[PATCH 2 of 2] revset: speed up existence checks for ordered filtered sets

[Yuya Nishihara]

On Sun, 20 Sep 2015 20:51:36 -0700, Durham Goode wrote:
> # HG changeset patch
> # User Durham Goode <durham at fb.com>
> # Date 1442793222 25200
> #      Sun Sep 20 16:53:42 2015 -0700
> # Node ID 46a81d56dfabfc6ed8e1c08dae41b840574b8835
> # Parent  9deecd57915da9ed217abafb81cf8cba506b2849
> revset: speed up existence checks for ordered filtered sets
> 
> Previously, calling 'if foo:' on a ordered filtered set would start iterating in
> whatever the current direction was and return if a value was available. If the
> current direction was ascending, but the set had a fastdesc available, this
> meant we did a lot more work than necessary.
> 
> If this was applied without my previous max/min fixes, it would improve max()
> performance (this was my first attempt at fixing the issue). Since those
> previous fixes went in though, this doesn't have a visible benefit in the
> benchmarks, but it does seem clearly better than it was before so I think it
> should still go in.
> 
> diff --git a/mercurial/revset.py b/mercurial/revset.py
> --- a/mercurial/revset.py
> +++ b/mercurial/revset.py
> @@ -3111,7 +3111,12 @@ class filteredset(abstractsmartset):
>          return lambda: self._iterfilter(it())
>  
>      def __nonzero__(self):
> -        for r in self:
> +        it = self
> +        fast = self.fastdesc or self.fastasc
> +        if fast:
> +            it = fast()
> +
> +        for r in it:
>              return True

I think the PATCH 1 is great, but I'm not sure about this.

If I understand it, this is fast only if the filtered revisions exist near
fastdesc side?

  condition = parents(. + .^)  # subset.fastdesc can reach the condition faster
  subset = fullreposet