[llvm-commits] patch: chrec * chrec = chrec.

[Joerg Sonnenberger]

On Mon, Oct 03, 2011 at 02:10:45PM -0700, Nick Lewycky wrote:
> On 3 October 2011 13:51, Joerg Sonnenberger <joerg at britannica.bec.de> wrote:
> > What exactly is the desired behavior in case of overflow?
> > E.g. do you want to get C(n,k) % (UINTMAX + 1)? In that case, your
> > formula doesn't compute correct results.
> >
> 
> In overflow the return value is garbage and Overflow is set. This method
> deserves a comment.

>From IRC: since n is supposed to be small, it might be better to just
check for n <= 34 and consider it as overflow otherwise. Essentially,
that's the largest number for which C(2n,n) < 2**32. In theory, it would
be possible to compute C(n,k) in the finite field by eliminating the
factors 2 in the divisor and implementing the division using the
multiplicative inverse at the end. That's quite a bit more work though.

Joerg