Re: [PATCH] fix aliasing for chars in global structures - take II

[Fergus Henderson <fjh at cs dot mu dot OZ dot AU>]

On 22-Mar-2002, Dan Nicolaescu <dann@godzilla.ICS.UCI.EDU> wrote:
> Fergus Henderson <fjh@cs.mu.oz.au> writes:
>   > Consider the following program:
...
>   > Isn't that program strictly conforming C, which should output `4'?
>   > And won't your patch mis-optimize it to output something
>   > different?
> 
> I don't think so. The the references to ps1->f1 and ps2->f2 will be in
> the same alias set.

Sorry, I got the example wrong.
Let's make it

 	struct first  {  unsigned char f1; int x; };
 	struct second {  unsigned int f2; double y; };

so that they are not in the same alias set,
and change the code to something like this:
 
 	void 
 	g (struct first *ps1, struct second *ps2)
 	{
 	  ps1->f1++;
 	  ps2->f2 = 0;
 	  ps1->f1++;
 	}
 
 	int 
 	main (void)
 	{
 	  void *p = calloc (sizeof(struct first) + sizeof(struct second), 1);
 	  g (p, p);
 	  printf("%d\n", ((struct first *) p)->f1);
 	  return 0;
 	}

This one should print "1", but I suspect that with your patch it will
print either "0" or "2".

-- 
Fergus Henderson <fjh@cs.mu.oz.au>  |  "I have always known that the pursuit
The University of Melbourne         |  of excellence is a lethal habit"
WWW: <http://www.cs.mu.oz.au/~fjh>  |     -- the last words of T. S. Garp.