[Bug testsuite/103270] [12 regression] gcc.dg/vect/pr96698.c inner loop turned from hot to cold after r12-4526

["luoxhu at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103270

--- Comment #2 from luoxhu at gcc dot gnu.org ---
(In reply to Richard Biener from comment #1)
> So you say this is a problem with loop header copying, that would mean the
> issue is really latent and general, no?  Header copying uses
> gimple_duplicate_sese_region and has no own profile updating.  I guess its
> profile updating code isn't designed to cope with copying a region with
> "side"-entries (we are ignoring the backedge here).  Not sure if we can
> somehow generally handle those (maybe we can learn from tracer or threader
> here).
> 
> Honza?

Yes, it seems to be a general issue in gimple_duplicate_sese_region, the inner
loop cfg was:

8
|
3<-- 
| \ |
5  4  

And it is modified by ch_base::copy_headers->gimple_duplicate_sese_region to(
entry edge is 8->3, exit edge is 3->4):

8
|
12
|
4<-- 
|   |
3---
|
5

bb 12 is copied block from bb 3 as new preheader, bb 3 is rotated to be new
exit of the loop, bb 3 and bb 12 are adjusted count to "total_count -
entry_count" (354334800) and "entry_count"(719407024), at last bb 3 and bb 4
will be merged to one block by gimple_merge_blocks later by TODO_cleanup_cfg
with much smaller
count than preheader.


gimple_duplicate_sese_region:

  if (total_count.initialized_p () && entry_count.initialized_p ())
    {
      scale_bbs_frequencies_profile_count (region, n_region,
                                           total_count - entry_count,
                                           total_count);
      scale_bbs_frequencies_profile_count (region_copy, n_region, entry_count,
                                           total_count);
    }


Obviously, region of bb 3's profile count shouldn't be decreased from
"total_count" to "total_count - entry_count", it executes at every execution of
the loop.  Simply adjust it back to total_count and region_copy to entry_count
will cause some other cases fail. And at the moment edge 3->4 is still not a
backedge now?