[Bug tree-optimization/106754] New: compute_control_dep_chain over-estimates domain

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106754

            Bug ID: 106754
           Summary: compute_control_dep_chain over-estimates domain
           Product: gcc
           Version: 13.0
            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: ---

The following testcase shows compute_control_dep_chain for init_use_preds
of the 'return u' uninitialized use compute a path with w != 0 && v != 1,
leaving out the !(i < 10) loop exit predicate because of how the iteration
terminating condition on the post-dominator walk works.  While there's a
special case for the loop exit edge itself, the following if (y) z = 1;
intermediate post-dominator will still make the iteration terminate early
before reaching if (v != 1) and eventually the target block.  The parent
iteration which entered the loop will proceed to find that but the loop
exit condition will be lost.

extern unsigned bar (void);
extern void quux (void);
int z;
unsigned foo (unsigned v, int y, int w)
{
  unsigned u;
  if (v != 1)
    u = bar ();

  // This variantion from uninit-pred-11.c causes compute_control_dep_chain
  // to run into a defect, producing z != 0 && v != 1, omitting !(i<10)
  // from the path predicate
  if (w)
    {
      for (int i = 0; i < 10; i++)
        quux ();

      if (y)
        z = 1;
      if (v != 1)
        return u;
    }

  return 0;
}