[LiveVariables] Switch Kill/Defs sets to be DenseSet(s).
The testcase in PR32984 shows a non linear compile time increase
after a change that made the LoopUnroll pass more aggressive
(increasing the threshold).
My profiling shows all the time of PHI elimination goes to
llvm::LiveVariables::addNewBlock. This is because we keep
Defs/Kills registers in a SmallSet and vfind(const T &V); is O(N).
Switching to a DenseSet reduces the time spent in the pass from
297 seconds to 97 seconds. Profiling still shows a lot of time is
spent iterating the data structure, so I guess there's room for
improvement.
Dan tells me GCC uses real set operations for live registers and
it takes no-time on this testcase. Matthias points out we might
want to switch all this to LiveIntervalAnalysis so it's not entirely
sure if a rewrite is worth it.
Differential Revision: https://reviews.llvm.org/D33088
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@302819
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