[PATCH] D27247: Parallelize ICF to make LLD's ICF really fast.

[Sean Silva via Phabricator via llvm-commits]

silvas accepted this revision.
silvas added a comment.
This revision is now accepted and ready to land.

This LGTM. Thanks for looking so closely at this! It's a very nice speedup!

>> do some sort of topological sorting (even approximate) and then do partial iterations which only sort only part of the array. (more generally, avoid revisiting sections that are unlikely to change this iteration). This can speed up the convergence since we avoid wasting work on nodes that won't change.
> 
> Interesting idea. I could imagine that we might be able to compute strongly connected component and toposort them to do some sort of optimization. I wouldn't do that in this patch, but that's probably worth trying.

We may not even need SCC's, as cycles are relatively rare. Just ordering by a simple DFS/BFS may give most of the benefit. So we may be able to get this "for free" from the work we do for --gc-sections.

(as far as parallelizing graph traversals, parallel graph algorithms have actually been studied quite a bit, although often in a distributed setting.
Some ones I remember seeing are:
https://github.com/MicrosoftResearch/Naiad/blob/release_0.5/Examples/DifferentialDataflow/ConnectedComponents.cs
https://github.com/MicrosoftResearch/Naiad/blob/release_0.5/Examples/DifferentialDataflow/StronglyConnectedComponents.cs
https://github.com/frankmcsherry/timely-dataflow/blob/master/examples/bfs.rs

Ideas from there might be useful inspiration for any parallel graph processing we have to do.
)


https://reviews.llvm.org/D27247