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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/105142] [12 Regression] Wrong code with -O2
 since r12-2591
Date: Mon, 04 Apr 2022 11:20:18 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D105142

--- Comment #10 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #9)
> Created attachment 52745 [details]
> avoid combining the conditions
>=20
> Like this prototype which fixes the testcase and avoids combining from st=
mts
> defined in the middle BB.  It could be enhanced by only considering not
> combining from stmts with undefined overflow behavior (or flow-sensitive
> info).
>=20
> This patch is also incomplete, maybe_fold_and_comparisons and
> maybe_fold_or_comparisons is used by if-conversion, ifcombine and reassoc
> and more importantly through recursion with itself and related
> simplifications
> in gimple-fold.cc.  It's not clear where to extract the context BB from a=
nd
> I didn't follow all the flow through the various routines.  Esp.
> if-conversion
> looks susceptible, reassoc might restrict itself to conditions from the s=
ame
> BB, not sure.

surprisingly(?) the patch has no negative effect on the testsuite (OTOH it =
is
incomplete and likely test coverage for vectorizer if-conversion is broader=
).

I'm unsure whether we want to rush things for GCC 12 and to what extent
we want to allow combination from the middle-block(s) (exposing, besides
undefined overflow, other flow-sensitive info that depends on the outer
condition being short-circuiting).=