Re: Do C++ signed types have modulo semantics?

[Nathan Sidwell <nathan at codesourcery dot com>]

Gabriel Dos Reis wrote:
> Michael Veksler <VEKSLER@il.ibm.com> writes:
>
> [...]
>
> | The code is not very simple, and different codes will get optimized
> | differently.
> | The user will have to learn how to write this piece of code differently for
> | each
> | processor to have best results.
> | 
> | int wrap_sum(int a, int b)
> | {
> |   if ( (a<0) != (b<0))
> |     return a+b; // Different sign, no overflow possible.
> |   unsigned sum = (unsigned) a + b;
> |   if (sum <= MAX_INT)
> |     return sum;
> |   sum -= MIN_INT;
> |   if (sum > MAX_INT) // can be eliminated for 2's complement
> |    abort(); // oops
> |   return (int)sum + MIN_INT;
> | }
> | 
> | It does not look too good optimization-wise.
>
> Thanks for providing this example.  This is the sort of thing I
> faced.  With pipeline process, it does harm.

Please explain why
int wrap_sum (int a, int b)
{
 return (int) ((unsigned)a + (unsigned)b));
}
is unacceptable (provided we're on the standard 2's complement machine where the 
mapping between negative ints and unsigned is *implementation defined* to be the 
sane mapping, which I might point out you've already assumed in the wrap_sum I 
quoted).

nathan

and whoever silently removed gcc@gcc.gnu.org, shame on you

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