Re: strict aliasing: how to swap pointers

[Andrew Haley <aph at redhat dot com>]

Evan Jones wrote:
> John Love-Jensen wrote:
>> void** is not void*.
> 
> Ah, of course. So with strict aliasing, GCC assumes that a void**
> pointer only points to variables of exactly type void*? This would
> explain my confusion. I was assuming that since T* is convertible to
> void*, void** can point to any T*. It makes sense that this is not true.
> 
> I wish I could get GCC to generate a "surprising" optimization for a
> void**/T** type pun, but I have been unable to do so. This, of course,
> doesn't mean that ignoring the warning is safe.
> 
> It seems to me that the lesson for me is that when doing low-level
> type-unsafe manipulation, void* pointers should be used. Since GCC must
> assume that void* pointers can point anywhere, it will not be able to
> optimize away the accesses to them, which is what I want in this case.
> Please correct me if I am wrong about this.

gcc will look through temporaries of type void* if the information is
visible.  So, trying to defeat the type system by using void* variable as
temporaries doesnt work because gcc will simply delete any pointless
temporaries.

so, this:

  int *sp = &int_var;
  void *vp = sp;
  *(short*)vp = 22;

is undefined behaviour, and may well not do what you want. 

Andrew.