Re: [PATCH] Fix PR26898, folding of X +- C1 CMP Y +- C2

["Joseph S. Myers" <joseph at codesourcery dot com>]

On Thu, 30 Mar 2006, Richard Guenther wrote:

> 2006-03-29  Richard Guenther  <rguenther@suse.de>
> 
> 	PR middle-end/26898
> 	* fold-const.c (fold_comparison): Fold signed comparisons
> 	of the form X +- C1 CMP Y +- C2.

This miscompiles:

int a = 0, b = __INT_MAX__ - 1;
extern void abort(void);
extern void exit(int);
int
main(void)
{
  if (a - 1 > b + 1)
    abort();
  exit(0);
}

The optimization is only valid if the two constants cancel to one of the 
same sign (as in the original "a + 1 < b + 1"; or "a + 1 < b + 2"); not if 
you end up adding two constants of the same sign, which may cause overflow 
where the original test didn't have any.  And for "a + 2 < b + 1" it's OK 
to change it to "a + 1 < b", but not to "a < b - 1", since "b - 1" may 
overflow.

-- 
Joseph S. Myers               http://www.srcf.ucam.org/~jsm28/gcc/
    jsm@polyomino.org.uk (personal mail)
    joseph@codesourcery.com (CodeSourcery mail)
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