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

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

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

--- Comment #9 from Segher Boessenkool <segher at gcc dot gnu.org> ---
Yeah.

Without a testcase we do not know what is going on.  Likely it is a testcase
with some very big basic block, which naturally gives very many combination
opportunities: the problem by nature is at least quadratic.  There are various
ways to limit the work done for this, all amounting to "just give up if the
problem is too big", just like we do in many other places.

It also is interesting to see when this started happening.  One of the external
PRs indicated this has happened for some years already -- so notably this is
not a regression -- but what change caused this then?  It can even be the 2-2
thing, if it started far enough back.  Or, the real reason why we need to know
when it started: possibly a bug was introduced.

In all cases, we need the testcase.

(The reason this does not happen on x86 is that so many things on x86 are
stored
in memory, and on less register-poor archs like 390 not.  Combine never does
dependencies via memory).