[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
diff --git a/llvm/lib/Analysis/CGSCCPassManager.cpp b/llvm/lib/Analysis/CGSCCPassManager.cpp
index 74b5d79..cb6d139 100644
--- a/llvm/lib/Analysis/CGSCCPassManager.cpp
+++ b/llvm/lib/Analysis/CGSCCPassManager.cpp
@@ -619,6 +619,14 @@
AM.invalidate(*C, PA);
}
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) {
// 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