[Bug tree-optimization/101548] New: dependence analysis fails with .MASK_STORE

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101548

            Bug ID: 101548
           Summary: dependence analysis fails with .MASK_STORE
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

We fail to vectorize (with -fno-trapping-math in case COND_ADD is not
available)

double a[1024], b[1024];

void foo ()
{
  for (int i = 0; i < 1024; ++i)
    if (b[i] < 3.)
      a[i] += b[i];
}

the loop is if-converted to

  <bb 3> [local count: 1063004409]:
  # i_13 = PHI <i_9(7), 0(15)>
  # ivtmp_6 = PHI <ivtmp_4(7), 1024(15)>
  _1 = b[i_13];
  _2 = a[i_13];
  _3 = _1 + _2;
  _24 = _1 < 3.0e+0;
  _25 = &a[i_13];
  .MASK_STORE (_25, 64B, _24, _3);
  i_9 = i_13 + 1;
  ivtmp_4 = ivtmp_6 - 1;
  if (ivtmp_4 != 0)
    goto <bb 7>; [98.99%]
  else
    goto <bb 6>; [1.01%]

but then dependence analysis fails:

(compute_affine_dependence
  ref_a: b[i_13], stmt_a: _1 = b[i_13];
  ref_b: *_25, stmt_b: .MASK_STORE (_25, 64B, _24, _3);
) -> no dependence
(compute_affine_dependence
  ref_a: a[i_13], stmt_a: _2 = a[i_13];
  ref_b: *_25, stmt_b: .MASK_STORE (_25, 64B, _24, _3);
) -> dependence analysis failed
t3.c:7:12: missed:   versioning for alias required: can't determine dependence
between a[i_13] and *_25
consider run-time aliasing test between a[i_13] and *_25

and then

t3.c:5:21: note:   === vect_prune_runtime_alias_test_list ===
t3.c:5:21: note:   can tell at compile time that a[i_13] and *_25 alias
t3.c:7:12: missed:   not vectorized: compilation time alias: _2 = a[i_13];
.MASK_STORE (_25, 64B, _24, _3);

and we give up.  But the may-dependence does not prevent vectorization.