[PATCH] D31843: [LCSSA] Try to not walk the dominator tree more than necessary

[Davide Italiano via Phabricator via llvm-commits]

davide created this revision.

PR31851 (although I hit the same issue on some other, internal testcases) shows LCSSA formation taking a long time.
This patch is an idea discussed on IRC and proposed by Danny to speed up the computation.
It seems a large chunk of time is spent walking the Dominator tree while walking over the basic blocks forming a loop. We do it to skip blocks not dominating any of the loop exits, as if this condition is true, no SSA name defined in the block can be used outside of it.
Turns out, and that's at the same time funny and sad, performing the check itself is more expensive than performing the work.

This patch speeds up the checking doing the following:

1. Walk the domtree in breadth-first order, keeping tracks of levels and numbering nodes according to the BF order.
2. Walk loops in domtree order, performing the dominance checks only on exits above the current BB.

(Danny suggests we could do better just looking at the subtrees, but that's probably an improvement for later).

The patch needs cleanup, and needs to be slightly reworked as the domtree computation is performed in `runOnFunction` but there are many loop passes which call `formLCSSARecursively()` as an utility and could benefit from this speedup (although they currently don't). So, consider it a WIP, but, goes without saying, comments welcome.

Figures: `opt -O2` on the test attached to PR31851

Baseline:

  real    1m45.731s
  user    1m45.533s
  sys     0m0.198s

Patch:

  real    1m35.519s
  user    1m35.318s
  sys     0m0.201s

Unless I measured this wrong (which is possible, given it's 2AM), it's a net 10% reduction in compile time.


https://reviews.llvm.org/D31843

Files:
  include/llvm/Analysis/LoopInfo.h
  lib/Transforms/Utils/LCSSA.cpp

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