[Bug rtl-optimization/101523] Huge number of combine attempts

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101523

--- Comment #22 from Andreas Krebbel <krebbel at gcc dot gnu.org> ---
I did a git bisect which ended up pointing at this commit, somewhere between
GCC 8 and 9:

commit c4c5ad1d6d1e1e1fe7a1c2b3bb097cc269dc7306 (bad)
Author: Segher Boessenkool <segher@kernel.crashing.org>
Date:   Mon Jul 30 15:18:17 2018 +0200

    combine: Allow combining two insns to two insns

    This patch allows combine to combine two insns into two.  This helps
    in many cases, by reducing instruction path length, and also allowing
    further combinations to happen.  PR85160 is a typical example of code
    that it can improve.

    This patch does not allow such combinations if either of the original
    instructions was a simple move instruction.  In those cases combining
    the two instructions increases register pressure without improving the
    code.  With this move test register pressure does no longer increase
    noticably as far as I can tell.

    (At first I also didn't allow either of the resulting insns to be a
    move instruction.  But that is actually a very good thing to have, as
    should have been obvious).

With this command line:
cc1plus -O2 -march=z196 -fpreprocessed Q111-8.ii -quiet

before:   20s compile-time and    21846 total combine attempts
after: > 5min compile-time and 43175686 total combine attempts