[RFC PATCH v2 1/4] tools/nolibc: x86-64: Use `rep movsb` for `memcpy()` and `memmove()`
From: Ammar Faizi
Date: Sat Sep 02 2023 - 01:55:55 EST
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:
```
0000000000401040 <memmove>:
401040: 48 89 d1 mov %rdx,%rcx
401043: 48 89 fa mov %rdi,%rdx
401046: 48 89 f8 mov %rdi,%rax
401049: 48 29 f2 sub %rsi,%rdx
40104c: 48 39 ca cmp %rcx,%rdx
40104f: 73 0f jae 401060 <memmove+0x20>
401051: 48 8d 7c 0f ff lea -0x1(%rdi,%rcx,1),%rdi
401056: 48 8d 74 0e ff lea -0x1(%rsi,%rcx,1),%rsi
40105b: fd std
40105c: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi)
40105e: fc cld
40105f: c3 ret
401060: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi)
401062: c3 ret
0000000000401063 <memcpy>:
401063: 48 89 f8 mov %rdi,%rax
401066: 48 89 d1 mov %rdx,%rcx
401069: f3 a4 rep movsb %ds:(%rsi),%es:(%rdi)
40106b: c3 ret
```
Link: https://lore.kernel.org/lkml/5a821292d96a4dbc84c96ccdc6b5b666@xxxxxxxxxxxxxxxx
Suggested-by: David Laight <David.Laight@xxxxxxxxxx>
Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
---
tools/include/nolibc/arch-x86_64.h | 35 ++++++++++++++++++++++++++++++
tools/include/nolibc/string.h | 4 ++++
2 files changed, 39 insertions(+)
diff --git a/tools/include/nolibc/arch-x86_64.h b/tools/include/nolibc/arch-x86_64.h
index e5ccb926c90306b6..297ffa364b2312eb 100644
--- a/tools/include/nolibc/arch-x86_64.h
+++ b/tools/include/nolibc/arch-x86_64.h
@@ -156,21 +156,56 @@
/* 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\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"
+);
+
#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