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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     if (phi->HasMergedIndex()) {
    122       block->RecordDeletedPhi(phi->merged_index());
    123     }
    124   }
    125 }
    126 
    127 } }  // namespace v8::internal
    128