[Bug tree-optimization/98925] New: Extend modref to handle return slot optimization

[slyfox at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=98925

            Bug ID: 98925
           Summary: Extend modref to handle return slot optimization
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: hubicka at gcc dot gnu.org
          Reporter: slyfox at gcc dot gnu.org
                CC: rguenth at gcc dot gnu.org
  Target Milestone: ---

Moving out https://gcc.gnu.org/PR98499#c9 to a separate enhancement PR:

Right now analyze_ssa_name_flags() does not perform detailed modref analysis of
return slot optimization and falls back to conservative assumption:

    analyze_ssa_name_flags()

          /* Return slot optiomization would require bit of propagation;
             give up for now.  */
          if (gimple_call_return_slot_opt_p (call)
              && gimple_call_lhs (call) != NULL_TREE
              && TREE_ADDRESSABLE (TREE_TYPE (gimple_call_lhs (call))))
            {
              if (dump_file)
                fprintf (dump_file, "%*s  Unhandled return slot opt\n",
                         depth * 4, "");
              lattice[index].merge (0);
            }

I don't know what would be a good test for it.

If I don't overstimate modref and alias analysis   destructor should disappear
completely in the example below (from https://gcc.gnu.org/PR98499#c4):

struct string {
  char * _M_buf;
  // local store
  char _M_local_buf[16];

  __attribute__((noinline)) string() : _M_buf(_M_local_buf) {}

  ~string() {
    if (_M_buf != _M_local_buf)
      __builtin_trap();
  }

  string(const string &__str); // no copies
};

// main.cc

__attribute__((noinline)) static string dir_name() { return string(); }
class Importer {
  string base_path;

public:
  __attribute__((noinline)) Importer() : base_path (dir_name()) {}
};

int main() {
  Importer imp;
}