Optimizing: Fix `InsertInputAt()`.

Test: m test-art-host-gtest
Test: testrunner.py --host --optimizing
Change-Id: I500faee42b02dbc72474e30fa2a3c0388ae86674

2 files changed, 33 insertions, 1 deletions

diff --git a/compiler/optimizing/nodes.cc b/compiler/optimizing/nodes.cc
index fc7d0a52ab..ece11435db 100644
--- a/compiler/optimizing/nodes.cc
+++ b/compiler/optimizing/nodes.cc
@@ -1541,12 +1541,14 @@ void HVariableInputSizeInstruction::AddInput(HInstruction* input) {
 
 void HVariableInputSizeInstruction::InsertInputAt(size_t index, HInstruction* input) {
   inputs_.insert(inputs_.begin() + index, HUserRecord<HInstruction*>(input));
-  input->AddUseAt(this, index);
   // Update indexes in use nodes of inputs that have been pushed further back by the insert().
   for (size_t i = index + 1u, e = inputs_.size(); i < e; ++i) {
     DCHECK_EQ(inputs_[i].GetUseNode()->GetIndex(), i - 1u);
     inputs_[i].GetUseNode()->SetIndex(i);
   }
+  // Add the use after updating the indexes. If the `input` is already used by `this`,
+  // the fixup after use insertion can use those indexes.
+  input->AddUseAt(this, index);
 }
 
 void HVariableInputSizeInstruction::RemoveInputAt(size_t index) {
diff --git a/compiler/optimizing/nodes_test.cc b/compiler/optimizing/nodes_test.cc
index 0302298b9c..5f71d053c6 100644
--- a/compiler/optimizing/nodes_test.cc
+++ b/compiler/optimizing/nodes_test.cc
@@ -197,6 +197,36 @@ TEST_F(NodeTest, AddInstruction) {
   ASSERT_TRUE(parameter->GetUses().HasExactlyOneElement());
 }
 
+TEST_F(NodeTest, InsertDuplicateInstructionAt) {
+  HBasicBlock* ret = InitEntryMainExitGraphWithReturnVoid();
+  HInstruction* const0 = graph_->GetIntConstant(0);
+  HInstruction* const1 = graph_->GetIntConstant(1);
+  HInstruction* const2 = graph_->GetIntConstant(0);
+  HInstruction* const3 = graph_->GetIntConstant(1);
+
+  // We should be able to insert a duplicate input to `HPhi`s if we want to
+  // make a graph transformation that adds another predecessor to a block.
+
+  // This used to accidentally end up with correct use information but unexpectedly
+  // using the old `HUseListNode<>` for the new use and the new one for the old use.
+  HPhi* phi1 = MakePhi(ret, {const0, const1});
+  struct AccessProtected : HVariableInputSizeInstruction {
+    using HVariableInputSizeInstruction::InputRecordAt;
+  };
+  const HUseListNode<HInstruction*>* old_use_node_before =
+      std::addressof(*(phi1->*&AccessProtected::InputRecordAt)(1u).GetUseNode());
+  phi1->InsertInputAt(1u, const1);  // Moves the old use from position 1 to position 2.
+  const HUseListNode<HInstruction*>* old_use_node_after =
+      std::addressof(*(phi1->*&AccessProtected::InputRecordAt)(2u).GetUseNode());
+  EXPECT_EQ(old_use_node_before, old_use_node_after);
+  EXPECT_EQ(1u, (phi1->*&AccessProtected::InputRecordAt)(1u).GetUseNode()->GetIndex());
+  EXPECT_EQ(2u, (phi1->*&AccessProtected::InputRecordAt)(2u).GetUseNode()->GetIndex());
+
+  // This used to hit a `DCHECK()`.
+  HPhi* phi2 = MakePhi(ret, {const2, const3, const3});
+  phi2->InsertInputAt(1u, const3);
+}
+
 TEST_F(NodeTest, ParentEnvironment) {
   HGraph* graph = CreateGraph();
   HBasicBlock* entry = new (GetAllocator()) HBasicBlock(graph);