[LLVMdev] On LLD performance

[David Blaikie]

On Mon, Mar 16, 2015 at 10:52 PM, Davide Italiano <davide at freebsd.org>
wrote:

> On Mon, Mar 16, 2015 at 1:54 AM, Davide Italiano <davide at freebsd.org>
> wrote:
> >
> > Shankar's parallel for per-se didn't introduce any performance benefit
> > (or regression).
> > If the change I propose is safe, I would like to see Shankar's change
> > in (and this on top of it).
> > I have other related changes coming next, but I would like to tackle
> > them one at a time.
> >
>
> Here's an update.
>
> After http://reviews.llvm.org/D8372 , I updated the profiling data.
>
> https://people.freebsd.org/~davide/llvm/lld-03162015.svg
> It seems now 85% of CPU time is spent inside
> FileArchive::buildTableOfContents().
> In particular, 35% of the samples are spent inserting into
> unordered_map, so there's maybe something we can do differently there
> (e.g. , Rui's proposal of a concurrent map doesn't seem that bad).
>

Anyone tried a DenseMap instead of an unordered_map? If you need pointer
validity to the elements, a DenseMap with unique_ptrs rather than direct
values could be an option. Chandler's usual argument here is that walking
the map is cheap with high locality (as in a DenseMap) even if the nodes
themselves involve indirection. Could be worth an experiment.


>
> Thanks,
>
> --
> Davide
>
> "There are no solved problems; there are only problems that are more
> or less solved" -- Henri Poincare
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