[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 #51 from Segher Boessenkool <segher at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #46)
> Maybe combine already knows that it just "keeps i2" rather than replacing it?

It never does that.  Instead, it thinks it is making a new I2, but it ends up
to be exactly the same instruction.  This is not a good thing to do, combine
can change the whole thing back to the previous shape for example, when it
feels like it (combine does not make canonical forms ever!)

> When !newi2pat we seem to delete i2.  Anyway, somebody more familiar with
> combine should produce a good(TM) patch.

Yes, the most common combinations delete I2, they combine 2->1 or 3->1 or 4->1.
When this isn't possible combine tries to combine to two instructions, it has
various strategies for this: the backend can do it explicitly (via a
define_split),
or it can break apart the expression that was the src in the one set that was
the ->1 result, hoping that the two instructions it gets that way are valid
insns.  It tries only one way to do this, and it isn't very smart about it,
just very heuristic.