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From: "amker at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug tree-optimization/100499] Different results with -fpeel-loops
 -ftree-loop-vectorize options
Date: Tue, 18 May 2021 11:33:37 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D100499

--- Comment #13 from bin cheng <amker at gcc dot gnu.org> ---
(In reply to Richard Biener from comment #10)
> (In reply to bin cheng from comment #9)
> > Seems we have a long standing bug in fold-const.c:multiple_of_p in case=
 of
> > wrapping types.  Take unsigned int as an example:
> >   (0xfffffffc * 0x3) % 0x3 =3D 0x1
> > But multiple_of_p returns true here.
> >=20
> > The same issue also stands for MINUS_EXPR and PLUS_EXPR.  Given
> > multiple_of_p is used elsewhere, the fix might break existing optimizat=
ions.
> > Especially, number of loop iterations is computed in unsigned types
>=20
> multiple_of_p is mostly used in contexts where overflow "cannot happen"
> (in TYPE/DECL_SIZE computation context), and in niter analysis it seems to
> be guarded similarly.  This restriction of multiple_of_p seems undocument=
ed,
Oh, I am not aware of this.  Actually my previous change to it seems broke =
this
assumption already.  Will see how to fix or revert the change.

> so fixing that might be good.
>=20
> Now, you don't say what's the chain of events that lead to a multiple_of_p
> call
> eventually leading to the wrong answer, but I guess it's the code added
> under the
>=20
> +  if (!niter->control.no_overflow
> +      && (integer_onep (s) || multiple_of_p (type, c, s)))
>=20
> check as !niter->control.no_overflow seems to suggest that the multiple_o=
f_p
> check is not properly guarded?=