Problem with -O3 and int to pointer (64bits)

[Benjamin Grange]

On 11/3/06, Andrew Haley <aph@redhat.com> wrote:
> Benjamin Grange writes:
>  > On 11/3/06, Andrew Haley <aph@redhat.com> wrote:
>  > > Benjamin Grange writes:
>  > >  > Do you see a problem in this line?:
>  > >  > (void *)  (unsigned long)(*(((unsigned int *)&a)));
>  > >
>  > > Yes, I can: it's not legal C++.  What are you really tring to do that
>  > > needs this?
>  > I'm trying to make this code which is not from me to work  with O3.
>  > In the real code, there is a list class which use  void * to save its elements.
>  > A lot of other list are derived from this one to avoid doing casting
>  > of void * to the original pointer. They all use real pointer, so no
>  > problem.
>
>  > But there is a special derived list which converts its elements
>  > into int and save the int as a void * in order to avoid doing heap
>  > allocation. Exactly like the code I have posted.
>
> If you know that the data will all fit within a void*, you can simply do
>
>   class A {
>   public:
>           A(int n) { x = n; }
>           int x;
>   };
>
>   union
>   {
>      int n;
>      void *p;
>   } u;
>
>    u.n = a.x;
>    return u.p;
>
> ... or whatever, depending on your pointer size.
>
>  > What make my code not legal C++?
>  >
>  > Will it be better if I do something like that for put():
>  > listA[i] = (void *)a.getInt();
>  > and get:
>  > return A((int)listA[i])
>
> OK, the rule is this: you cannot access an object of type A through a
> pointer that is not type compatible with A.
>
> So, if you declare:
>
>    short *p;
>    int a = 9;
>    p = (short*)&a;
>    return p;
>
> then your code will not work, and current versions of gcc will warn you:
>
>  $ gcc /mnt/zebedee/p.c -O2 -Wall
> /mnt/zebedee/p.c: In function 'main':
> /mnt/zebedee/p.c:7: warning: dereferencing type-punned pointer will break strict-aliasing rules
Ok thank you. I will use the getInt() version since it's legal C++ and
pointer size independent.
-- 
Benjamin