On Fri, Jan 6, 2023 at 10:43 PM Ammar Faizi wrote: > 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> Reviewed-by: Alviro Iskandar Setiawan <alviro.iskandar@xxxxxxxxxxx>
