https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92346
--- Comment #2 from wierton <141242068 at smail dot nju.edu.cn> ---
(In reply to Andrew Pinski from comment #1)
> This code is clearly undefined.
>
> First you say eax is in eax and then then you say the register gets
> clobbered by the inline-asm (and is set by the inline-asm).
> But you only set the lower bits of the eax to be 1 in the inline-asm.
>
> In the -O0 case, what is happening is eax is both the output register and
> the clobbered.
> In the -O2 case, the eax is not used as the output register but rather a
> different one as eax would conflict with the clobber register.
>
> You can see that effect by outputing %0 and %1 in the inline-asm template.
>
> Basically the inline-asm is broken and is not expected to work the way you
> expected it to work.
I feel so sorry for my previous manner of speaking, finding a bug in big
project such as gcc makes me a little excited. I simplified the code as
following:
```
#include <stdio.h>
int set_eax(int val) { return val; }
int main() {
int ax, tmp;
register int eax asm("eax") = set_eax(0x23232323);
asm volatile("movl $1, %%eax\n" ::: "eax");
printf("%08x\n", eax);
return 0;
}
```
This code can reproduce the phenomenon and there are no output constraints. I
do understand that the declaration %eax in eax is conflict with the inline
assembly clobber list, but in compiler such as clang, this dependency will be
detected and clang will extraly generate a move instruction to save the eax and
the resume it. I think gcc is a mostly widely used compiler, we as users expect
this compiler becomes better and better, and this feature will help improve the
usability, so I post it here. Very sorry for previous post.
The binary code generated by clang:
```
8048451: 89 45 f0 mov %eax,-0x10(%ebp) // save eax
8048454: b8 01 00 00 00 mov $0x1,%eax // inline assembly
8048459: 8d 05 00 85 04 08 lea 0x8048500,%eax
804845f: 8b 4d f0 mov -0x10(%ebp),%ecx // restore the value to
ecx
8048462: 89 04 24 mov %eax,(%esp)
8048465: 89 4c 24 04 mov %ecx,0x4(%esp)
8048469: e8 52 fe ff ff call 80482c0 <printf@plt>
```
