[petsc-users] matsetvaluesblocked4_

[Jed Brown]

Mark Adams <mfadams at lbl.gov> writes:

> The existing code seems to have *a line (low=i+1) *that seems to be
> trying to exploit consecutive indices but it is not quite right, I don't
> think,
>
> This is the existing code fragment (this code has been cloned many times
> and there are several instances of this kernel).
>
> I've *added code* that I think might make this do the right thing.
>
>      if (col <= lastcol) low = 0;
>       else high = nrow;
>       lastcol = col;
>       while (high-low > 5) {
>         t = (low+high)/2;
>
> *if (rp[low] == col) high = low+1;else *       if (rp[t] > col) high = t;
>         else low = t;

Replacing a single comparison per bsearch iteration with two doesn't
seem like a good choice to me.

>       }
>       for (i=low; i<high; i++) {
>         if (rp[i] > col) break; *// delete this check if you don't add new
> columns*
>         if (rp[i] == col) {
>  ap[i] += value;
>
> *  low = i + 1;*          goto noinsert;

I was thinking of a fast-path like while (rp[i] == in[l]) ap[i++] = in[l++];

A bit more logic is needed to avoid running off the end of either array.