------- Comment #3 from pinskia at gmail dot com 2009-07-07 03:44 -------
Subject: Re: New: dereferencing type-punned pointer warnings cannot be
disabled
Sent from my iPhone
On Jul 6, 2009, at 6:12 PM, "mikulas at artax dot karlin dot mff dot
cuni dot cz" <gcc-bugzi...@gcc.gnu.org> wrote:
> Gcc became recently (4.4) very bad regarding false positive type-
> punned
> warnings. In previous versions, the warnings could be suppressed by
> casting to
> (void *), in 3.x and 4.1 it worked perfectly, in 4.3 it still worked
> somehow
> (except in -Wstrict-aliasing=3 mode), in 4.4 there are cases where
> it doesn't
> work at all.
>
> I don't want to completely disable the warnings with -Wno-strict-
> aliasing (this
> could leave bugs unnotified), but I need a method to disable them on
> case-by-case basis once I verified that the code in question is
> correct.
>
> Simple example, compile with -O2 -Wall:
>
> extern int c;
> int a(void)
> {
> return *(short *)(void *)&c;
> }
This is a very bad example of a false positive as you are acessing an
int as a short; that is undefined. I will look at your code later on,
my laptop for home is currently broken.
>
> In 4.4 the warning can't be disabled at all! The (void *) cast
> doesn't suppress
> the warning and none of three options to -Wstrict-aliasing helps. In
> 4.3 the
> cast to (void *) suppressed the warning in -Wstrict-aliasing 1,2
> modes (and
> didn't suppress it in the default mode 3), in 4.4 the warning can't be
> suppressed at all.
>
> Gcc developers tried to made these warnings more "intelligent" with
> less false
> positives, but unfortunatelly they completely broke the method to
> disable them
> in the specific case. For me, false positives are not a major
> problem --- when
> I get a false positive, I just read the code, check it and if I
> conclude that
> it's OK, I disable the warning with (void *).
>
> But if there's no way to disable false positives, it makes the
> warnings
> completely useless.
>
>
> --
> Summary: dereferencing type-punned pointer warnings cannot
> be
> disabled
> Product: gcc
> Version: 4.4.1
> Status: UNCONFIRMED
> Severity: major
> Priority: P3
> Component: regression
> AssignedTo: unassigned at gcc dot gnu dot org
> ReportedBy: mikulas at artax dot karlin dot mff dot cuni dot cz
> GCC build triplet: i686-pc-linux-gnu
> GCC host triplet: i686-pc-linux-gnu
> GCC target triplet: i686-pc-linux-gnu
>
>
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40665
>
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40665
