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

[edwintorok at gmail dot com]

------- 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?

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.

> 
> I have no opinion on the different levels of warnings (I think this case
> should be unconditionally).
> 


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