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From: "rguenth 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 11:27:05 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D109154

--- Comment #64 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Tamar Christina from comment #63)
> > > It looks like `-fno-tree-pre` does the trick, but then of course, mes=
ses up
> > > elsewhere.  The conditional statement seem to stay in the most compli=
cated
> > > form possible in scalar code.
> > >=20
> > > I'll try to track down what to turn off and experiment with a pre2 af=
ter
> > > 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.
>=20
> 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
> feasible and better to pass on additional information from pre to ifcvt to
> indicate that the operation was created from a common block.
>=20
> In which case ifcvt could move the cond to just before the first shared
> statement?

I don't think PRE "knows" where the operation was created from since it's
transforms from a global dataflow problem solution.

Btw, what's the testcase your last examples are from?=