On Jun 29, 2005, "Dave Korn" <[EMAIL PROTECTED]> wrote: > In fact, doesn't this suggest that in _most_ circumstances, *saturation* > would be the best behaviour?
If that's for user-introduced overflows, it could be useful in some cases, but I wouldn't go as far as `most'. For compiler-introduced overflows (e.g., re-association), wrap semantics enable some optimizations that you just couldn't perform otherwise. Consider the already-mentioned example: (a + b) + c if you re-group this as: a + (b + c) and `b + c' overflows, and then the addition with a underflows, that's not a problem, since modulo semantics will give you the correct result. Turning: a * b + a * c into a * (b + c) might be a problem should b + c overflow, but if you use modulo semantics, you'll always get the correct result for cases in which none of the original operations overflowed. What you must be careful to avoid, however, is deriving further assumptions from `(b + c) does not overflow'. Since it's a compiler-introduced operation in both cases, that's only valid if you can assume modulo semantics, assuming it doesn't overflow will lead you to wrong conclusions. -- Alexandre Oliva http://www.lsd.ic.unicamp.br/~oliva/ Red Hat Compiler Engineer [EMAIL PROTECTED], gcc.gnu.org} Free Software Evangelist [EMAIL PROTECTED], gnu.org}
