[llvm][fix-irreducible] ensure that loop subtree under child is correctly reconnected...

The modified function was incorrectly (not unnecessarily) ignoring grandchild
loops, and this change fixes the bug. In particular, this fixes the handling of
the loop { inner, body }. The TODO in the same function is talking about the b1
self loop, which may be "unnecessarily" lost, but that is a different issue.

diff --git a/llvm/lib/Transforms/Utils/FixIrreducible.cpp b/llvm/lib/Transforms/Utils/FixIrreducible.cpp
index e0ebf11..24539bd 100644 (file)
--- a/llvm/lib/Transforms/Utils/FixIrreducible.cpp
+++ b/llvm/lib/Transforms/Utils/FixIrreducible.cpp
@@ -138,10 +138,18 @@ static void reconnectChildLoops(LoopInfo &LI, Loop *ParentLoop, Loop *NewLoop,
     // not be necessary if we can retain such backedges.
     if (Headers.count(Child->getHeader())) {
       for (auto BB : Child->blocks()) {
+        if (LI.getLoopFor(BB) != Child)
+          continue;
         LI.changeLoopFor(BB, NewLoop);
         LLVM_DEBUG(dbgs() << "moved block from child: " << BB->getName()
                           << "\n");
       }
+      std::vector<Loop *> GrandChildLoops;
+      std::swap(GrandChildLoops, Child->getSubLoopsVector());
+      for (auto GrandChildLoop : GrandChildLoops) {
+        GrandChildLoop->setParentLoop(nullptr);
+        NewLoop->addChildLoop(GrandChildLoop);
+      }
       LI.destroy(Child);
       LLVM_DEBUG(dbgs() << "subsumed child loop (common header)\n");
       continue;