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From: "tnfchris at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug tree-optimization/109154] [13/14 regression] jump threading
 de-optimizes nested floating point comparisons
Date: Mon, 10 Jul 2023 11:02:45 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109154

--- Comment #63 from Tamar Christina <tnfchris at gcc dot gnu.org> ---
> > It looks like `-fno-tree-pre` does the trick, but then of course, messe=
s up
> > elsewhere.  The conditional statement seem to stay in the most complica=
ted
> > form possible in scalar code.
> >=20
> > I'll try to track down what to turn off and experiment with a pre2 after
> > vect.
> > Is before predcom a good place?
>=20
> I would avoid putting it into the loop pipeline.  Instead I'd turn the
> FRE pass that runs after tracer into PRE.  Maybe conditional on whether
> there are any loops.
>=20
> Note it's not so easy to "tame" PRE, the existing things happen at
> elimination time in eliminate_dom_walker::eliminate_stmt.  I would
> experiment with restricting the use of inserted PHIs in innermost(!)
> loops containing invariants, maybe only if the number of PHI args is
> more than two ... (but that's somewhat artificial).
>=20
> That said, I'm not really convinced this is a good idea.

I hear you.. there's also the added complexity that this likely only is
beneficial for fully masked architectures.  I wonder, if it might be feasib=
le
and better to pass on additional information from pre to ifcvt to indicate =
that
the operation was created from a common block.

In which case ifcvt could move the cond to just before the first shared
statement?=