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[RemoveDuplicatePHINodes] Start over after removing a PHI.

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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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@246694 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Benjamin Kramer 2015-09-02 19:52:23 +00:00
parent 97cac33e22
commit 7c393ccd97
2 changed files with 42 additions and 0 deletions
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5
lib/Transforms/Utils/Local.cpp
View File

@ -874,6 +874,11 @@ bool llvm::EliminateDuplicatePHINodes(BasicBlock *BB) {
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();
}
}

37
unittests/Transforms/Utils/Local.cpp
View File

@ -58,3 +58,40 @@ TEST(Local, RecursivelyDeleteDeadPHINodes) {
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());
}