From: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
Clang and GCC generate an insane vectorized memset() in nolibc.c.
liburing doesn't need such a powerful memset(). Add an empty inline ASM
to prevent the compilers from over-optimizing the memset().
Just for comparison, see the following Assembly code (generated by
Clang).
Before this patch:
```
0000000000003a00 <__uring_memset>:
3a00: mov %rdi,%rax
3a03: test %rdx,%rdx
3a06: je 3b2c <__uring_memset+0x12c>
3a0c: cmp $0x8,%rdx
3a10: jae 3a19 <__uring_memset+0x19>
3a12: xor %ecx,%ecx
3a14: jmp 3b20 <__uring_memset+0x120>
3a19: movzbl %sil,%r8d
3a1d: cmp $0x20,%rdx
3a21: jae 3a2a <__uring_memset+0x2a>
3a23: xor %ecx,%ecx
3a25: jmp 3ae0 <__uring_memset+0xe0>
3a2a: mov %rdx,%rcx
3a2d: and $0xffffffffffffffe0,%rcx
3a31: movd %r8d,%xmm0
3a36: punpcklbw %xmm0,%xmm0
3a3a: pshuflw $0x0,%xmm0,%xmm0
3a3f: pshufd $0x0,%xmm0,%xmm0
3a44: lea -0x20(%rcx),%rdi
3a48: mov %rdi,%r10
3a4b: shr $0x5,%r10
3a4f: inc %r10
3a52: mov %r10d,%r9d
3a55: and $0x3,%r9d
3a59: cmp $0x60,%rdi
3a5d: jae 3a63 <__uring_memset+0x63>
3a5f: xor %edi,%edi
3a61: jmp 3aa9 <__uring_memset+0xa9>
3a63: and $0xfffffffffffffffc,%r10
3a67: xor %edi,%edi
3a69: nopl 0x0(%rax)
3a70: movdqu %xmm0,(%rax,%rdi,1)
3a75: movdqu %xmm0,0x10(%rax,%rdi,1)
3a7b: movdqu %xmm0,0x20(%rax,%rdi,1)
3a81: movdqu %xmm0,0x30(%rax,%rdi,1)
3a87: movdqu %xmm0,0x40(%rax,%rdi,1)
3a8d: movdqu %xmm0,0x50(%rax,%rdi,1)
3a93: movdqu %xmm0,0x60(%rax,%rdi,1)
3a99: movdqu %xmm0,0x70(%rax,%rdi,1)
3a9f: sub $0xffffffffffffff80,%rdi
3aa3: add $0xfffffffffffffffc,%r10
3aa7: jne 3a70 <__uring_memset+0x70>
3aa9: test %r9,%r9
3aac: je 3ad6 <__uring_memset+0xd6>
3aae: lea (%rdi,%rax,1),%r10
3ab2: add $0x10,%r10
3ab6: shl $0x5,%r9
3aba: xor %edi,%edi
3abc: nopl 0x0(%rax)
3ac0: movdqu %xmm0,-0x10(%r10,%rdi,1)
3ac7: movdqu %xmm0,(%r10,%rdi,1)
3acd: add $0x20,%rdi
3ad1: cmp %rdi,%r9
3ad4: jne 3ac0 <__uring_memset+0xc0>
3ad6: cmp %rdx,%rcx
3ad9: je 3b2c <__uring_memset+0x12c>
3adb: test $0x18,%dl
3ade: je 3b20 <__uring_memset+0x120>
3ae0: mov %rcx,%rdi
3ae3: mov %rdx,%rcx
3ae6: and $0xfffffffffffffff8,%rcx
3aea: movd %r8d,%xmm0
3aef: punpcklbw %xmm0,%xmm0
3af3: pshuflw $0x0,%xmm0,%xmm0
3af8: nopl 0x0(%rax,%rax,1)
3b00: movq %xmm0,(%rax,%rdi,1)
3b05: add $0x8,%rdi
3b09: cmp %rdi,%rcx
3b0c: jne 3b00 <__uring_memset+0x100>
3b0e: cmp %rdx,%rcx
3b11: je 3b2c <__uring_memset+0x12c>
3b13: data16 data16 data16 cs nopw 0x0(%rax,%rax,1)
3b20: mov %sil,(%rax,%rcx,1)
3b24: inc %rcx
3b27: cmp %rcx,%rdx
3b2a: jne 3b20 <__uring_memset+0x120>
3b2c: ret
3b2d: nopl (%rax)
```
After this patch:
```
0000000000003424 <__uring_memset>:
3424: mov %rdi,%rax
3427: test %rdx,%rdx
342a: je 343a <__uring_memset+0x16>
342c: xor %ecx,%ecx
342e: mov %sil,(%rax,%rcx,1)
3432: inc %rcx
3435: cmp %rcx,%rdx
3438: jne 342e <__uring_memset+0xa>
343a: ret
```
Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx>
---
src/nolibc.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/src/nolibc.c b/src/nolibc.c
index 3207e33..ac81575 100644
--- a/src/nolibc.c
+++ b/src/nolibc.c
@@ -12,9 +12,16 @@ void *__uring_memset(void *s, int c, size_t n)
size_t i;
unsigned char *p = s;
- for (i = 0; i < n; i++)
+ for (i = 0; i < n; i++) {
p[i] = (unsigned char) c;
+ /*
+ * An empty inline ASM to avoid auto-vectorization
+ * because it's too bloated for liburing.
+ */
+ __asm__ volatile ("");
+ }
+
return s;
}
--
Ammar Faizi
