[Bug middle-end/40135] using alias-set zero for union accesses necessary because of RTL alias oracle

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40135

--- Comment #6 from Richard Biener <rguenth at gcc dot gnu.org> ---
For reference I see for example

FAIL: gcc.target/i386/avx2-vpblendd128-2.c execution test
FAIL: gcc.target/i386/avx2-vpblendd256-2.c execution test
FAIL: gcc.target/i386/avx2-vpblendw-2.c execution test
FAIL: gcc.target/i386/avx2-vpbroadcastd128-2.c execution test
FAIL: gcc.target/i386/avx2-vpbroadcastd256-2.c execution test
FAIL: gcc.target/i386/avx2-vpbroadcastw128-2.c execution test
FAIL: gcc.target/i386/avx2-vpbroadcastw256-2.c execution test
FAIL: gcc.target/i386/avx2-vpermd-2.c execution test
FAIL: gcc.target/i386/avx2-vpermps-2.c execution test
FAIL: gcc.target/i386/avx2-vpmaxsd-2.c execution test
FAIL: gcc.target/i386/avx2-vpmaxsw-2.c execution test
FAIL: gcc.target/i386/avx2-vpmaxud-2.c execution test
FAIL: gcc.target/i386/avx2-vpmaxuw-2.c execution test
FAIL: gcc.target/i386/avx2-vpminsd-2.c execution test
FAIL: gcc.target/i386/avx2-vpminsw-2.c execution test
FAIL: gcc.target/i386/avx2-vpminud-2.c execution test
FAIL: gcc.target/i386/avx2-vpminuw-2.c execution test
FAIL: gcc.target/i386/avx2-vpmuldq-2.c execution test
FAIL: gcc.target/i386/avx2-vpmulhrsw-2.c execution test
FAIL: gcc.target/i386/avx2-vpmulhuw-2.c execution test
FAIL: gcc.target/i386/avx2-vpmulhw-2.c execution test
FAIL: gcc.target/i386/avx2-vpmulld-2.c execution test
...
FAIL: gcc.target/i386/avx2-vpunpckhdq-2.c execution test
FAIL: gcc.target/i386/avx2-vpunpckhwd-2.c execution test
FAIL: gcc.target/i386/avx2-vpunpckldq-2.c execution test
FAIL: gcc.target/i386/avx2-vpunpcklwd-2.c execution test

but for example gcc.target/i386/avx2-vpblendd128-2.c contains "invalid"
type-punning through unions:

static void
init_pblendd128 (int *src1, int *src2, int seed)
{
  int i, sign = 1;

  for (i = 0; i < 4; i++)
    {
      src1[i] = (i + seed) * (i + seed) * sign;
      src2[i] = (i + seed + 20) * sign;
      sign = -sign;
    }
}
...
static void
avx2_test (void)
{
  union128i_d src1, src2, dst;
  int dst_ref[4];
  int i;

  for (i = 0; i < NUM; i++)
    {
      init_pblendd128 (src1.a, src2.a, i);

      dst.x = _mm_blend_epi32 (src1.x, src2.x, MASK);

which stores into src1/src2 via a pointer access and only reads via a direct
access to the union.

That's not how the GCC extension specifies union type-punning. I've filed
PR63211 for that.

Here the stores do not end up in alias-set zero and with removing the
c-common.c hack the loads also end up not using alias-set zero.  Without
the fix the loads use alias-set zero and thus keep the int-stores live.

There is also no easy must-alias to identify here as the stores happen
in a loop.