[Bug libstdc++/29286] [4.0/4.1/4.2/4.3 Regression] placement new does not change the dynamic type as it should

["mark at codesourcery dot com" <gcc-bugzilla at gcc dot gnu dot org>]

------- Comment #109 from mark at codesourcery dot com  2007-05-22 17:19 -------
Subject: Re:  [4.0/4.1/4.2/4.3 Regression] placement
 new does not change the dynamic type as it should

gdr at cs dot tamu dot edu wrote:

> Consider the following instead
> 
>    // tu-1.C
>    void f(int* p) {
>       *p = 90;
>       // ...
>       *(double *) p = 8.3748;
>    };
> 
> Is the above code invalid, independent of context?   I don't think
> you can find a wording in the standard that says it is invalid.

IMO, the standard is just not clear with respect to aliasing.  We cannot
rely upon it, except as a guide.  As I've said throughout this thread,
it doesn't make sense to try to do "close reading" of the standard for
aliasing issues because it just wasn't written with those issues in
mind, just as there are all of the famous memory model issues in C.

In any case, I consider the code above invalid.

> Indeed, consider this:
> 
>    // tu-2.C
>    void f(int*);
>    void g() {
>       union {
>         int i;
>         double d;
>       } t;
> 
>      t.i = 42;
>      f(&t);
>      cout << t.d << endl;
>    }
> 
> I believe we can all agree the definition of g is valid.

No, I do not.  And GCC historically has not; you are only allowed to use
the union for type-punning if the accesses are through the union
directly.  That was the decision we made a long time ago regarding TBAA,
and it even appears in the manual; the -fstrict-aliasing documentation says:

"The practice of reading from a different union member than the one most
recently written to (called ``type-punning'') is common.  Even with
@option{-fstrict-aliasing}, type-punning is allowed, provided the memory
is accessed through the union type.  So, the code above will work as
expected.  However, this code might not:
@smallexample
int f() @{
  a_union t;
  int* ip;
  t.d = 3.0;
  ip = &t.i;
  return *ip;
@}
@end smallexample"

The point here is that the compiler is allowed to decide that t.d does
not alias "*ip" because the latter is not a direct access through the union.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=29286