On Wed, Jan 13, 2016 at 10:00:08PM -0700, Dave Johansen wrote:
> I was working on packaging rr [1] and one of the tests [2] fails to build
> when optimizations are turned on. I've reduced it to the following and
> still been able to reproduce the issue:
>
> static const float xmm0 = 10;
>
> int main() {
>   __asm__ __volatile__(
> #if __i386__
>       "movss xmm0, %xmm0\n\t"
> #elif __x86_64__
>       "movss xmm0(%rip), %xmm0\n\t"
> #else
> #error unexpected architecture
> #endif
>       );
>
>   return 0;
> }
>
> Here's the output on F23 x86_64:
> $ gcc fxregs.c -O0
> $ gcc fxregs.c -O1
> /tmp/cccdze3O.o: In function `main':
> fxregs.c:(.text+0x4): undefined reference to `xmm0'
> collect2: error: ld returned 1 exit status
>
> Is this a gcc bug or is there something that I need to do in the build to
> get the tests to build without error?

I think it's a bug in your test.  GCC is just optimizing away the xmm0
variable because it cannot see "inside" the asm to know that it is
being used.