On 03/16/2012 12:16 AM, Jakub Jelinek wrote: > On Fri, Mar 16, 2012 at 12:03:08AM +0100, Bernd Schmidt wrote: >> On 03/15/2012 11:12 PM, Zdenek Dvorak wrote: >> >>> the reason unsigned integer types are prefered is that possible overflows >>> during the computation have defined semantics. With pointer types, the >>> intermediate steps of the computations could have undefined behavior, >>> possibly >>> confusing further optimizations. Is the patch with this regard? >> >> It's trying to use sizetype for pointer offset computations. As far as I >> can tell that's supposed to be an unsigned type, so it should be OK. I >> think the final POINTER_PLUS_EXPRs we make can't overflow in valid programs. > > In the IL before ivopts it shouldn't for valid programs, but what ivopts > makes out of it often would, that is why it uses unsigned integers instead.
Well, what are our rules for whether overflow on POINTER_PLUS_EXPR is defined or not? A quick search through the headers and docs doesn't turn up anything. Would there be a downside to defining it as wrapping? Can you show an example where a POINTER_PLUS_EXPR produced by ivopts would overflow? Bernd
