On 12/12/2012 3:29 PM, Timon Gehr wrote:
On 12/12/2012 10:25 PM, Walter Bright wrote:
On 12/12/2012 4:51 AM, Araq wrote:
...
So how does D improve on C's model? If signed integers are required to
wrap
around in D (no undefined behaviour), you also prevent some otherwise
possible
optimizations (there is a reason it's still undefined behaviour in C).
D requires 2's complement arithmetic, it does not support 1's complement
as C does.
I think what he is talking about is that in C, if after a few steps of inlining
and constant propagation you end up with something like:
int x;
// ...
if(x>x+1) {
// lots and lots of code
}else return 0;
Then a C compiler will assume that the addition does not overflow and reduce the
code to 'return 0;', whereas a D compiler will not apply this optimization as it
might change the semantics of valid D programs.
You're right in that the D optimizer does not take advantage of C "undefined
behavior" in its optimizations. The article mentioned that many bugs were caused
not by the actual wraparound behavior, but by aggressive C optimizers that
interpreted "undefined behavior" as not having to account for those cases.
