[Bug tree-optimization/123113] New: factor_out_conditional_operation could be improved to handle `EDGE_COUNT (merge->preds) != 2`

[pinskia at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123113

            Bug ID: 123113
           Summary: factor_out_conditional_operation could be improved to
                    handle `EDGE_COUNT (merge->preds) != 2`
           Product: gcc
           Version: 16.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: enhancement
          Priority: P3
         Component: tree-optimization
          Assignee: pinskia at gcc dot gnu.org
          Reporter: pinskia at gcc dot gnu.org
  Target Milestone: ---

While thinking about the recent removal of forwarder blocks, I found that
factor_out_conditional_operation does not handle  `EDGE_COUNT (merge->preds) !=
2` as it does not insert a new block. I think we should be able to do it.

Take:
```
int f(int a, int b, int c, int d)
{
  if (a)
  {
    if (b){
      c = c > d ? c : d;
      c = -c;
    }
    else {
      c = c > d ? c : d;
      c = -c;
    }
  }
  return c;
}
```

This should be optimized at -O1 but currently we don't. -O2 works because of
PRE/code hoisting.

Note this is not a regression but I am filing this for future work for GCC 17.
Since merge_phi has been in the pipeline before phiopt; this is not a
regression which was exposed by the remove forwarders patch either.