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      1 // Copyright 2014 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/compiler/common-operator.h"
      6 #include "src/compiler/generic-node-inl.h"
      7 #include "src/compiler/graph-inl.h"
      8 #include "src/compiler/js-context-specialization.h"
      9 #include "src/compiler/js-operator.h"
     10 #include "src/compiler/node-aux-data-inl.h"
     11 #include "src/compiler/node-matchers.h"
     12 #include "src/compiler/node-properties-inl.h"
     13 
     14 namespace v8 {
     15 namespace internal {
     16 namespace compiler {
     17 
     18 class ContextSpecializationVisitor : public NullNodeVisitor {
     19  public:
     20   explicit ContextSpecializationVisitor(JSContextSpecializer* spec)
     21       : spec_(spec) {}
     22 
     23   GenericGraphVisit::Control Post(Node* node) {
     24     switch (node->opcode()) {
     25       case IrOpcode::kJSLoadContext: {
     26         Reduction r = spec_->ReduceJSLoadContext(node);
     27         if (r.Changed() && r.replacement() != node) {
     28           NodeProperties::ReplaceWithValue(node, r.replacement());
     29           node->RemoveAllInputs();
     30         }
     31         break;
     32       }
     33       case IrOpcode::kJSStoreContext: {
     34         Reduction r = spec_->ReduceJSStoreContext(node);
     35         if (r.Changed() && r.replacement() != node) {
     36           NodeProperties::ReplaceWithValue(node, r.replacement());
     37           node->RemoveAllInputs();
     38         }
     39         break;
     40       }
     41       default:
     42         break;
     43     }
     44     return GenericGraphVisit::CONTINUE;
     45   }
     46 
     47  private:
     48   JSContextSpecializer* spec_;
     49 };
     50 
     51 
     52 void JSContextSpecializer::SpecializeToContext() {
     53   NodeProperties::ReplaceWithValue(context_,
     54                                    jsgraph_->Constant(info_->context()));
     55 
     56   ContextSpecializationVisitor visitor(this);
     57   jsgraph_->graph()->VisitNodeInputsFromEnd(&visitor);
     58 }
     59 
     60 
     61 Reduction JSContextSpecializer::ReduceJSLoadContext(Node* node) {
     62   DCHECK_EQ(IrOpcode::kJSLoadContext, node->opcode());
     63 
     64   HeapObjectMatcher<Context> m(NodeProperties::GetValueInput(node, 0));
     65   // If the context is not constant, no reduction can occur.
     66   if (!m.HasValue()) {
     67     return Reducer::NoChange();
     68   }
     69 
     70   ContextAccess access = OpParameter<ContextAccess>(node);
     71 
     72   // Find the right parent context.
     73   Context* context = *m.Value().handle();
     74   for (int i = access.depth(); i > 0; --i) {
     75     context = context->previous();
     76   }
     77 
     78   // If the access itself is mutable, only fold-in the parent.
     79   if (!access.immutable()) {
     80     // The access does not have to look up a parent, nothing to fold.
     81     if (access.depth() == 0) {
     82       return Reducer::NoChange();
     83     }
     84     const Operator* op = jsgraph_->javascript()->LoadContext(
     85         0, access.index(), access.immutable());
     86     node->set_op(op);
     87     Handle<Object> context_handle = Handle<Object>(context, info_->isolate());
     88     node->ReplaceInput(0, jsgraph_->Constant(context_handle));
     89     return Reducer::Changed(node);
     90   }
     91   Handle<Object> value =
     92       Handle<Object>(context->get(access.index()), info_->isolate());
     93 
     94   // Even though the context slot is immutable, the context might have escaped
     95   // before the function to which it belongs has initialized the slot.
     96   // We must be conservative and check if the value in the slot is currently the
     97   // hole or undefined. If it is neither of these, then it must be initialized.
     98   if (value->IsUndefined() || value->IsTheHole()) {
     99     return Reducer::NoChange();
    100   }
    101 
    102   // Success. The context load can be replaced with the constant.
    103   // TODO(titzer): record the specialization for sharing code across multiple
    104   // contexts that have the same value in the corresponding context slot.
    105   return Reducer::Replace(jsgraph_->Constant(value));
    106 }
    107 
    108 
    109 Reduction JSContextSpecializer::ReduceJSStoreContext(Node* node) {
    110   DCHECK_EQ(IrOpcode::kJSStoreContext, node->opcode());
    111 
    112   HeapObjectMatcher<Context> m(NodeProperties::GetValueInput(node, 0));
    113   // If the context is not constant, no reduction can occur.
    114   if (!m.HasValue()) {
    115     return Reducer::NoChange();
    116   }
    117 
    118   ContextAccess access = OpParameter<ContextAccess>(node);
    119 
    120   // The access does not have to look up a parent, nothing to fold.
    121   if (access.depth() == 0) {
    122     return Reducer::NoChange();
    123   }
    124 
    125   // Find the right parent context.
    126   Context* context = *m.Value().handle();
    127   for (int i = access.depth(); i > 0; --i) {
    128     context = context->previous();
    129   }
    130 
    131   const Operator* op = jsgraph_->javascript()->StoreContext(0, access.index());
    132   node->set_op(op);
    133   Handle<Object> new_context_handle = Handle<Object>(context, info_->isolate());
    134   node->ReplaceInput(0, jsgraph_->Constant(new_context_handle));
    135 
    136   return Reducer::Changed(node);
    137 }
    138 
    139 }  // namespace compiler
    140 }  // namespace internal
    141 }  // namespace v8
    142