[Bug tree-optimization/78485] New: Missed scalarization in simple read-from-clobber case

[glisse at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=78485

            Bug ID: 78485
           Summary: Missed scalarization in simple read-from-clobber case
           Product: gcc
           Version: 7.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: glisse at gcc dot gnu.org
  Target Milestone: ---

#include <initializer_list>
static std::initializer_list<int> f(int i) { return { i }; }
int main() { auto c=f(-1); return 1+c.begin()[0]; }

This remains as

  D.3093 ={v} {CLOBBER};
  _1 = MEM[(const int *)&D.3093];
  _3 = _1 + 1;

instead of becoming

  _3 = SR.10_5(D) + 1;

so we don't warn about it. Ideally something like PRE/DOM would handle it in
the general case, but in this specific case, SRA would be sufficient. However,
it does not seem interested in scalarizing this variable:

! Disqualifying D.3093 - No scalar replacements to be created.

I am hoping that a minor tweak to the heuristic could let it handle this case.
We still wouldn't warn (see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60517#c18) but we would be
close...