On 10/31/2013 08:02 AM, David Brown wrote:
> On 30/10/13 16:56, Andrew Haley wrote:
>> On 10/30/2013 03:23 PM, David Brown wrote:
>>> I believe that's only a minor reason for making signed overflows
>>> undefined behaviour. If it were a matter of implementation, I think it
>>> would have been made "implementation defined" rather than "undefined",
>>> so that two's complement machines could use wrapping while other systems
>>> (which are very rare now) could have other defined overflow behaviour.
>>>
>>> The main reason (again, this is my guess - I don't know what the C
>>> standards folk /actually/ thought) is that there is no obvious "correct"
>>> behaviour for signed overflow
>>
>> A more important reason that having signed overflow undefined allows
>> a lot of powerful loop optimization algorithms.
>
> Didn't I mention that? I certainly meant to!
>
> There was a recent discussion on comp.lang.c about this. I don't know
> how one could (or should) grade the relative importance of reasons, but
> certainly by having signed overflow as undefined you can enable many
> nice optimisations. These don't really turn up in "normal" code
They do:
for (int i = 0; i < 5; i ++)
a(p + i * 10);
->
for (int p1 = p; p1 < p + 50; p1 += 10)
a(p1);
Andrew.
