[Bug tree-optimization/103990] 541.leela_r slower by 4.5-6% with PGO+LTO -Ofast -march=native in the first week of January 2022

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103990

Richard Biener <rguenth at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |ASSIGNED
   Last reconfirmed|                            |2022-01-12
     Ever confirmed|0                           |1
           Assignee|unassigned at gcc dot gnu.org      |rguenth at gcc dot gnu.org

--- Comment #1 from Richard Biener <rguenth at gcc dot gnu.org> ---
OK, so the only effect I can think of is that simple_dce_from_worklist can end
up removing the last stmt in a BB and thus _eventually_ expose BB merging CFG
cleanup opportunities.  I also notice that while tail_merge_optimize altered
todo by clearing TODO_cleanup_cfg, PRE just did (and still does)

-  todo |= tail_merge_optimize (todo);
+  todo |= tail_merge_optimize (todo, need_crit_edge_split);

so it would have retained TODO_cleanup_cfg, something we now do not.  The
code is all somewhat of a mess due to the embedded tail-merge and I tried
to do as little changes as possible this late in the cycle.

I'll try to reproduce and see if keeping TODO_cleanup_cfg around helps.