[llvm] r246694 - [RemoveDuplicatePHINodes] Start over after removing a PHI.

[Benjamin Kramer via llvm-commits]

Author: d0k
Date: Wed Sep  2 14:52:23 2015
New Revision: 246694

URL: http://llvm.org/viewvc/llvm-project?rev=246694&view=rev
Log:
[RemoveDuplicatePHINodes] Start over after removing a PHI.

This makes RemoveDuplicatePHINodes more effective and fixes an assertion
failure. Triggering the assertions requires a DenseSet reallocation
so this change only contains a constructive test.

I'll explain the issue with a small example. In the following function
there's a duplicate PHI, %4 and %5 are identical. When this is found
the DenseSet in RemoveDuplicatePHINodes contains %2, %3 and %4.

define void @F() {
  br label %1

; <label>:1                                       ; preds = %1, %0
  %2 = phi i32 [ 42, %0 ], [ %4, %1 ]
  %3 = phi i32 [ 42, %0 ], [ %5, %1 ]
  %4 = phi i32 [ 42, %0 ], [ 23, %1 ]
  %5 = phi i32 [ 42, %0 ], [ 23, %1 ]
  br label %1
}

after RemoveDuplicatePHINodes runs the function looks like this. %3 has
changed and is now identical to %2, but RemoveDuplicatePHINodes never
saw this.

define void @F() {
  br label %1

; <label>:1                                       ; preds = %1, %0
  %2 = phi i32 [ 42, %0 ], [ %4, %1 ]
  %3 = phi i32 [ 42, %0 ], [ %4, %1 ]
  %4 = phi i32 [ 42, %0 ], [ 23, %1 ]
  br label %1
}

If the DenseSet does a reallocation now it will reinsert all
keys and stumble over %3 now having a different hash value than it had
when inserted into the map for the first time. This change clears the
set whenever a PHI is deleted and starts the progress from the
beginning, allowing %3 to be deleted and avoiding inconsistent DenseSet
state. This potentially has a negative performance impact because
it rescans all PHIs, but I don't think that this ever makes a difference
in practice.

Modified:
    llvm/trunk/lib/Transforms/Utils/Local.cpp
    llvm/trunk/unittests/Transforms/Utils/Local.cpp

Modified: llvm/trunk/lib/Transforms/Utils/Local.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/Utils/Local.cpp?rev=246694&r1=246693&r2=246694&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/Utils/Local.cpp (original)
+++ llvm/trunk/lib/Transforms/Utils/Local.cpp Wed Sep  2 14:52:23 2015
@@ -874,6 +874,11 @@ bool llvm::EliminateDuplicatePHINodes(Ba
       PN->replaceAllUsesWith(*Inserted.first);
       PN->eraseFromParent();
       Changed = true;
+
+      // The RAUW can change PHIs that we already visited. Start over from the
+      // beginning.
+      PHISet.clear();
+      I = BB->begin();
     }
   }
 

Modified: llvm/trunk/unittests/Transforms/Utils/Local.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/unittests/Transforms/Utils/Local.cpp?rev=246694&r1=246693&r2=246694&view=diff
==============================================================================
--- llvm/trunk/unittests/Transforms/Utils/Local.cpp (original)
+++ llvm/trunk/unittests/Transforms/Utils/Local.cpp Wed Sep  2 14:52:23 2015
@@ -58,3 +58,40 @@ TEST(Local, RecursivelyDeleteDeadPHINode
   delete bb0;
   delete bb1;
 }
+
+TEST(Local, RemoveDuplicatePHINodes) {
+  LLVMContext &C(getGlobalContext());
+  IRBuilder<> B(C);
+
+  std::unique_ptr<Function> F(
+      Function::Create(FunctionType::get(B.getVoidTy(), false),
+                       GlobalValue::ExternalLinkage, "F"));
+  BasicBlock *Entry(BasicBlock::Create(C, "", F.get()));
+  BasicBlock *BB(BasicBlock::Create(C, "", F.get()));
+  BranchInst::Create(BB, Entry);
+
+  B.SetInsertPoint(BB);
+
+  AssertingVH<PHINode> P1 = B.CreatePHI(Type::getInt32Ty(C), 2);
+  P1->addIncoming(B.getInt32(42), Entry);
+
+  PHINode *P2 = B.CreatePHI(Type::getInt32Ty(C), 2);
+  P2->addIncoming(B.getInt32(42), Entry);
+
+  AssertingVH<PHINode> P3 = B.CreatePHI(Type::getInt32Ty(C), 2);
+  P3->addIncoming(B.getInt32(42), Entry);
+  P3->addIncoming(B.getInt32(23), BB);
+
+  PHINode *P4 = B.CreatePHI(Type::getInt32Ty(C), 2);
+  P4->addIncoming(B.getInt32(42), Entry);
+  P4->addIncoming(B.getInt32(23), BB);
+
+  P1->addIncoming(P3, BB);
+  P2->addIncoming(P4, BB);
+  BranchInst::Create(BB, BB);
+
+  // Verify that we can eliminate PHIs that become duplicates after chaning PHIs
+  // downstream.
+  EXPECT_TRUE(EliminateDuplicatePHINodes(BB));
+  EXPECT_EQ(3U, BB->size());
+}