[Bug tree-optimization/123518] New: gimple_make_forwarder_block should not create degenerated phis

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=123518

            Bug ID: 123518
           Summary: gimple_make_forwarder_block should not create
                    degenerated phis
           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: ---

So currently gimple_make_forwarder_block will create degenerated phis nodes in
some cases.
make_forwarder_block happens for a few different reasons.
One is creating a loop pre-header or creating a single latch of the loop
header.


E.g we go from:
  # BS_INC_1_45 = PHI <_87(18), 0(4), _87(12)>
  # .MEM_46 = PHI <.MEM_36(18), .MEM_30(4), .MEM_27(12)>

to
  # BS_INC_1_86 = PHI <_87(22), _87(12)>

This is unlike PR 123417 which is just about loop latches.