Re: strict aliasing: how to swap pointers

[Evan Jones <evanj@MIT.EDU>]

On Apr 30, 2008, at 4:55 , Andrew Haley wrote:
> We have no way to know, since you didn't provide us with the source of
> exchange(), and that's where the aliasing violation, if any, would
> occur.

Oops! Sorry, I meant to. The simplest implementation is a swap using a 
temporary:

void* exchange(void** ptr, void* next) {
  void* old = *ptr;
  *ptr = next;
  return old;
}

> However, the answer is almost certainly no.  I don't know why you're
> trying to do something so simple in such a difficult way.  If you really
> want to do this in standard portable C, the easiest way is a macro:

I have implementations of exchange() that either use a temporary, a 
compare-and-swap, a load-linked/store-conditional, or a mutex. There is 
some project-specific configuration to link against the appropriate 
implementation.

On Apr 30, 2008, at 12:18, Sergei Organov wrote:
> Now, here is a solution that does not break strict aliasing rules:
>
> $ cat alias.cc
> #include <cstdio>
> #include <cstring>
>
> using namespace std;
>
> void swap(void* p1, void* p2)
> {
>   void* t;
>   memcpy(&t, p1, sizeof(t));
>   memcpy(p1, p2, sizeof(t));
>   memcpy(p2, &t, sizeof(t));
> }

This seems reasonable. In my version, I would simply return t, rather 
than using the last memcpy.

What is confusing to me about the strict aliasing warning is that I 
thought GCC must conservatively assume that a void* pointer can point to 
anything, since T* is convertible to void*. Hence, it seems to me that 
casting a T** to void** should not result in a type-punning warning.

Additionally I'm confused because the warning is quite "fragile." This 
causes the warning:

void** voidptrptr = reinterpret_cast<void**>(&intptr);

This does not:

int** intptrptr = &intptr;
void** voidptrptr = reinterpret_cast<void**>(intptrptr);

To me, these appear to be equivalent.

Thanks for the help, I appreciate it.

Evan Jones

--
Evan Jones
http://evanjones.ca/