[LoopRotate] Fix DomTree update logic for unreachable nodes. Fix PR33701.

Summary:
LoopRotate manually updates the DoomTree by iterating over all predecessors of a basic block and computing the Nearest Common Dominator.

When a predecessor happens to be unreachable, `DT.findNearestCommonDominator` returns nullptr.

This patch teaches LoopRotate to handle this case and fixes [[ https://bugs.llvm.org/show_bug.cgi?id=33701 | PR33701 ]].

In the future, LoopRotate should be taught to use the new incremental API for updating the DomTree.

Reviewers: dberlin, davide, uabelho, grosser

Subscribers: efriedma, mzolotukhin

Differential Revision: https://reviews.llvm.org/D35074

llvm-svn: 307828

2 files changed, 43 insertions, 4 deletions

diff --git a/llvm/lib/Transforms/Scalar/LoopRotation.cpp b/llvm/lib/Transforms/Scalar/LoopRotation.cpp
index 7312d97f8ef..3506ac343d5 100644
--- a/llvm/lib/Transforms/Scalar/LoopRotation.cpp
+++ b/llvm/lib/Transforms/Scalar/LoopRotation.cpp
@@ -485,10 +485,22 @@ bool LoopRotate::rotateLoop(Loop *L, bool SimplifiedLatch) {
           DomTreeNode *Node = HeaderChildren[I];
           BasicBlock *BB = Node->getBlock();
 
-          pred_iterator PI = pred_begin(BB);
-          BasicBlock *NearestDom = *PI;
-          for (pred_iterator PE = pred_end(BB); PI != PE; ++PI)
-            NearestDom = DT->findNearestCommonDominator(NearestDom, *PI);
+          BasicBlock *NearestDom = nullptr;
+          for (BasicBlock *Pred : predecessors(BB)) {
+            // Consider only reachable basic blocks.
+            if (!DT->getNode(Pred))
+              continue;
+
+            if (!NearestDom) {
+              NearestDom = Pred;
+              continue;
+            }
+
+            NearestDom = DT->findNearestCommonDominator(NearestDom, Pred);
+            assert(NearestDom && "No NearestCommonDominator found");
+          }
+
+          assert(NearestDom && "Nearest dominator not found");
 
           // Remember if this changes the DomTree.
           if (Node->getIDom()->getBlock() != NearestDom) {
diff --git a/llvm/test/Transforms/LoopRotate/pr33701.ll b/llvm/test/Transforms/LoopRotate/pr33701.ll
new file mode 100644
index 00000000000..ed162b12098
--- /dev/null
+++ b/llvm/test/Transforms/LoopRotate/pr33701.ll
@@ -0,0 +1,27 @@
+; RUN: opt < %s -loop-rotate -verify-dom-info -verify-loop-info -disable-output
+
+define void @func() {
+bb0:
+  br label %bb1
+
+bb1:                                              ; preds = %bb4, %bb0
+  %0 = phi i16 [ %2, %bb4 ], [ 0, %bb0 ]
+  %1 = icmp sle i16 %0, 2
+  br i1 %1, label %bb2, label %bb5
+
+bb2:                                              ; preds = %bb1
+  br i1 undef, label %bb6, label %bb4
+
+bb3:                                              ; No predecessors!
+  br label %bb6
+
+bb4:                                              ; preds = %bb2
+  %2 = add i16 undef, 1
+  br label %bb1
+
+bb5:                                              ; preds = %bb1
+  br label %bb6
+
+bb6:                                              ; preds = %bb5, %bb3, %bb2
+  unreachable
+}