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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/hydrogen-store-elimination.h"
      6 #include "src/hydrogen-instructions.h"
      7 
      8 namespace v8 {
      9 namespace internal {
     10 
     11 #define TRACE(x) if (FLAG_trace_store_elimination) PrintF x
     12 
     13 // Performs a block-by-block local analysis for removable stores.
     14 void HStoreEliminationPhase::Run() {
     15   GVNFlagSet flags;  // Use GVN flags as an approximation for some instructions.
     16   flags.RemoveAll();
     17 
     18   flags.Add(kArrayElements);
     19   flags.Add(kArrayLengths);
     20   flags.Add(kStringLengths);
     21   flags.Add(kBackingStoreFields);
     22   flags.Add(kDoubleArrayElements);
     23   flags.Add(kDoubleFields);
     24   flags.Add(kElementsPointer);
     25   flags.Add(kInobjectFields);
     26   flags.Add(kExternalMemory);
     27   flags.Add(kStringChars);
     28   flags.Add(kTypedArrayElements);
     29 
     30   for (int i = 0; i < graph()->blocks()->length(); i++) {
     31     unobserved_.Rewind(0);
     32     HBasicBlock* block = graph()->blocks()->at(i);
     33     if (!block->IsReachable()) continue;
     34     for (HInstructionIterator it(block); !it.Done(); it.Advance()) {
     35       HInstruction* instr = it.Current();
     36       if (instr->CheckFlag(HValue::kIsDead)) continue;
     37 
     38       // TODO(titzer): eliminate unobserved HStoreKeyed instructions too.
     39       switch (instr->opcode()) {
     40         case HValue::kStoreNamedField:
     41           // Remove any unobserved stores overwritten by this store.
     42           ProcessStore(HStoreNamedField::cast(instr));
     43           break;
     44         case HValue::kLoadNamedField:
     45           // Observe any unobserved stores on this object + field.
     46           ProcessLoad(HLoadNamedField::cast(instr));
     47           break;
     48         default:
     49           ProcessInstr(instr, flags);
     50           break;
     51       }
     52     }
     53   }
     54 }
     55 
     56 
     57 void HStoreEliminationPhase::ProcessStore(HStoreNamedField* store) {
     58   HValue* object = store->object()->ActualValue();
     59   int i = 0;
     60   while (i < unobserved_.length()) {
     61     HStoreNamedField* prev = unobserved_.at(i);
     62     if (aliasing_->MustAlias(object, prev->object()->ActualValue()) &&
     63         prev->CanBeReplacedWith(store)) {
     64       // This store is guaranteed to overwrite the previous store.
     65       prev->DeleteAndReplaceWith(NULL);
     66       TRACE(("++ Unobserved store S%d overwritten by S%d\n",
     67              prev->id(), store->id()));
     68       unobserved_.Remove(i);
     69     } else {
     70       // TODO(titzer): remove map word clearing from folded allocations.
     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 v8::internal
    122