1 /* 2 * Copyright (C) 2014 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #include "ssa_builder.h" 18 19 #include "nodes.h" 20 #include "ssa_type_propagation.h" 21 22 namespace art { 23 24 void SsaBuilder::BuildSsa() { 25 // 1) Visit in reverse post order. We need to have all predecessors of a block visited 26 // (with the exception of loops) in order to create the right environment for that 27 // block. For loops, we create phis whose inputs will be set in 2). 28 for (HReversePostOrderIterator it(*GetGraph()); !it.Done(); it.Advance()) { 29 VisitBasicBlock(it.Current()); 30 } 31 32 // 2) Set inputs of loop phis. 33 for (size_t i = 0; i < loop_headers_.Size(); i++) { 34 HBasicBlock* block = loop_headers_.Get(i); 35 for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) { 36 HPhi* phi = it.Current()->AsPhi(); 37 for (size_t pred = 0; pred < block->GetPredecessors().Size(); pred++) { 38 HInstruction* input = ValueOfLocal(block->GetPredecessors().Get(pred), phi->GetRegNumber()); 39 phi->AddInput(input); 40 } 41 } 42 } 43 44 // 3) Propagate types of phis. 45 SsaTypePropagation type_propagation(GetGraph()); 46 type_propagation.Run(); 47 48 // 4) Clear locals. 49 // TODO: Move this to a dead code eliminator phase. 50 for (HInstructionIterator it(GetGraph()->GetEntryBlock()->GetInstructions()); 51 !it.Done(); 52 it.Advance()) { 53 HInstruction* current = it.Current(); 54 if (current->AsLocal() != nullptr) { 55 current->GetBlock()->RemoveInstruction(current); 56 } 57 } 58 } 59 60 HInstruction* SsaBuilder::ValueOfLocal(HBasicBlock* block, size_t local) { 61 return GetLocalsFor(block)->Get(local); 62 } 63 64 void SsaBuilder::VisitBasicBlock(HBasicBlock* block) { 65 current_locals_ = GetLocalsFor(block); 66 67 if (block->IsLoopHeader()) { 68 // If the block is a loop header, we know we only have visited the pre header 69 // because we are visiting in reverse post order. We create phis for all initialized 70 // locals from the pre header. Their inputs will be populated at the end of 71 // the analysis. 72 for (size_t local = 0; local < current_locals_->Size(); local++) { 73 HInstruction* incoming = ValueOfLocal(block->GetLoopInformation()->GetPreHeader(), local); 74 if (incoming != nullptr) { 75 HPhi* phi = new (GetGraph()->GetArena()) HPhi( 76 GetGraph()->GetArena(), local, 0, Primitive::kPrimVoid); 77 block->AddPhi(phi); 78 current_locals_->Put(local, phi); 79 } 80 } 81 // Save the loop header so that the last phase of the analysis knows which 82 // blocks need to be updated. 83 loop_headers_.Add(block); 84 } else if (block->GetPredecessors().Size() > 0) { 85 // All predecessors have already been visited because we are visiting in reverse post order. 86 // We merge the values of all locals, creating phis if those values differ. 87 for (size_t local = 0; local < current_locals_->Size(); local++) { 88 bool one_predecessor_has_no_value = false; 89 bool is_different = false; 90 HInstruction* value = ValueOfLocal(block->GetPredecessors().Get(0), local); 91 92 for (size_t i = 0, e = block->GetPredecessors().Size(); i < e; ++i) { 93 HInstruction* current = ValueOfLocal(block->GetPredecessors().Get(i), local); 94 if (current == nullptr) { 95 one_predecessor_has_no_value = true; 96 break; 97 } else if (current != value) { 98 is_different = true; 99 } 100 } 101 102 if (one_predecessor_has_no_value) { 103 // If one predecessor has no value for this local, we trust the verifier has 104 // successfully checked that there is a store dominating any read after this block. 105 continue; 106 } 107 108 if (is_different) { 109 HPhi* phi = new (GetGraph()->GetArena()) HPhi( 110 GetGraph()->GetArena(), local, block->GetPredecessors().Size(), Primitive::kPrimVoid); 111 for (size_t i = 0; i < block->GetPredecessors().Size(); i++) { 112 HInstruction* value = ValueOfLocal(block->GetPredecessors().Get(i), local); 113 phi->SetRawInputAt(i, value); 114 } 115 block->AddPhi(phi); 116 value = phi; 117 } 118 current_locals_->Put(local, value); 119 } 120 } 121 122 // Visit all instructions. The instructions of interest are: 123 // - HLoadLocal: replace them with the current value of the local. 124 // - HStoreLocal: update current value of the local and remove the instruction. 125 // - Instructions that require an environment: populate their environment 126 // with the current values of the locals. 127 for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) { 128 it.Current()->Accept(this); 129 } 130 } 131 132 void SsaBuilder::VisitLoadLocal(HLoadLocal* load) { 133 load->ReplaceWith(current_locals_->Get(load->GetLocal()->GetRegNumber())); 134 load->GetBlock()->RemoveInstruction(load); 135 } 136 137 void SsaBuilder::VisitStoreLocal(HStoreLocal* store) { 138 current_locals_->Put(store->GetLocal()->GetRegNumber(), store->InputAt(1)); 139 store->GetBlock()->RemoveInstruction(store); 140 } 141 142 void SsaBuilder::VisitInstruction(HInstruction* instruction) { 143 if (!instruction->NeedsEnvironment()) { 144 return; 145 } 146 HEnvironment* environment = new (GetGraph()->GetArena()) HEnvironment( 147 GetGraph()->GetArena(), current_locals_->Size()); 148 environment->Populate(*current_locals_); 149 instruction->SetEnvironment(environment); 150 } 151 152 } // namespace art 153
