https://gcc.gnu.org/bugzilla/show_bug.cgi?id=96062
--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Joseph C. Sible from comment #0)
> Consider this C code:
>
> long ps4_syscall0(long n) {
> long ret;
> int carry;
> __asm__ __volatile__(
> "syscall"
> : "=a"(ret), "=@ccc"(carry)
> : "a"(n)
> : "rcx", "r8", "r9", "r10", "r11", "memory"
> );
> return carry ? -ret : ret;
> }
>
> With "-O3", it results in this assembly:
>
> ps4_syscall0:
> movq %rdi, %rax
> syscall
> setc %dl
> movq %rax, %rdi
> movzbl %dl, %edx
> negq %rdi
> testl %edx, %edx
> cmovne %rdi, %rax
> ret
>
> On modern Intel CPUs, doing "setc %dl" creates a false dependency on rdx.
> Doing "movzbl %dl, %edx" doesn't do anything to fix that. Here's some ways
> that we could improve this code, without having to fall back to a
> conditional branch:
>
> 1. Get rid of "movzbl %dl, %edx" (since it doesn't help), and then do "testb
> %dl, %dl" instead of "testl %edx, %edx".
Just declare "_Bool carry". There is no need for int.
