bug#44754: Extreme performance degradation in GNU grep 3.4 / 3.6

[Jim Meyering]

On Thu, Nov 26, 2020 at 9:03 AM Jim Meyering <jim@meyering.net> wrote:
>
> On Wed, Nov 25, 2020 at 3:12 PM Jim Meyering <jim@meyering.net> wrote:
> > Thank you for the fine bug report.
> > The grep-3.6 bug you've exposed is due to the fact that your input
> > triggers excessive hash collisions when using the code modeled after
> > gnulib/lib/hash-pjw.c. That made the new pattern-preprocessing phase
> > take O(N^2) time for N patterns. In the attached, I've switched grep
> > to use the djb2 hash function, and that resolves the problem. I'll
> > also add a NEWS entry and a test before pushing this.
>
> Timings suggest that grep-3.6's preprocessing came closer to O(N^3).
> Here's an example that would take 2-3 days with grep-3.6 and only
> seconds with this fix:
>
>   : | grep -Ff <(seq 6400000 | tr 0-9 A-J)
>
> Here's a complete patch.
> I'll push it later today.

Pushed along with two gnulib-related changes.