[Bug middle-end/39895] gcc-4.4 -Wstrict-aliasing and -Wstrict-aliasing=3 behaves like -Wstrict-aliasing=2 in gcc-4.3

[rguenther at suse dot de]

------- 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.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=39895