[compiler] Generalize JSContextSpecialization.

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/js-context-specialization.h"
 
 #include "src/compiler/common-operator.h"
 #include "src/compiler/js-graph.h"
 #include "src/compiler/js-operator.h"
 #include "src/compiler/node-matchers.h"
@@ skipping 10 matching lines @@
     case IrOpcode::kJSLoadContext:
       return ReduceJSLoadContext(node);
     case IrOpcode::kJSStoreContext:
       return ReduceJSStoreContext(node);
     default:
       break;
   }
   return NoChange();
 }
 
+namespace {
 
-MaybeHandle<Context> JSContextSpecialization::GetSpecializationContext(
-    Node* node) {
-  DCHECK(node->opcode() == IrOpcode::kJSLoadContext ||
-         node->opcode() == IrOpcode::kJSStoreContext);
-  Node* const object = NodeProperties::GetContextInput(node);
-  return NodeProperties::GetSpecializationContext(object, context());
+Node* GetOuterContext(Node* node, size_t* depth) {
+  Node* context = NodeProperties::GetContextInput(node);
+  while (*depth > 0 && IrOpcode::IsContextExtendingOpcode(context->opcode())) {
+    context = NodeProperties::GetContextInput(context);
+    (*depth)--;
+  }
+  return context;
+}  // XXX: Move to node-properties?

[Michael Starzinger, 2017/01/13 13:36:26]

nit: s/XXX/TODO/ here. Also maybe move to in front of the function.

[neis, 2017/01/13 14:08:02]

I actually moved it to NodeProperties now.

+
+}  // anonymous namespace
+
+Reduction JSContextSpecialization::StrengthenJSLoadContext(Node* node,

[Michael Starzinger, 2017/01/13 13:36:26]

random non-actionable rambling: As discussed offline, the name "strengthen"
confused me at first. In my interpretation of terminology this performs a
"reduction of strength". My interpretation might be completely wrong though.

[neis, 2017/01/13 14:08:02]

Renamed to SimplifyJS...

+                                                           Node* new_context,
+                                                           size_t new_depth) {
+  DCHECK_EQ(IrOpcode::kJSLoadContext, node->opcode());
+  const ContextAccess& access = ContextAccessOf(node->op());
+  DCHECK_LE(new_depth, access.depth());
+
+  if (new_depth == access.depth() &&
+      new_context == NodeProperties::GetContextInput(node)) {
+    return NoChange();
+  }
+
+  const Operator* op = jsgraph_->javascript()->LoadContext(
+      new_depth, access.index(), access.immutable());
+  NodeProperties::ReplaceContextInput(node, new_context);
+  NodeProperties::ChangeOp(node, op);
+  return Changed(node);
 }
 
+Reduction JSContextSpecialization::StrengthenJSStoreContext(Node* node,
+                                                            Node* new_context,
+                                                            size_t new_depth) {
+  DCHECK_EQ(IrOpcode::kJSStoreContext, node->opcode());
+  const ContextAccess& access = ContextAccessOf(node->op());
+  DCHECK_LE(new_depth, access.depth());
+
+  if (new_depth == access.depth() &&
+      new_context == NodeProperties::GetContextInput(node)) {
+    return NoChange();
+  }
+
+  const Operator* op =
+      jsgraph_->javascript()->StoreContext(new_depth, access.index());
+  NodeProperties::ReplaceContextInput(node, new_context);
+  NodeProperties::ChangeOp(node, op);
+  return Changed(node);
+}
 
 Reduction JSContextSpecialization::ReduceJSLoadContext(Node* node) {
   DCHECK_EQ(IrOpcode::kJSLoadContext, node->opcode());
 
-  // Get the specialization context from the node.
-  Handle<Context> context;
-  if (!GetSpecializationContext(node).ToHandle(&context)) return NoChange();
+  const ContextAccess& access = ContextAccessOf(node->op());
+  size_t depth = access.depth();
 
-  // Find the right parent context.
-  const ContextAccess& access = ContextAccessOf(node->op());
-  for (size_t i = access.depth(); i > 0; --i) {
-    context = handle(context->previous(), isolate());
+  // First walk up the context chain in the graph until we reduce the depth to 0
+  // or hit a node that does not have a CreateXYZContext operator.
+  Node* outer = GetOuterContext(node, &depth);
+
+  Handle<Context> concrete;
+  if (!NodeProperties::GetSpecializationContext(outer, context())
+           .ToHandle(&concrete)) {
+    // We do not have a concrete context object, so we can only partially reduce
+    // the load by folding-in the outer context node.
+    return StrengthenJSLoadContext(node, outer, depth);
   }
 
-  // If the access itself is mutable, only fold-in the parent.
+  // Now walk up the concrete context chain for the remaining depth.
+  for (; depth > 0; --depth) {
+    concrete = handle(concrete->previous(), isolate());
+  }
+
   if (!access.immutable()) {
-    // The access does not have to look up a parent, nothing to fold.
-    if (access.depth() == 0) {
-      return NoChange();
-    }
-    const Operator* op = jsgraph_->javascript()->LoadContext(
-        0, access.index(), access.immutable());
-    NodeProperties::ReplaceContextInput(node, jsgraph_->Constant(context));
-    NodeProperties::ChangeOp(node, op);
-    return Changed(node);
+    // We found the requested context object but since the context slot is
+    // mutable we can only partially reduce the load.
+    return StrengthenJSLoadContext(node, jsgraph()->Constant(concrete), depth);
   }
-  Handle<Object> value =
-      handle(context->get(static_cast<int>(access.index())), 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.
+  // We must be conservative and check if the value in the slot is currently
+  // the hole or undefined. Only if it is neither of these, can we be sure that
+  // it won't change anymore.
+  Handle<Object> value(concrete->get(static_cast<int>(access.index())),
+                       isolate());
   if (value->IsUndefined(isolate()) || value->IsTheHole(isolate())) {
-    return NoChange();
+    return StrengthenJSLoadContext(node, jsgraph()->Constant(concrete), depth);
   }
 
   // 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.
   Node* constant = jsgraph_->Constant(value);
   ReplaceWithValue(node, constant);
   return Replace(constant);
 }
 
 
 Reduction JSContextSpecialization::ReduceJSStoreContext(Node* node) {
   DCHECK_EQ(IrOpcode::kJSStoreContext, node->opcode());
 
-  // Get the specialization context from the node.
-  Handle<Context> context;
-  if (!GetSpecializationContext(node).ToHandle(&context)) return NoChange();
+  const ContextAccess& access = ContextAccessOf(node->op());
+  size_t depth = access.depth();
 
-  // The access does not have to look up a parent, nothing to fold.
-  const ContextAccess& access = ContextAccessOf(node->op());
-  if (access.depth() == 0) {
-    return NoChange();
+  // First walk up the context chain in the graph until we reduce the depth to 0
+  // or hit a node that does not have a CreateXYZContext operator.
+  Node* outer = GetOuterContext(node, &depth);
+
+  Handle<Context> concrete;
+  if (!NodeProperties::GetSpecializationContext(outer, context())
+           .ToHandle(&concrete)) {
+    // We do not have a concrete context object, so we can only partially reduce
+    // the load by folding-in the outer context node.
+    return StrengthenJSStoreContext(node, outer, depth);
   }
 
-  // Find the right parent context.
-  for (size_t i = access.depth(); i > 0; --i) {
-    context = handle(context->previous(), isolate());
+  // Now walk up the concrete context chain for the remaining depth.
+  for (; depth > 0; --depth) {
+    concrete = handle(concrete->previous(), isolate());
   }
 
-  NodeProperties::ReplaceContextInput(node, jsgraph_->Constant(context));
-  NodeProperties::ChangeOp(node, javascript()->StoreContext(0, access.index()));
-  return Changed(node);
+  return StrengthenJSStoreContext(node, jsgraph()->Constant(concrete), depth);
 }
 
 
 Isolate* JSContextSpecialization::isolate() const {
   return jsgraph()->isolate();
 }
 
 
 JSOperatorBuilder* JSContextSpecialization::javascript() const {
   return jsgraph()->javascript();
 }
 
 }  // namespace compiler
 }  // namespace internal
 }  // namespace v8