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?


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