[Bug c/40141] New: accessing aliased __m128 miscompiles

["kretz at kde dot org" <gcc-bugzilla at gcc dot gnu dot org>]

Testcase:

#include <emmintrin.h>
#include <stdio.h>

typedef float floatA __attribute__((__may_alias__));

int main()
{
    __m128 x = _mm_setzero_ps();
    int i;
    for (i = 0; i < 4; ++i) {
        const float xx = ((floatA*)&x)[i];
        if (xx != 0.f) {
            printf("%d: %f\n", i, xx);
            return -1;
        }
    }
    return 0;
}

This fails with -O2, as well as with -O2 -fno-strict-aliasing. It works with
-O1 and -O0.

The generated assembly shows that the stack is created but not initialized and
then read from and checked whether it's 0.

If instead of
        const float xx = ((floatA*)&x)[i];
you write
        const float xx = ((floatA*)&x)[0];
(or any other constant between 0 and 3) then the testcase doesn't fail.

Tested versions:
4.2.4: doesn't fail
4.3.2: fails
4.3.3: fails
4.4.0: doesn't fail


-- 
           Summary: accessing aliased __m128 miscompiles
           Product: gcc
           Version: 4.3.3
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: kretz at kde dot org
 GCC build triplet: x86_64-unknown-linux-gnu
  GCC host triplet: x86_64-unknown-linux-gnu
GCC target triplet: x86_64-unknown-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40141