[Bug target/62041] New: vector fneg codegen uses a subtract instead of an xor (x86-64)

["spatel at rotateright dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62041

            Bug ID: 62041
           Summary: vector fneg codegen uses a subtract instead of an xor
                    (x86-64)
           Product: gcc
           Version: 4.9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: spatel at rotateright dot com

$ cat fneg.c
#include <xmmintrin.h>

__m128 fneg4(__m128 x) {
    return _mm_sub_ps(_mm_set1_ps(-0.0), x);
}

$ ~gcc49/local/bin/gcc -march=core-avx2 -O2 -S fneg.c -o - 
...
_fneg4:
LFB513:
    vmovaps    LC0(%rip), %xmm1
    vsubps    %xmm0, %xmm1, %xmm0
    ret
...
LC0:
    .long    2147483648
    .long    2147483648
    .long    2147483648
    .long    2147483648

------------------------------------

Instead of generating 'vsubps' here, it would be better to generate 'vxorps'
because we know we're just flipping the sign bit of each element. This is what
gcc does for the scalar version of this code.

Note that there is no difference if I use -ffast-math with this testcase. With
-ffast-math enabled, we should generate the same 'xorps' code even if the
"-0.0" is "+0.0". Again, that's what the scalar codegen does, so I think this
is just a deficiency when generating vector code.

I can file the -ffast-math case as a separate bug if that would be better.