[llvm] [CycleAnalysis] Methods to verify cycles and their nesting. (PR #102300)

[Matt Arsenault via llvm-commits]

================
@@ -119,6 +119,101 @@ auto GenericCycle<ContextT>::getCyclePredecessor() const -> BlockT * {
   return Out;
 }
 
+/// \brief Verify that this is actually a well-formed cycle in the CFG.
+template <typename ContextT> void GenericCycle<ContextT>::verifyCycle() const {
+#ifndef NDEBUG
+  assert(!Blocks.empty() && "Cycle cannot be empty.");
+  DenseSet<BlockT *> Blocks;
+  for (BlockT *BB : blocks()) {
+    assert(Blocks.insert(BB).second); // duplicates in block list?
+  }
+  assert(!Entries.empty() && "Cycle must have one or more entries.");
+
+  DenseSet<BlockT *> Entries;
+  for (BlockT *Entry : entries()) {
+    assert(Entries.insert(Entry).second); // duplicate entry?
+    assert(contains(Entry));
+  }
+
+  // Setup for using a depth-first iterator to visit every block in the cycle.
+  SmallVector<BlockT *, 8> ExitBBs;
+  getExitBlocks(ExitBBs);
+  df_iterator_default_set<BlockT *> VisitSet;
+  VisitSet.insert(ExitBBs.begin(), ExitBBs.end());
+
+  // Keep track of the BBs visited.
+  SmallPtrSet<BlockT *, 8> VisitedBBs;
+
+  // Check the individual blocks.
+  for (BlockT *BB : depth_first_ext(getHeader(), VisitSet)) {
+    assert(llvm::any_of(llvm::children<BlockT *>(BB),
+                        [&](BlockT *B) { return contains(B); }) &&
+           "Cycle block has no in-cycle successors!");
+
+    assert(llvm::any_of(llvm::inverse_children<BlockT *>(BB),
+                        [&](BlockT *B) { return contains(B); }) &&
+           "Cycle block has no in-cycle predecessors!");
+
+    DenseSet<BlockT *> OutsideCyclePreds;
+    for (BlockT *B : llvm::inverse_children<BlockT *>(BB))
+      if (!contains(B))
+        OutsideCyclePreds.insert(B);
+
+    if (Entries.contains(BB)) {
+      assert(!OutsideCyclePreds.empty() && "Entry is unreachable!");
+    } else if (!OutsideCyclePreds.empty()) {
+      // A non-entry block shouldn't be reachable from outside the cycle,
+      // though it is permitted if the predecessor is not itself actually
+      // reachable.
+      BlockT *EntryBB = &BB->getParent()->front();
+      for (BlockT *CB : depth_first(EntryBB))
+        assert(!OutsideCyclePreds.contains(CB) &&
+               "Non-entry block reachable from outside!");
+    }
+    assert(BB != &getHeader()->getParent()->front() &&
+           "Cycle contains function entry block!");
+
+    VisitedBBs.insert(BB);
+  }
+
+  if (VisitedBBs.size() != getNumBlocks()) {
+    dbgs() << "The following blocks are unreachable in the cycle: ";
+    for (auto *BB : Blocks) {
+      if (!VisitedBBs.count(BB)) {
+        dbgs() << *BB << "\n";
+      }
+    }
+    assert(false && "Unreachable block in cycle");
----------------
arsenm wrote:

It's always a stylistic choice, and llvm_unreachable is nicer 

https://github.com/llvm/llvm-project/pull/102300