Order of constructors in Data.Map.Internal

[Carter Schonwald]

This is about first run  branch prediction on cpus and how constructor
order can influence code gen of case expressions.

This is more Andreask’S wheelhouse rather than David’s.  In fact Andreas
has some work to make things like this easier to do in progress.

Johan did some super detailed benchmarking when he did this and other
work.  Perhaps he can share with you more about how he did it. I ccd him on
this email.

On Mon, Jan 28, 2019 at 4:44 PM chessai . <chessai1996 at gmail.com> wrote:

> When looking through the source of Data.Map.Internal, there's a note:
>
> -- [Note: Order of constructors]
> -- ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
> -- The order of constructors of Map matters when considering performance.
> -- Currently in GHC 7.0, when type has 2 constructors, a forward
> conditional
> -- jump is made when successfully matching second constructor. Successful
> match
> -- of first constructor results in the forward jump not taken.
> -- On GHC 7.0, reordering constructors from Tip | Bin to Bin | Tip
> -- improves the benchmark by up to 10% on x86.
>
> I've been curious about this for a while now. GHC 7.0 was released
> quite a long time ago, so I wonder if this is still the case? David
> might know.
> _______________________________________________
> Libraries mailing list
> Libraries at haskell.org
> http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.haskell.org/pipermail/libraries/attachments/20190203/cfbc4fe7/attachment.html>