[PATCH] D65310: [JumpThreading] In updatePredecessorProfileMetadata, stop searching predecessor when the current bb is an unreachable single bb loop

[Wei Mi via Phabricator via llvm-commits]

wmi created this revision.
wmi added reviewers: davidxl, efriedma.
Herald added subscribers: jfb, hiraditya.
Herald added a project: LLVM.

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 single bb loop happens to be on the predecessor block chain, keeping following the predecessor block will run into an infinite loop.

The patch fixes it.


Repository:
  rL LLVM

https://reviews.llvm.org/D65310

Files:
  llvm/lib/Transforms/Scalar/JumpThreading.cpp
  llvm/test/Transforms/JumpThreading/unreachable-single-bb-loop.ll


Index: llvm/test/Transforms/JumpThreading/unreachable-single-bb-loop.ll
===================================================================
--- /dev/null
+++ llvm/test/Transforms/JumpThreading/unreachable-single-bb-loop.ll
@@ -0,0 +1,35 @@
+; RUN: opt -jump-threading -S < %s | FileCheck %s
+; RUN: opt -passes=jump-threading -S < %s | FileCheck %s
+; Check the unreachable single bb loop won't cause infinite loop
+; in jump-threading when it tries to update the predecessors'
+; profile metadata from a phi node.
+
+define void @test() {
+; CHECK-LABEL: @test()
+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
+}
+
+declare i32 @a()
+
+!0 = !{!"branch_weights", i32 2146410443, i32 1073205}
Index: llvm/lib/Transforms/Scalar/JumpThreading.cpp
===================================================================
--- llvm/lib/Transforms/Scalar/JumpThreading.cpp
+++ llvm/lib/Transforms/Scalar/JumpThreading.cpp
@@ -231,6 +231,9 @@
       auto *SinglePredBB = PredBB->getSinglePredecessor();
       if (!SinglePredBB)
         return {nullptr, nullptr};
+      // For unreachable one block loop, stop searching.
+      if (PredBB == SinglePredBB)
+        return {PredBB, SuccBB};
       SuccBB = PredBB;
       PredBB = SinglePredBB;
     }
@@ -253,7 +256,9 @@
       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.


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