[Bug tree-optimization/103680] Jump threading and switch corrupts profile

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103680

--- Comment #5 from Jan Hubicka <hubicka at gcc dot gnu.org> ---
The cfgcleanup logic is consistent assuming that your profile was consistent on
the input (i.e. read from profile feedback). If you 
 1) read profile
 2) do optimization and prove that given if conditional is true
then you should also have 100% probability on the "true" edge so doing nothing
in cfgcleanup is correct.

Now of course what can happen is that you guess profile or
 1) read profile
 2) duplicate code
 3) prove if conditonal always true in one of the copy.
In this case fixing up profile locally is not possible (since it is also wrong
in the other copy), so we opt doing nothing which keeps errors sort of
contained and we need to live that profile is somethimes inconsistent.

So cfgcleanup behaviour is by design.

However if you do threading there is way to update the profile and logic for
that iis n update_bb_profile_for_threading.  If guessed profile was consistent
with the thread, it will update profile well and it will drop message to a dump
file otherwise.

Now the problem is that each time profiling code is updated the interface to
this function is lost.  I tried to get it fixed but got lost in the new code.

/* An edge originally destinating BB of COUNT has been proved to
   leave the block by TAKEN_EDGE.  Update profile of BB such that edge E can be
   redirected to destination of TAKEN_EDGE.

   This function may leave the profile inconsistent in the case TAKEN_EDGE
   frequency or count is believed to be lower than COUNT
   respectively.  */
void
update_bb_profile_for_threading (basic_block bb, 
                                 profile_count count, edge taken_edge)

So the interface is quite simple.  I have to re-read the new updating code
since I no longer recall where I got lost, but perhaps if you are familiar with
it, you can write in the update?