[llvm] [BasicBlockUtils] Fixed LoopInfo update in UpdateAnalysisInformation() (PR #177147)

[Robert Imschweiler via llvm-commits]

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@@ -1054,6 +1054,28 @@ BasicBlock *llvm::SplitBlock(BasicBlock *Old, BasicBlock::iterator SplitPt,
   return SplitBlockImpl(Old, SplitPt, DTU, /*DT=*/nullptr, LI, MSSAU, BBName);
 }
 
+static bool hasReachableLoopEntry(const Loop &L, const DominatorTree &DT) {
+  for (const auto Pred : inverse_children<const BasicBlock *>(L.getHeader()))
+    if (!L.contains(Pred) && DT.isReachableFromEntry(Pred))
+      return true;
+
+  return false;
+}
+
+// Destroy the loop.
+static void Destroy(Loop *L, LoopInfo &LI) {
+  auto *ParentL = L->getParentLoop();
+  for (auto *BB : L->getBlocks())
+    LI.changeLoopFor(BB, ParentL);
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ro-i wrote:

What's with sub-loops? Wouldn't they get corrupted by changing all the blocks to `ParentL` unconditionally?
Maybe have a look at:
https://github.com/llvm/llvm-project/blob/1eeb2eccf8b2397c49aaeefde61257c5be6fc905/llvm/lib/Transforms/Utils/FixIrreducible.cpp#L196-L211

https://github.com/llvm/llvm-project/pull/177147