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From: "dmalcolm at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug target/107565] [12/13 Regression]
 -Wanalyzer-use-of-uninitialized-value false positive with rdrand
Date: Tue, 08 Nov 2022 14:06:54 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D107565

--- Comment #2 from David Malcolm <dmalcolm at gcc dot gnu.org> ---
(In reply to Andrew Pinski from comment #1)
> Confirmed.
>       else if (!fndecl_has_gimple_body_p (callee_fndecl)
>                && (!(callee_fndecl_flags & (ECF_CONST | ECF_PURE)))
>                && !fndecl_built_in_p (callee_fndecl))
>         unknown_side_effects =3D true;
>=20
> The last part is part of the problem I think. At least here.

I think the problem is here, in the analyzer: I think the analyzer is here
making the assumption that builtins that haven't been explicitly handled do=
n't
have side effects (such as writing through the input pointers), which is
clearly wrong for this builtin.


> Yes maybe we should have another builtin which returns a "_Complex unsign=
ed
> long long" here which is folded into for __builtin_ia32_rdrand*_step to
> remove the need to the address too.
>=20
> I am going to declare this one as a target issue but there might be other
> builtins which are harder to do the "_Complex" trick.

Andrew: I see you've marked this as a target missed-optimization bug, but
arguably there's still an analyzer bug here.  Should we reassign this back =
to
the analyzer, or perhaps make a clone of the bug so that we can cover the t=
wo
aspects of this separately?=