------- Comment #3 from rguenther at suse dot de 2009-04-25 14:06 -------
Subject: Re: gcc-4.4 -Wstrict-aliasing and
-Wstrict-aliasing=3 behaves like -Wstrict-aliasing=2 in gcc-4.3
On Sat, 25 Apr 2009, edwintorok at gmail dot com wrote:
>
>
> ------- Comment #2 from edwintorok at gmail dot com 2009-04-25 13:49 -------
> (In reply to comment #1)
> > Casting through a union (2)
> >
> > describes an invalid way of doing type-punning.
>
> There is also a citation from C99 on that page:
> "An object shall have its stored value accessed only by an lvalue expression
> that has one of the following types:
>
> * a type compatible with the effective type of the object,
> * a qualified version of a type compatible with the effective type of the
> object,
> * a type that is the signed or unsigned type corresponding to the
> effective
> type of the object,
> * a type that is the signed or unsigned type corresponding to a qualified
> version of the effective type of the object,
> * an aggregate or union type that includes one of the aforementioned types
> among its members (including, recursively, a member of a subaggregate or
> contained union), or
> * a character type."
>
> I'm casting to a union that has both types as members, why doesn't that fit
> under the 5th case in the above quote?
Because it is certainly backwards.
> Also there is a warning for foo(), but there is no warning for bar(), but I
> think they are exactly the same things wrt to violating or not the aliasing
> rules.
>
> >
> > The only conforming and portable way is
> >
> > unsigned bar(char *x)
> > {
> > unsigned un;
> > memcpy (&un, x, sizeof (un));
> > return un;
> > }
>
> That may be too slow for me, I'll go with a static inline function that takes
> a
> void* instead of a macro that does the cast.
The above is properly optimized. Why do you think that an inline
function taking void * would fix anything?
Richard.
--
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39895
