[Bug middle-end/102997] [12 Regression] 45% 454.calculix regression with LTO+PGO -march=native -Ofast on Zen since r12-4526-gd8edfadfc7a9795b65177a50ce44fd348858e844

["hubicka at kam dot mff.cuni.cz" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102997

--- Comment #21 from hubicka at kam dot mff.cuni.cz ---
> to also allow to thread through a loop path not crossing the latch but
> at least for the issue of "breaking loops" the loops_crossed stuff shouldn't
> be necessary.  It might still prevent us from doing such transforms when
> it involves peeling.

I am not sure what is the set of tranforms we want to forbid in general.
I think main situation we want to prevent is turning single-entry loop
to irreducible loop by forwarding loop entry edges differently.

For that it should be safe to trhead if the path enters a loop and later
leaves. For that we need to keep track of loops we entered in stack and
pop them when we leave it and check that stack is empty at the end.  (or
do stackless bookeeping but it may happen that we start in one loop,
leave it and then enter different loop and stop in it which is stil
unsafe so just comparing loop nest of entry and exit block is not right
thing to do)

It seems to me that it should be also OK to thread if we thread all
entry edges to the loop (in particular if it is only one) which should
be easy to check by looking into other edges entering the path being
threaded and checking that they are from loop and not from outside
(since the outside edges will keep the old path alive and we end up with
duplicated entry point).

I may miss something obvious, but I would say that all other threads
involving loops should be fine and better informed than peeling since
threader knows that the path in question is special.