Re: [PATCH] Fix the one entry mem{{,p}cpy,move,set} optimization aliasing issues (PR middle-end/29272)

[Paolo Bonzini <paolo dot bonzini at lu dot unisi dot ch>]

> Actually wait a minute a second time, these testcases are all invalid
> because other reasons.  We are using memory after they have been
> "dealloced"  Richard, do you have an example which does not have that
> issue because it seems like they are all invalid.

How in the world can the C compiler know what is "dealloc"-ing?  There 
is no such thing as __attribute__ ((free)) and even if it was there, 
it's not attached to dealloc_one.  As far as the compiler is concerned, 
the code is just playing with a linked list.

The only way in which freeing affects aliasing, is that malloc will 
return something that does not alias anything, and thus overwrites of 
the malloc-ed object via a dangling pointer will be missed by the 
aliasing machine.  That is:

  free (a);
  b = malloc (10); /* suppose b = a */
  b->x = 3;
  a->x = 5;        /* dangling pointer, but not invalid code */
  return b->x;     /* compiler may return 3 */

Paolo