[early patch] Speed up decl chaining

[Chandler Carruth]

On Fri, Oct 18, 2013 at 8:35 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> > Currently decl chaining is O(n). We use a circular singly linked list
> > that points to the previous element and has a bool to say if we are
> > the first element (and actually point to the last).
> >
> > Adding a new decl is O(n) because we have to find the first element by
> > walking the prev links. One way to make this O(1) that is sure to work
> > is a doubly linked list, but that would be very wasteful in memory.
>

What about just storing both the first and the last and keeping the
direction as it is now?


> >
> > What this patch does is reverse the list so that a decl points to the
> > next decl (or to the first if it is the last). With this chaining
> > becomes O(1). The flip side is that getPreviousDecl is now O(n).
>
> The patch still needs work, but I managed to convert all calls to
> getPreviousDecl in the above testcase. In my machine the clang -cc1
> time on it goes from  4.819s to 0.865s now (5.57x faster).
>
>
> Cheers,
> Rafael
>
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