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      1 // Copyright 2013 the V8 project authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #include "src/crankshaft/hydrogen-store-elimination.h"
      6 
      7 #include "src/crankshaft/hydrogen-instructions.h"
      8 #include "src/objects-inl.h"
      9 
     10 namespace v8 {
     11 namespace internal {
     12 
     13 #define TRACE(x) if (FLAG_trace_store_elimination) PrintF x
     14 
     15 // Performs a block-by-block local analysis for removable stores.
     16 void HStoreEliminationPhase::Run() {
     17   GVNFlagSet flags;  // Use GVN flags as an approximation for some instructions.
     18   flags.RemoveAll();
     19 
     20   flags.Add(kArrayElements);
     21   flags.Add(kArrayLengths);
     22   flags.Add(kStringLengths);
     23   flags.Add(kBackingStoreFields);
     24   flags.Add(kDoubleArrayElements);
     25   flags.Add(kDoubleFields);
     26   flags.Add(kElementsPointer);
     27   flags.Add(kInobjectFields);
     28   flags.Add(kExternalMemory);
     29   flags.Add(kStringChars);
     30   flags.Add(kTypedArrayElements);
     31 
     32   for (int i = 0; i < graph()->blocks()->length(); i++) {
     33     unobserved_.Rewind(0);
     34     HBasicBlock* block = graph()->blocks()->at(i);
     35     if (!block->IsReachable()) continue;
     36     for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
     37       HInstruction* instr = it.Current();
     38       if (instr->CheckFlag(HValue::kIsDead)) continue;
     39 
     40       switch (instr->opcode()) {
     41         case HValue::kStoreNamedField:
     42           // Remove any unobserved stores overwritten by this store.
     43           ProcessStore(HStoreNamedField::cast(instr));
     44           break;
     45         case HValue::kLoadNamedField:
     46           // Observe any unobserved stores on this object + field.
     47           ProcessLoad(HLoadNamedField::cast(instr));
     48           break;
     49         default:
     50           ProcessInstr(instr, flags);
     51           break;
     52       }
     53     }
     54   }
     55 }
     56 
     57 
     58 void HStoreEliminationPhase::ProcessStore(HStoreNamedField* store) {
     59   HValue* object = store->object()->ActualValue();
     60   int i = 0;
     61   while (i < unobserved_.length()) {
     62     HStoreNamedField* prev = unobserved_.at(i);
     63     if (aliasing_->MustAlias(object, prev->object()->ActualValue()) &&
     64         prev->CanBeReplacedWith(store)) {
     65       // This store is guaranteed to overwrite the previous store.
     66       prev->DeleteAndReplaceWith(NULL);
     67       TRACE(("++ Unobserved store S%d overwritten by S%d\n",
     68              prev->id(), store->id()));
     69       unobserved_.Remove(i);
     70     } else {
     71       i++;
     72     }
     73   }
     74   // Only non-transitioning stores are removable.
     75   if (!store->has_transition()) {
     76     TRACE(("-- Might remove store S%d\n", store->id()));
     77     unobserved_.Add(store, zone());
     78   }
     79 }
     80 
     81 
     82 void HStoreEliminationPhase::ProcessLoad(HLoadNamedField* load) {
     83   HValue* object = load->object()->ActualValue();
     84   int i = 0;
     85   while (i < unobserved_.length()) {
     86     HStoreNamedField* prev = unobserved_.at(i);
     87     if (aliasing_->MayAlias(object, prev->object()->ActualValue()) &&
     88         load->access().Equals(prev->access())) {
     89       TRACE(("-- Observed store S%d by load L%d\n", prev->id(), load->id()));
     90       unobserved_.Remove(i);
     91     } else {
     92       i++;
     93     }
     94   }
     95 }
     96 
     97 
     98 void HStoreEliminationPhase::ProcessInstr(HInstruction* instr,
     99     GVNFlagSet flags) {
    100   if (unobserved_.length() == 0) return;  // Nothing to do.
    101   if (instr->CanDeoptimize()) {
    102     TRACE(("-- Observed stores at I%d (%s might deoptimize)\n",
    103            instr->id(), instr->Mnemonic()));
    104     unobserved_.Rewind(0);
    105     return;
    106   }
    107   if (instr->CheckChangesFlag(kNewSpacePromotion)) {
    108     TRACE(("-- Observed stores at I%d (%s might GC)\n",
    109            instr->id(), instr->Mnemonic()));
    110     unobserved_.Rewind(0);
    111     return;
    112   }
    113   if (instr->DependsOnFlags().ContainsAnyOf(flags)) {
    114     TRACE(("-- Observed stores at I%d (GVN flags of %s)\n",
    115            instr->id(), instr->Mnemonic()));
    116     unobserved_.Rewind(0);
    117     return;
    118   }
    119 }
    120 
    121 }  // namespace internal
    122 }  // namespace v8
    123