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src/compiler/js-context-specialization.cc

+// Copyright 2014 the V8 project authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "src/compiler/common-operator.h"
+#include "src/compiler/generic-node-inl.h"
+#include "src/compiler/js-context-specialization.h"
+#include "src/compiler/js-operator.h"
+#include "src/compiler/node-aux-data-inl.h"
+#include "src/compiler/node-matchers.h"
+#include "src/compiler/node-properties-inl.h"
+
+namespace v8 {
+namespace internal {
+namespace compiler {
+
+// TODO(titzer): factor this out to a common routine with js-typed-lowering.
+static void ReplaceEffectfulWithValue(Node* node, Node* value) {
+  Node* effect = NodeProperties::GetEffectInput(node);
+
+  // Requires distinguishing between value and effect edges.
+  UseIter iter = node->uses().begin();
+  while (iter != node->uses().end()) {
+    if (NodeProperties::IsEffectEdge(iter.edge())) {
+      iter = iter.UpdateToAndIncrement(effect);
+    } else {
+      iter = iter.UpdateToAndIncrement(value);
+    }
+  }
+}
+
+
+void JSContextSpecializer::SpecializeToContext() {
+  ValueMatcher<Handle<Context> > match(context_);
+
+  // Iterate over all uses of the context and try to replace {LoadContext}
+  // nodes with their values from the constant context.
+  UseIter iter = match.node()->uses().begin();
+  while (iter != match.node()->uses().end()) {
+    Node* use = *iter;
+    if (use->opcode() == IrOpcode::kJSLoadContext) {
+      Reduction r = ReduceJSLoadContext(use);
+      if (r.Changed() && r.replacement() != use) {
+        ReplaceEffectfulWithValue(use, r.replacement());
+      }
+    }
+    ++iter;
+  }
+}
+
+
+Reduction JSContextSpecializer::ReduceJSLoadContext(Node* node) {
+  ASSERT_EQ(IrOpcode::kJSLoadContext, node->opcode());
+
+  ContextAccess access =
+      static_cast<Operator1<ContextAccess>*>(node->op())->parameter();
+
+  // Find the right parent context.
+  Context* context = *info_->context();
+  for (int i = access.depth(); i > 0; --i) {
+    context = context->previous();
+  }
+
+  // If the access itself is mutable, only fold-in the parent.
+  if (!access.immutable()) {
+    // The access does not have to look up a parent, nothing to fold.
+    if (access.depth() == 0) {
+      return Reducer::NoChange();
+    }
+    Operator* op = jsgraph_->javascript()->LoadContext(0, access.index(),
+                                                       access.immutable());
+    node->set_op(op);
+    Handle<Object> context_handle = Handle<Object>(context, info_->isolate());
+    node->ReplaceInput(0, jsgraph_->Constant(context_handle));
+    return Reducer::Changed(node);
+  }
+  Handle<Object> value =
+      Handle<Object>(context->get(access.index()), info_->isolate());
+
+  // Even though the context slot is immutable, the context might have escaped
+  // before the function to which it belongs has initialized the slot.
+  // We must be conservative and check if the value in the slot is currently the
+  // hole or undefined. If it is neither of these, then it must be initialized.
+  if (value->IsUndefined() || value->IsTheHole()) return Reducer::NoChange();
+
+  // Success. The context load can be replaced with the constant.
+  // TODO(titzer): record the specialization for sharing code across multiple
+  // contexts that have the same value in the corresponding context slot.
+  return Reducer::Replace(jsgraph_->Constant(value));
+}
+}
+}
+}  // namespace v8::internal::compiler