On 31/10/13 17:51, Andrew Haley wrote:
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.
I hadn't thought of that sort of thing - thanks. Maybe I'll annoy the
comp.lang.c folks by resurrecting that thread with more cases!
David
