov %rdi,%rax 401341: 48 89 fa mov %rdi,%rdx 401344: 48 29 f2 sub %rsi,%rdx 401347: 48 39 ca cmp %rcx,%rdx 40134a: 72 03 jb 40134f <memcpy+0x14> 40134c: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi) 40134e: c3 ret 40134f: 48 8d 7c 0f ff lea -0x1(%rdi,%rcx,1),%rdi 401354: 48 8d 74 0e ff lea -0x1(%rsi,%rcx,1),%rsi 401359: fd std 40135a: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi) 40135c: fc cld 40135d: c3 ret ```
v3: - Make memmove as an alias for memcpy (Willy). - Make the forward copy the likely case (Alviro).
v2: - Fix the broken memmove implementation (David).
Link: https://lore.kernel.org/lkml/20230902062237.GA23141@1wt.eu Link: https://lore.kernel.org/lkml/5a821292d96a4dbc84c96ccdc6b5b666@AcuMS.aculab.com Suggested-by: David Laight <David.Laight@aculab.com> Signed-off-by: Ammar Faizi <ammarfaizi2@gnuweeb.org> --- tools/include/nolibc/arch-x86_64.h | 29 +++++++++++++++++++++++++++++ tools/include/nolibc/string.h | 4 ++++ 2 files changed, 33 insertions(+)
diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h index e5ccb926c90306b6..aece7d8951535a36 100644 --- a/tools/include/nolibc/arch-x86_64.h +++ b/tools/include/nolibc/arch-x86_64.h @@ -156,21 +156,50 @@ /* startup code */ /* * x86-64 System V ABI mandates: * 1) %rsp must be 16-byte aligned right before the function call. * 2) The deepest stack frame should be zero (the %rbp). * */ 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); + +__asm__ ( +".section .text.nolibc_memmove_memcpy\n" +".weak memmove\n" +".weak memcpy\n" +"memmove:\n" +"memcpy:\n" + "movq %rdx, %rcx\n\t" + "movq %rdi, %rax\n\t" + "movq %rdi, %rdx\n\t" + "subq %rsi, %rdx\n\t" + "cmpq %rcx, %rdx\n\t" + "jb .Lbackward_copy\n\t" + "rep movsb\n\t" + "retq\n" +".Lbackward_copy:" + "leaq -1(%rdi, %rcx, 1), %rdi\n\t" + "leaq -1(%rsi, %rcx, 1), %rsi\n\t" + "std\n\t" + "rep movsb\n\t" + "cld\n\t" + "retq\n" +); + #endif /* _NOLIBC_ARCH_X86_64_H */ diff --git a/tools/include/nolibc/string.h b/tools/include/nolibc/string.h index 0c2e06c7c4772bc6..6eca267ec6fa7177 100644 --- a/tools/include/nolibc/string.h +++ b/tools/include/nolibc/string.h @@ -32,70 +32,74 @@ void *_nolibc_memcpy_up(void *dst, const void *src, size_t len) { size_t pos = 0; while (pos < len) { ((char *)dst)[pos] = ((const char *)src)[pos]; pos++; } return dst; } static __attribute__((unused)) void *_nolibc_memcpy_down(void *dst, const void *src, size_t len) { while (len) { len--; ((char *)dst)[len] = ((const char *)src)[len]; } return dst; } +#ifndef NOLIBC_ARCH_HAS_MEMMOVE /* might be ignored by the compiler without -ffreestanding, then found as * missing. */ __attribute__((weak,unused,section(".text.nolibc_memmove"))) void *memmove(void *dst, const void *src, size_t len) { size_t dir, pos; pos = len; dir = -1; if (dst < src) { pos = -1; dir = 1; } 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 */ /* 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; } static __attribute__((unused)) char *strchr(const char *s, int c) { -- Ammar Faizi
| |