[JumpThreading] Stop searching predecessor when the current bb is in a

unreachable loop.

updatePredecessorProfileMetadata in jumpthreading tries to find the
first dominating predecessor block for a PHI value by searching upwards
the predecessor block chain.

But jumpthreading may see some temporary IR state which contains
unreachable bb not being cleaned up. If an unreachable loop happens to
be on the predecessor block chain, keeping chasing the predecessor
block will run into an infinite loop.

The patch fixes it.

Differential Revision: https://reviews.llvm.org/D65310

llvm-svn: 367154

diff --git a/llvm/lib/Transforms/Scalar/JumpThreading.cpp b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
index b86bf2f..459be21 100644 (file)
--- a/llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ b/llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -224,13 +224,21 @@ static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
          BasicBlock *PhiBB) -> std::pair<BasicBlock *, BasicBlock *> {
     auto *PredBB = IncomingBB;
     auto *SuccBB = PhiBB;
+    SmallPtrSet<BasicBlock *, 16> Visited;
     while (true) {
       BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
       if (PredBr && PredBr->isConditional())
         return {PredBB, SuccBB};
+      Visited.insert(PredBB);
       auto *SinglePredBB = PredBB->getSinglePredecessor();
       if (!SinglePredBB)
         return {nullptr, nullptr};
+
+      // Stop searching when SinglePredBB has been visited. It means we see
+      // an unreachable loop.
+      if (Visited.count(SinglePredBB))
+        return {nullptr, nullptr};
+
       SuccBB = PredBB;
       PredBB = SinglePredBB;
     }
@@ -253,7 +261,9 @@ static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
       return;
 
     BasicBlock *PredBB = PredOutEdge.first;
-    BranchInst *PredBr = cast<BranchInst>(PredBB->getTerminator());
+    BranchInst *PredBr = dyn_cast<BranchInst>(PredBB->getTerminator());
+    if (!PredBr)
+      return;
 
     uint64_t PredTrueWeight, PredFalseWeight;
     // FIXME: We currently only set the profile data when it is missing.

diff --git a/llvm/test/Transforms/JumpThreading/unreachable-loops.ll b/llvm/test/Transforms/JumpThreading/unreachable-loops.ll
new file mode 100644 (file)
index 0000000..3f75aea
--- /dev/null
+++ b/llvm/test/Transforms/JumpThreading/unreachable-loops.ll
@@ -0,0 +1,63 @@
+; RUN: opt -jump-threading -S < %s | FileCheck %s
+; RUN: opt -passes=jump-threading -S < %s | FileCheck %s
+; Check the unreachable loop won't cause infinite loop
+; in jump-threading when it tries to update the predecessors'
+; profile metadata from a phi node.
+
+define void @unreachable_single_bb_loop() {
+; CHECK-LABEL: @unreachable_single_bb_loop()
+bb:
+  %tmp = call i32 @a()
+  %tmp1 = icmp eq i32 %tmp, 1
+  br i1 %tmp1, label %bb5, label %bb8
+
+; unreachable single bb loop.
+bb2:                                              ; preds = %bb2
+  %tmp4 = icmp ne i32 %tmp, 1
+  switch i1 %tmp4, label %bb2 [
+    i1 0, label %bb5
+    i1 1, label %bb8
+  ]
+
+bb5:                                              ; preds = %bb2, %bb
+  %tmp6 = phi i1 [ %tmp1, %bb ], [ false, %bb2 ]
+  br i1 %tmp6, label %bb8, label %bb7, !prof !0
+
+bb7:                                              ; preds = %bb5
+  br label %bb8
+
+bb8:                                              ; preds = %bb8, %bb7, %bb5, %bb2
+  ret void
+}
+
+define void @unreachable_multi_bbs_loop() {
+; CHECK-LABEL: @unreachable_multi_bbs_loop()
+bb:
+  %tmp = call i32 @a()
+  %tmp1 = icmp eq i32 %tmp, 1
+  br i1 %tmp1, label %bb5, label %bb8
+
+; unreachable two bbs loop.
+bb3:                                              ; preds = %bb2
+  br label %bb2
+
+bb2:                                              ; preds = %bb3
+  %tmp4 = icmp ne i32 %tmp, 1
+  switch i1 %tmp4, label %bb3 [
+    i1 0, label %bb5
+    i1 1, label %bb8
+  ]
+
+bb5:                                              ; preds = %bb2, %bb
+  %tmp6 = phi i1 [ %tmp1, %bb ], [ false, %bb2 ]
+  br i1 %tmp6, label %bb8, label %bb7, !prof !0
+
+bb7:                                              ; preds = %bb5
+  br label %bb8
+
+bb8:                                              ; preds = %bb8, %bb7, %bb5, %bb2
+  ret void
+}
+declare i32 @a()
+
+!0 = !{!"branch_weights", i32 2146410443, i32 1073205}