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From: "rguenth at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/106902] [11/12/13/14 Regression] Program compiled with
 -O3 -mfma produces different result
Date: Fri, 12 May 2023 06:27:26 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D106902

--- Comment #21 from Richard Biener <rguenth at gcc dot gnu.org> ---
(In reply to Alexander Monakov from comment #20)
> I missed it the first time around, but placing PAREN_EXPR around the
> complete expression won't work: nothing will prevent GCC from duplicating
> evaluations of the sub-expressions, and then randomly forming FMAs like
> here. It would just bury this class of bugs deeper.

Hmm, true.

> Now that we are in stage1, can we make some kind of progress here? Is the=
re
> any buy-in for:
>=20
> 1. Implementing fp-contract=3Don via GENERIC folding?
> 2. Defaulting to fp-contract=3Don instead of fp-contract=3Dfast under -st=
d=3Dgnu*?
> 3. Enabling fp-contract=3Dfast under -ffast-math?

Sounds reasonable.  Though I wouldn't use GENERIC folding but instead
some folding-like code in c-family/ that for example would get invoked
by genericization or via the gimplification hook?  If we'd add GENERIC
folding in fold-const.cc or match.pd the chance is that it will pick up
FMAs "late".=