[PATCH] D28594: NewGVN: Move leaders around properly to ensure we have a canonical dominating leader. Fixes PR 31613.

[Daniel Berlin via Phabricator via llvm-commits]

dberlin created this revision.
dberlin added a reviewer: davide.
dberlin added a subscriber: llvm-commits.

This is a testcase where phi node cycling happens, and because we do
not order the leaders by domination or anything similar, the leader
keeps changing.

Using std::set for the members is too expensive, and we actually don't
need them sorted all the time, only at leader changes.

We could keep both a set and a vector, and keep them mostly sorted and
resort as necessary, or use a set and a fibheap, but all of this seems
premature.

After running some statistics, we are able to avoid the vast majority
of sorting by keeping a "next leader" field.  Most congruence classes only have
leader changes once or twice during GVN.


https://reviews.llvm.org/D28594

Files:
  lib/Transforms/Scalar/NewGVN.cpp
  test/Transforms/NewGVN/pr31613.ll

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