[Bug middle-end/65947] New: Vectorizer misses conditional assignment of constant

[alalaw01 at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65947

            Bug ID: 65947
           Summary: Vectorizer misses conditional assignment of constant
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: alalaw01 at gcc dot gnu.org
  Target Milestone: ---

This testcase:

int a[32];

int main(int argc, char **argv)
{
  int res = 3;
  for (int i = 0; i < 32; i++)
    if (a[i]) res = 7;
  return res;
}

does not vectorize at -O3 on x86_64 or aarch64. tree-if-conversion succeeds,
giving a loop of form:

<bb 3>:
  # res_10 = PHI <res_1(4), 3(2)>
  # i_11 = PHI <i_6(4), 0(2)>
  # ivtmp_9 = PHI <ivtmp_2(4), 32(2)>
  _5 = a[i_11];
  res_1 = _5 != 0 ? 7 : res_10;
  i_6 = i_11 + 1;
  ivtmp_2 = ivtmp_9 - 1;
  if (ivtmp_2 != 0)
    goto <bb 4>;
  else
    goto <bb 5>;

  <bb 4>:
  goto <bb 3>;

but -fdump-tree-vect-details shows:

test.c:9:3: note: Analyze phi: res_10 = PHI <res_1(4), 3(2)>
test.c:9:3: note: reduction: not commutative/associative: res_1 = _5 != 0 ? 7 :
res_10;
test.c:9:3: note: Unknown def-use cycle pattern.
...
test.c:9:3: note: vect_is_simple_use: operand res_10
test.c:9:3: note: def_stmt: res_10 = PHI <res_1(4), 3(2)>
test.c:9:3: note: Unsupported pattern.
test.c:9:3: note: not vectorized: unsupported use in stmt.
test.c:9:3: note: unexpected pattern.