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[PM] Add a comment clarifying what a particular predicate is doing. 

This came up as a point of confusion while working on a fundamental
problem with the combination of CGSCC iteration and the inliner.

llvm-svn: 309662
This commit is contained in:
Chandler Carruth 2017-08-01 06:40:11 +00:00
parent 85da7543f9
commit 3c6a820ce3
1 changed files with 8 additions and 0 deletions
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8
llvm/lib/Analysis/CGSCCPassManager.cpp
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@ -619,6 +619,14 @@ LazyCallGraph::SCC &llvm::updateCGAndAnalysisManagerForFunctionPass(
AM.invalidate(*C, PA); AM.invalidate(*C, PA);
} }
auto NewSCCIndex = RC->find(*C) - RC->begin(); auto NewSCCIndex = RC->find(*C) - RC->begin();
// If we have actually moved an SCC to be topologically "below" the current
// one due to merging, we will need to revisit the current SCC after
// visiting those moved SCCs.
//
// It is critical that we *do not* revisit the current SCC unless we
// actually move SCCs in the process of merging because otherwise we may
// form a cycle where an SCC is split apart, merged, split, merged and so
// on infinitely.
if (InitialSCCIndex < NewSCCIndex) { if (InitialSCCIndex < NewSCCIndex) {
// Put our current SCC back onto the worklist as we'll visit other SCCs // Put our current SCC back onto the worklist as we'll visit other SCCs
// that are now definitively ordered prior to the current one in the // that are now definitively ordered prior to the current one in the