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// 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/linkage.h"
#include "src/compiler/node-matchers.h"
#include "src/compiler/node-properties.h"
#include "src/contexts-inl.h"
namespace v8 {
namespace internal {
namespace compiler {
Reduction JSContextSpecialization::Reduce(Node* node) {
switch (node->opcode()) {
case IrOpcode::kParameter:
return ReduceParameter(node);
case IrOpcode::kJSLoadContext:
return ReduceJSLoadContext(node);
case IrOpcode::kJSStoreContext:
return ReduceJSStoreContext(node);
default:
break;
}
return NoChange();
}
Reduction JSContextSpecialization::ReduceParameter(Node* node) {
DCHECK_EQ(IrOpcode::kParameter, node->opcode());
int const index = ParameterIndexOf(node->op());
if (index == Linkage::kJSCallClosureParamIndex) {
// Constant-fold the function parameter {node}.
Handle<JSFunction> function;
if (closure().ToHandle(&function)) {
Node* value = jsgraph()->HeapConstant(function);
return Replace(value);
}
}
return NoChange();
}
Reduction JSContextSpecialization::SimplifyJSLoadContext(Node* node,
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::SimplifyJSStoreContext(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);
}
namespace {
bool IsContextParameter(Node* node) {
DCHECK_EQ(IrOpcode::kParameter, node->opcode());
Node* const start = NodeProperties::GetValueInput(node, 0);
DCHECK_EQ(IrOpcode::kStart, start->opcode());
int const index = ParameterIndexOf(node->op());
// The context is always the last parameter to a JavaScript function, and
// {Parameter} indices start at -1, so value outputs of {Start} look like
// this: closure, receiver, param0, ..., paramN, context.
return index == start->op()->ValueOutputCount() - 2;
}
// Given a context {node} and the {distance} from that context to the target
// context (which we want to read from or store to), try to return a
// specialization context. If successful, update {distance} to whatever
// distance remains from the specialization context.
MaybeHandle<Context> GetSpecializationContext(Node* node, size_t* distance,
Maybe<OuterContext> maybe_outer) {
switch (node->opcode()) {
case IrOpcode::kHeapConstant:
return Handle<Context>::cast(OpParameter<Handle<HeapObject>>(node));
case IrOpcode::kParameter: {
OuterContext outer;
if (maybe_outer.To(&outer) && IsContextParameter(node) &&
*distance >= outer.distance) {
*distance -= outer.distance;
return outer.context;
}
break;
}
default:
break;
}
return MaybeHandle<Context>();
}
} // anonymous namespace
Reduction JSContextSpecialization::ReduceJSLoadContext(Node* node) {
DCHECK_EQ(IrOpcode::kJSLoadContext, node->opcode());
const ContextAccess& access = ContextAccessOf(node->op());
size_t depth = access.depth();
// First walk up the context chain in the graph as far as possible.
Node* context = NodeProperties::GetOuterContext(node, &depth);
Handle<Context> concrete;
if (!GetSpecializationContext(context, &depth, outer()).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 SimplifyJSLoadContext(node, context, depth);
}
// Now walk up the concrete context chain for the remaining depth.
for (; depth > 0; --depth) {
concrete = handle(concrete->previous(), isolate());
}
if (!access.immutable()) {
// We found the requested context object but since the context slot is
// mutable we can only partially reduce the load.
return SimplifyJSLoadContext(node, jsgraph()->Constant(concrete), depth);
}
// 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. 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 SimplifyJSLoadContext(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());
const ContextAccess& access = ContextAccessOf(node->op());
size_t depth = access.depth();
// 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* context = NodeProperties::GetOuterContext(node, &depth);
Handle<Context> concrete;
if (!GetSpecializationContext(context, &depth, outer()).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 SimplifyJSStoreContext(node, context, depth);
}
// Now walk up the concrete context chain for the remaining depth.
for (; depth > 0; --depth) {
concrete = handle(concrete->previous(), isolate());
}
return SimplifyJSStoreContext(node, jsgraph()->Constant(concrete), depth);
}
Isolate* JSContextSpecialization::isolate() const {
return jsgraph()->isolate();
}
} // namespace compiler
} // namespace internal
} // namespace v8
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