[Bug target/49133] New: [4.6 Regression] modification of aliased __m128d miscompiles

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

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49133

           Summary: [4.6 Regression] modification of aliased __m128d
                    miscompiles
           Product: gcc
           Version: 4.6.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: kretz@kde.org


Compile the following testcase with "g++ -msse2 -O2"

#include <xmmintrin.h>

typedef double double_a __attribute__((__may_alias__));

struct V
{
    __m128d data;
};

int main()
{
    V a;
    __m128d b;
    b      = _mm_set_pd(1., 0.);
    a.data = _mm_set_pd(1., 0.);
    a.data = _mm_add_pd(a.data, _mm_and_pd(_mm_cmpeq_pd(a.data,
_mm_set1_pd(0.)), _mm_set1_pd(2.)));
    reinterpret_cast<double_a *>(&a.data)[1] += 1.;
    b = _mm_add_pd(b, _mm_and_pd(_mm_cmpeq_pd(b, _mm_set1_pd(0.)),
_mm_set1_pd(1.)));
    b = _mm_add_pd(b, _mm_and_pd(_mm_cmpeq_pd(b, _mm_set1_pd(1.)),
_mm_set1_pd(1.)));
    if (_mm_movemask_pd(_mm_cmpeq_pd(a.data, b)) != 0x3) {
        abort();
    }
    return 0;
}

GCC 4.6.[01] calculate the correct values for a.data[0] and a.data[1] but fail
to combine the results correctly. I.e. the resulting shufpd $0x1 is wrong. GCC
4.5.x uses unpacklpd, which gives the correct result, but emits unnecessary
stores to the stack.