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

[Daniel Jacobowitz]

On Sat, Mar 23, 2002 at 06:21:55AM +1100, Fergus Henderson wrote:
> 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".

At this point you're well into illegal C, I think.  Why do you believe
you can pass the same memory area as a `struct first' and a `struct
second', and legally access them, when they do not share a common
prefix?

-- 
Daniel Jacobowitz                           Carnegie Mellon University
MontaVista Software                         Debian GNU/Linux Developer