http://gcc.gnu.org/bugzilla/show_bug.cgi?id=57156
--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> 2013-05-03
12:27:05 UTC ---
(In reply to comment #5)
> (In reply to comment #4)
> > I wouldn't know how to counter this for the _mm_cmpeq_epi8 case
>
> Actually, I have yet to find something in the standard that says using an
> uninitialized __m128 is undefined behavior. ;)
> Thus the only standard that would define the rules here would be Intel's, no?
> And from reading their docs I understand that calling an intrinsics maps to
> the
> behavior you'd get when using the associated instruction. In this case
> comparing any XMM register with itself will result in what I expect.
The standard says that using uninitialized variables invoke undefined
behavior. That doesn't magically exclude types that are not explicitely
covered by the standard. Doing
int a;
_Bool true = a == a;
is undefined as well, so is
int a;
int zero = a ^ a;
You cannot translate optimized assembler back to C in this way.
Well, you can. Sth like
register int a asm("eax");
int zero = a ^ a;
should do it.
