On Fri, Jan 6, 2023 at 10:43 PM Ammar Faizi wrote: > Use a struct initializer instead of memset(). It simplifies the C code > plus effectively reduces the code size. > > Extra bonus on x86-64. It reduces the stack allocation because it > doesn't need to allocate stack for the local variable @range. It can > just use 128 bytes of redzone below the `%rsp` (redzone is only > available in a leaf function). > > Before this patch: > > ``` > 0000000000003910 <io_uring_register_file_alloc_range>: > 3910: push %rbp > 3911: push %r15 > 3913: push %r14 > 3915: push %rbx > 3916: sub $0x18,%rsp > 391a: mov %edx,%r14d > 391d: mov %esi,%ebp > 391f: mov %rdi,%rbx > 3922: lea 0x8(%rsp),%r15 > 3927: mov $0x10,%edx > 392c: mov %r15,%rdi > 392f: xor %esi,%esi > 3931: call 3a00 <__uring_memset> > 3936: mov %ebp,0x8(%rsp) > 393a: mov %r14d,0xc(%rsp) > 393f: mov 0xc4(%rbx),%edi > 3945: mov $0x1ab,%eax > 394a: mov $0x19,%esi > 394f: mov %r15,%rdx > 3952: xor %r10d,%r10d > 3955: syscall > 3957: add $0x18,%rsp > 395b: pop %rbx > 395c: pop %r14 > 395e: pop %r15 > 3960: pop %rbp > 3961: ret > 3962: cs nopw 0x0(%rax,%rax,1) > 396c: nopl 0x0(%rax) > ``` > > After this patch: > > ``` > 0000000000003910 <io_uring_register_file_alloc_range>: > 3910: mov %esi,-0x10(%rsp) # set range.off > 3914: mov %edx,-0xc(%rsp) # set range.len > 3918: movq $0x0,-0x8(%rsp) # zero the resv > 3921: mov 0xc4(%rdi),%edi > 3927: lea -0x10(%rsp),%rdx > 392c: mov $0x1ab,%eax > 3931: mov $0x19,%esi > 3936: xor %r10d,%r10d > 3939: syscall > 393b: ret > ``` > > Signed-off-by: Ammar Faizi <ammarfaizi2@xxxxxxxxxxx> Reviewed-by: Alviro Iskandar Setiawan <alviro.iskandar@xxxxxxxxxxx>
