[PATCH liburing v1 1/2] nolibc: Fix bloated memset due to unexpected vectorization

[Ammar Faizi <ammarfaizi2@gnuweeb.org>]

From: Ammar Faizi <ammarfaizi2@gnuweeb.org>

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@gnuweeb.org>
---
 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