[Bug tree-optimization/111150] New: (vec CMP vec) != (vec CMP vec) should just produce (vec CMP vec) ^ (vec CMP vec)

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=111150

            Bug ID: 111150
           Summary: (vec CMP vec) != (vec CMP vec) should just produce
                    (vec CMP vec) ^ (vec CMP vec)
           Product: gcc
           Version: 14.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: pinskia at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

Take:
```
typedef int v4si __attribute((__vector_size__(4*sizeof(int))));
v4si f2_(v4si a, v4si b, v4si c, v4si d)
{
  v4si X = a == b;
  v4si Y = c == d;
  return (X != Y);
}

```
Currently we produce on x86_64 -O2:
```
        pcmpeqd %xmm1, %xmm0
        pcmpeqd %xmm3, %xmm2
        pxor    %xmm1, %xmm1
        pcmpeqd %xmm2, %xmm0
        pcmpeqd %xmm1, %xmm0
```
But we should produce:
```
        pcmpeqd %xmm1, %xmm0
        pcmpeqd %xmm3, %xmm2
        pxor    %xmm2, %xmm0
```

In forwprop1 we have:
```
  _1 = a_4(D) == b_5(D);
  X_6 = VEC_COND_EXPR <_1, { -1, -1, -1, -1 }, { 0, 0, 0, 0 }>;
  _2 = c_7(D) == d_8(D);
  Y_9 = VEC_COND_EXPR <_2, { -1, -1, -1, -1 }, { 0, 0, 0, 0 }>;
  _3 = X_6 != Y_9;
```
But we can push != further up since -1 != 0 is true.

basically in scalar we have:
(a ? -1 : 0) != (b ? -1 : 0)

but we know that is really `(a^b) ? -1 : 0`

Now in theory you could do the full expansion for this but I think we should
just handle the case where we have -1 and 0.