[llvm-commits] [RFC] Use topological sorting when writing the type table

[Chris Lattner]

On Mar 22, 2011, at 10:35 PM, Rafael Ávila de Espíndola wrote:

> I am trying to reduce the time of LTO builds a bit. I started profiling
> gnu ar (which uses libLTO via the gold plugin) and noticed that a lot of
> time was spent in type unification.
> 
> Type unification is known to be a problem when linking, but I was
> surprised to see it during 'ar' since no module was being merged. It
> turns out that it was because of forward references, which are handled
> in the reader by creating opaque types.
> 
> I changed the writer to do a topological sort of the types before
> writing. The time impact is pretty impressive. The timing for producing
> libgklayout.a in the firefox build (a 49MB file) goes from
> 
> real	2m4.186s
> user	2m3.591s
> sys	0m0.211s
> 
> to
> 
> real	0m23.944s
> user	0m23.698s
> sys	0m0.185s
> 
> I have attached the proof of concept patch that does it. Would a not as
> copy and stl heavy patch implementing the same idea be OK?

Yes, I'm ok with the idea in principle if the patch were reworked.  We used to do something similar but dropped it along the way.  FWIW, I'm hoping to have some time in the next 6 months to do the type system rewrite, which will hopefully define this away.

-Chris