On Wed, 21 Apr 2010 15:40:19 +0200 [email protected] wrote:
> Hello,
>
> Still about integer arithmetic.
...
> Conclusion (apparently): gcc always translate div involving power of two
> to binary manipulations, while (apparently) ken-cc does not.
gcc may choose to implement / with appropriate shifts and
what not but the result must be equivalent. It certainly
can't replace %2 with >>1 for negative numbers.
Try this:
int f(int x) { return x / 2; }
int g(int x) { return x >> 1; }
main(int argc, char** argv)
{
int x = argc > 1? atoi(argv[1]) : -7;
printf("%d %d\n", -7/2, -7>>1);
printf("%d %d\n", f(-7), g(-7));
printf("%d %d\n", f(x), g(x));
}
This will give you
-3 -4
-3 -4
-3 -4
I don't have a way to test what kencc does but I would be
surprised if the result is any different....
>> is an arithmetic shift for signed numbers (the sign bit is
replicated). The clearest way to see this is to compare
-1/2 and -1>>1
The first gives you 0. The second gives you -1 (on twos
complement machines).
IIRC, Pascal's div operator behaves the same as C's / for
integers. May be the problem is somehow related to mod?
> Conclusion: I will have to replace in METAFONT all div involving power
> of two to binary operations, since if I replace in some places and not
> in others, I wreak havoc the algorithms since computations are not done
> the same way for combined chunks.
Replacing div 2^N of a signed number by >>N seems like a mistake.
