[CycleInfo] skip unreachable predecessors (llvm#101316)

If an unreachable block B branches to a block S inside a cycle, it may
cause S to be incorrectly treated as an entry to the cycle. We avoid
that by skipping unreachable predecessors when locating entries.

llvm/include/llvm/ADT/GenericCycleImpl.h

@@ -134,6 +134,8 @@ template <typename ContextT> class GenericCycleInfoCompute {
     DFSInfo() = default;
     explicit DFSInfo(unsigned Start) : Start(Start) {}
 
+    explicit operator bool() const { return Start; }
+
     /// Whether this node is an ancestor (or equal to) the node \p Other
     /// in the DFS tree.
     bool isAncestorOf(const DFSInfo &Other) const {
@@ -231,6 +233,8 @@ void GenericCycleInfoCompute<ContextT>::run(BlockT *EntryBlock) {
 
     for (BlockT *Pred : predecessors(HeaderCandidate)) {
       const DFSInfo PredDFSInfo = BlockDFSInfo.lookup(Pred);
+      // This automatically ignores unreachable predecessors since they have
+      // zeros in their DFSInfo.
       if (CandidateInfo.isAncestorOf(PredDFSInfo))
         Worklist.push_back(Pred);
     }
@@ -257,6 +261,10 @@ void GenericCycleInfoCompute<ContextT>::run(BlockT *EntryBlock) {
         const DFSInfo PredDFSInfo = BlockDFSInfo.lookup(Pred);
         if (CandidateInfo.isAncestorOf(PredDFSInfo)) {
           Worklist.push_back(Pred);
+        } else if (!PredDFSInfo) {
+          // Ignore an unreachable predecessor. It will will incorrectly cause
+          // Block to be treated as a cycle entry.
+          LLVM_DEBUG(errs() << " skipped unreachable predecessor.\n");
         } else {
           IsEntry = true;
         }

llvm/test/Analysis/CycleInfo/unreachable-predecessor.ll

@@ -0,0 +1,23 @@
+; RUN: opt < %s -disable-output -passes='print<cycles>' 2>&1 | FileCheck %s
+; CHECK-LABEL: CycleInfo for function: unreachable
+; CHECK:    depth=1: entries(loop.body) loop.latch inner.block
+define void @unreachable(i32 %n) {
+entry:
+  br label %loop.body
+
+loop.body:
+  br label %inner.block
+
+; This branch should not cause %inner.block to appear as an entry.
+unreachable.block:
+  br label %inner.block
+
+inner.block:
+  br i1 undef, label %loop.exit, label %loop.latch
+
+loop.latch:
+  br label %loop.body
+
+loop.exit:
+  ret void
+}