ssa_phi_elimination.cc revision 77b022dfb8e73564b00c4724f7078cb1d5a57a65
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_phi_elimination.h" 18 19namespace art { 20 21void SsaDeadPhiElimination::Run() { 22 MarkDeadPhis(); 23 EliminateDeadPhis(); 24} 25 26void SsaDeadPhiElimination::MarkDeadPhis() { 27 // Add to the worklist phis referenced by non-phi instructions. 28 for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) { 29 HBasicBlock* block = it.Current(); 30 for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) { 31 HPhi* phi = inst_it.Current()->AsPhi(); 32 // Set dead ahead of running through uses. The phi may have no use. 33 phi->SetDead(); 34 for (HUseIterator<HInstruction*> use_it(phi->GetUses()); !use_it.Done(); use_it.Advance()) { 35 HUseListNode<HInstruction*>* current = use_it.Current(); 36 HInstruction* user = current->GetUser(); 37 if (!user->IsPhi()) { 38 worklist_.Add(phi); 39 phi->SetLive(); 40 break; 41 } 42 } 43 } 44 } 45 46 // Process the worklist by propagating liveness to phi inputs. 47 while (!worklist_.IsEmpty()) { 48 HPhi* phi = worklist_.Pop(); 49 for (HInputIterator it(phi); !it.Done(); it.Advance()) { 50 HInstruction* input = it.Current(); 51 if (input->IsPhi() && input->AsPhi()->IsDead()) { 52 worklist_.Add(input->AsPhi()); 53 input->AsPhi()->SetLive(); 54 } 55 } 56 } 57} 58 59void SsaDeadPhiElimination::EliminateDeadPhis() { 60 // Remove phis that are not live. Visit in post order so that phis 61 // that are not inputs of loop phis can be removed when they have 62 // no users left (dead phis might use dead phis). 63 for (HPostOrderIterator it(*graph_); !it.Done(); it.Advance()) { 64 HBasicBlock* block = it.Current(); 65 HInstruction* current = block->GetFirstPhi(); 66 HInstruction* next = nullptr; 67 HPhi* phi; 68 while (current != nullptr) { 69 phi = current->AsPhi(); 70 next = current->GetNext(); 71 if (phi->IsDead()) { 72 // Make sure the phi is only used by other dead phis. 73 if (kIsDebugBuild) { 74 for (HUseIterator<HInstruction*> use_it(phi->GetUses()); !use_it.Done(); 75 use_it.Advance()) { 76 HInstruction* user = use_it.Current()->GetUser(); 77 DCHECK(user->IsLoopHeaderPhi()) << user->GetId(); 78 DCHECK(user->AsPhi()->IsDead()) << user->GetId(); 79 } 80 } 81 // Remove the phi from use lists of its inputs. 82 for (size_t i = 0, e = phi->InputCount(); i < e; ++i) { 83 phi->RemoveAsUserOfInput(i); 84 } 85 // Remove the phi from environments that use it. 86 for (HUseIterator<HEnvironment*> use_it(phi->GetEnvUses()); !use_it.Done(); 87 use_it.Advance()) { 88 HUseListNode<HEnvironment*>* user_node = use_it.Current(); 89 HEnvironment* user = user_node->GetUser(); 90 user->SetRawEnvAt(user_node->GetIndex(), nullptr); 91 } 92 // Delete it from the instruction list. 93 block->RemovePhi(phi, /*ensure_safety=*/ false); 94 } 95 current = next; 96 } 97 } 98} 99 100void SsaRedundantPhiElimination::Run() { 101 // Add all phis in the worklist. Order does not matter for correctness, and 102 // neither will necessarily converge faster. 103 for (HReversePostOrderIterator it(*graph_); !it.Done(); it.Advance()) { 104 HBasicBlock* block = it.Current(); 105 for (HInstructionIterator inst_it(block->GetPhis()); !inst_it.Done(); inst_it.Advance()) { 106 worklist_.Add(inst_it.Current()->AsPhi()); 107 } 108 } 109 110 while (!worklist_.IsEmpty()) { 111 HPhi* phi = worklist_.Pop(); 112 113 // If the phi has already been processed, continue. 114 if (!phi->IsInBlock()) { 115 continue; 116 } 117 118 if (phi->InputCount() == 0) { 119 DCHECK(phi->IsCatchPhi()); 120 DCHECK(phi->IsDead()); 121 continue; 122 } 123 124 // Find if the inputs of the phi are the same instruction. 125 HInstruction* candidate = phi->InputAt(0); 126 // A loop phi cannot have itself as the first phi. Note that this 127 // check relies on our simplification pass ensuring the pre-header 128 // block is first in the list of predecessors of the loop header. 129 DCHECK(!phi->IsLoopHeaderPhi() || phi->GetBlock()->IsLoopPreHeaderFirstPredecessor()); 130 DCHECK_NE(phi, candidate); 131 132 for (size_t i = 1; i < phi->InputCount(); ++i) { 133 HInstruction* input = phi->InputAt(i); 134 // For a loop phi, if the input is the phi, the phi is still candidate for 135 // elimination. 136 if (input != candidate && input != phi) { 137 candidate = nullptr; 138 break; 139 } 140 } 141 142 // If the inputs are not the same, continue. 143 if (candidate == nullptr) { 144 continue; 145 } 146 147 // The candidate may not dominate a phi in a catch block. 148 if (phi->IsCatchPhi() && !candidate->StrictlyDominates(phi)) { 149 continue; 150 } 151 152 // Because we're updating the users of this phi, we may have new candidates 153 // for elimination. Add phis that use this phi to the worklist. 154 for (HUseIterator<HInstruction*> it(phi->GetUses()); !it.Done(); it.Advance()) { 155 HUseListNode<HInstruction*>* current = it.Current(); 156 HInstruction* user = current->GetUser(); 157 if (user->IsPhi()) { 158 worklist_.Add(user->AsPhi()); 159 } 160 } 161 162 phi->ReplaceWith(candidate); 163 phi->GetBlock()->RemovePhi(phi); 164 } 165} 166 167} // namespace art 168