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| 1 // Copyright 2013 the V8 project authors. All rights reserved. |
| 2 // Redistribution and use in source and binary forms, with or without |
| 3 // modification, are permitted provided that the following conditions are |
| 4 // met: |
| 5 // |
| 6 // * Redistributions of source code must retain the above copyright |
| 7 // notice, this list of conditions and the following disclaimer. |
| 8 // * Redistributions in binary form must reproduce the above |
| 9 // copyright notice, this list of conditions and the following |
| 10 // disclaimer in the documentation and/or other materials provided |
| 11 // with the distribution. |
| 12 // * Neither the name of Google Inc. nor the names of its |
| 13 // contributors may be used to endorse or promote products derived |
| 14 // from this software without specific prior written permission. |
| 15 // |
| 16 // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS |
| 17 // "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT |
| 18 // LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR |
| 19 // A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT |
| 20 // OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, |
| 21 // SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT |
| 22 // LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, |
| 23 // DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY |
| 24 // THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT |
| 25 // (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE |
| 26 // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. |
| 27 |
| 28 #include "hydrogen-dce.h" |
| 29 #include "v8.h" |
| 30 |
| 31 namespace v8 { |
| 32 namespace internal { |
| 33 |
| 34 bool HDeadCodeEliminationPhase::MarkLive(HValue* ref, HValue* instr) { |
| 35 if (instr->CheckFlag(HValue::kIsLive)) return false; |
| 36 instr->SetFlag(HValue::kIsLive); |
| 37 |
| 38 if (FLAG_trace_dead_code_elimination) { |
| 39 HeapStringAllocator allocator; |
| 40 StringStream stream(&allocator); |
| 41 if (ref != NULL) { |
| 42 ref->PrintTo(&stream); |
| 43 } else { |
| 44 stream.Add("root "); |
| 45 } |
| 46 stream.Add(" -> "); |
| 47 instr->PrintTo(&stream); |
| 48 PrintF("[MarkLive %s]\n", *stream.ToCString()); |
| 49 } |
| 50 |
| 51 return true; |
| 52 } |
| 53 |
| 54 |
| 55 void HDeadCodeEliminationPhase::MarkLiveInstructions() { |
| 56 ZoneList<HValue*> worklist(graph()->blocks()->length(), zone()); |
| 57 |
| 58 // Mark initial root instructions for dead code elimination. |
| 59 for (int i = 0; i < graph()->blocks()->length(); ++i) { |
| 60 HBasicBlock* block = graph()->blocks()->at(i); |
| 61 for (HInstructionIterator it(block); !it.Done(); it.Advance()) { |
| 62 HInstruction* instr = it.Current(); |
| 63 if (instr->CannotBeEliminated() && MarkLive(NULL, instr)) { |
| 64 worklist.Add(instr, zone()); |
| 65 } |
| 66 } |
| 67 for (int j = 0; j < block->phis()->length(); j++) { |
| 68 HPhi* phi = block->phis()->at(j); |
| 69 if (phi->CannotBeEliminated() && MarkLive(NULL, phi)) { |
| 70 worklist.Add(phi, zone()); |
| 71 } |
| 72 } |
| 73 } |
| 74 |
| 75 // Transitively mark all inputs of live instructions live. |
| 76 while (!worklist.is_empty()) { |
| 77 HValue* instr = worklist.RemoveLast(); |
| 78 for (int i = 0; i < instr->OperandCount(); ++i) { |
| 79 if (MarkLive(instr, instr->OperandAt(i))) { |
| 80 worklist.Add(instr->OperandAt(i), zone()); |
| 81 } |
| 82 } |
| 83 } |
| 84 } |
| 85 |
| 86 |
| 87 void HDeadCodeEliminationPhase::RemoveDeadInstructions() { |
| 88 ZoneList<HPhi*> worklist(graph()->blocks()->length(), zone()); |
| 89 |
| 90 // Remove any instruction not marked kIsLive. |
| 91 for (int i = 0; i < graph()->blocks()->length(); ++i) { |
| 92 HBasicBlock* block = graph()->blocks()->at(i); |
| 93 for (HInstructionIterator it(block); !it.Done(); it.Advance()) { |
| 94 HInstruction* instr = it.Current(); |
| 95 if (!instr->CheckFlag(HValue::kIsLive)) { |
| 96 // Instruction has not been marked live; assume it is dead and remove. |
| 97 // TODO(titzer): we don't remove constants because some special ones |
| 98 // might be used by later phases and are assumed to be in the graph |
| 99 if (!instr->IsConstant()) instr->DeleteAndReplaceWith(NULL); |
| 100 } else { |
| 101 // Clear the liveness flag to leave the graph clean for the next DCE. |
| 102 instr->ClearFlag(HValue::kIsLive); |
| 103 } |
| 104 } |
| 105 // Collect phis that are dead and remove them in the next pass. |
| 106 for (int j = 0; j < block->phis()->length(); j++) { |
| 107 HPhi* phi = block->phis()->at(j); |
| 108 if (!phi->CheckFlag(HValue::kIsLive)) { |
| 109 worklist.Add(phi, zone()); |
| 110 } else { |
| 111 phi->ClearFlag(HValue::kIsLive); |
| 112 } |
| 113 } |
| 114 } |
| 115 |
| 116 // Process phis separately to avoid simultaneously mutating the phi list. |
| 117 while (!worklist.is_empty()) { |
| 118 HPhi* phi = worklist.RemoveLast(); |
| 119 HBasicBlock* block = phi->block(); |
| 120 phi->DeleteAndReplaceWith(NULL); |
| 121 block->RecordDeletedPhi(phi->merged_index()); |
| 122 } |
| 123 } |
| 124 |
| 125 } } // namespace v8::internal |
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