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

[Joerg Sonnenberger]

On Sun, Oct 02, 2011 at 04:50:59PM -0700, Nick Lewycky wrote:
> Index: lib/Analysis/ScalarEvolution.cpp
> ===================================================================
> --- lib/Analysis/ScalarEvolution.cpp	(revision 140963)
> +++ lib/Analysis/ScalarEvolution.cpp	(working copy)
> @@ -1812,6 +1812,29 @@
>    return S;
>  }
>  
> +static unsigned umul_ov(unsigned i, unsigned j, bool &Overflow) {
> +  unsigned k = i*j;
> +  if (j > 1 && k / j != i) Overflow = true;
> +  return k;
> +}
> +
> +static unsigned Choose(unsigned n, unsigned k, bool &Overflow) {
> +  // This method can overflow internally even if the final result would fit.
> +
> +  if (n == 0) return 1;
> +  if (k >= n) return 1;
> +
> +  if (k > n/2)
> +    k = n-k;
> +
> +  unsigned r = 1;
> +  for (unsigned i = 1; i <= k; ++i) {
> +    r = umul_ov(r, n-(i-1), Overflow);
> +    r /= i;
> +  }
> +  return r;
> +}
> +

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.

Joerg