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// Copyright 2012 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.
// Live Edit feature implementation.
// User should be able to change script on already running VM. This feature
// matches hot swap features in other frameworks.
// The basic use-case is when user spots some mistake in function body
// from debugger and wishes to change the algorithm without restart.
// A single change always has a form of a simple replacement (in pseudo-code):
// script.source[positions, positions+length] = new_string;
// Implementation first determines, which function's body includes this
// change area. Then both old and new versions of script are fully compiled
// in order to analyze, whether the function changed its outer scope
// expectations (or number of parameters). If it didn't, function's code is
// patched with a newly compiled code. If it did change, enclosing function
// gets patched. All inner functions are left untouched, whatever happened
// to them in a new script version. However, new version of code will
// instantiate newly compiled functions.
#include "src/allocation.h"
#include "src/ast/ast-traversal-visitor.h"
namespace v8 {
namespace internal {
class JavaScriptFrame;
// This class collects some specific information on structure of functions
// in a particular script.
// The primary interest of the Tracker is to record function scope structures
// in order to analyze whether function code may be safely patched (with new
// code successfully reading existing data from function scopes). The Tracker
// also collects compiled function codes.
class LiveEditFunctionTracker
: public AstTraversalVisitor<LiveEditFunctionTracker> {
// Traverses the entire AST, and records information about all
// FunctionLiterals for further use by LiveEdit code patching. The collected
// information is returned as a serialized array.
static Handle<JSArray> Collect(FunctionLiteral* node, Handle<Script> script,
Zone* zone, Isolate* isolate);
friend AstTraversalVisitor<LiveEditFunctionTracker>;
void VisitFunctionLiteral(FunctionLiteral* node);
LiveEditFunctionTracker(Handle<Script> script, Zone* zone, Isolate* isolate);
void FunctionStarted(FunctionLiteral* fun);
void FunctionDone(Handle<SharedFunctionInfo> shared, Scope* scope);
Handle<Object> SerializeFunctionScope(Scope* scope);
Handle<Script> script_;
Zone* zone_;
Isolate* isolate_;
Handle<JSArray> result_;
int len_;
int current_parent_index_;
class LiveEdit : AllStatic {
static void InitializeThreadLocal(Debug* debug);
MUST_USE_RESULT static MaybeHandle<JSArray> GatherCompileInfo(
Handle<Script> script,
Handle<String> source);
static void ReplaceFunctionCode(Handle<JSArray> new_compile_info_array,
Handle<JSArray> shared_info_array);
static void FixupScript(Handle<Script> script, int max_function_literal_id);
static void FunctionSourceUpdated(Handle<JSArray> shared_info_array,
int new_function_literal_id);
// Updates script field in FunctionSharedInfo.
static void SetFunctionScript(Handle<JSValue> function_wrapper,
Handle<Object> script_handle);
static void PatchFunctionPositions(Handle<JSArray> shared_info_array,
Handle<JSArray> position_change_array);
// For a script updates its source field. If old_script_name is provided
// (i.e. is a String), also creates a copy of the script with its original
// source and sends notification to debugger.
static Handle<Object> ChangeScriptSource(Handle<Script> original_script,
Handle<String> new_source,
Handle<Object> old_script_name);
// In a code of a parent function replaces original function as embedded
// object with a substitution one.
static void ReplaceRefToNestedFunction(Handle<JSValue> parent_function_shared,
Handle<JSValue> orig_function_shared,
Handle<JSValue> subst_function_shared);
// Find open generator activations, and set corresponding "result" elements to
static bool FindActiveGenerators(Handle<FixedArray> shared_info_array,
Handle<FixedArray> result, int len);
// Checks listed functions on stack and return array with corresponding
// FunctionPatchabilityStatus statuses; extra array element may
// contain general error message. Modifies the current stack and
// has restart the lowest found frames and drops all other frames above
// if possible and if do_drop is true.
static Handle<JSArray> CheckAndDropActivations(
Handle<JSArray> old_shared_array, Handle<JSArray> new_shared_array,
bool do_drop);
// Restarts the call frame and completely drops all frames above it.
// Return error message or nullptr.
static const char* RestartFrame(JavaScriptFrame* frame);
// A copy of this is in liveedit.js.
enum FunctionPatchabilityStatus {
// Compares 2 strings line-by-line, then token-wise and returns diff in form
// of array of triplets (pos1, pos1_end, pos2_end) describing list
// of diff chunks.
static Handle<JSArray> CompareStrings(Handle<String> s1,
Handle<String> s2);
// Architecture-specific constant.
static const bool kFrameDropperSupported;
// A general-purpose comparator between 2 arrays.
class Comparator {
// Holds 2 arrays of some elements allowing to compare any pair of
// element from the first array and element from the second array.
class Input {
virtual int GetLength1() = 0;
virtual int GetLength2() = 0;
virtual bool Equals(int index1, int index2) = 0;
virtual ~Input() {}
// Receives compare result as a series of chunks.
class Output {
// Puts another chunk in result list. Note that technically speaking
// only 3 arguments actually needed with 4th being derivable.
virtual void AddChunk(int pos1, int pos2, int len1, int len2) = 0;
virtual ~Output() {}
// Finds the difference between 2 arrays of elements.
static void CalculateDifference(Input* input,
Output* result_writer);
// Simple helper class that creates more or less typed structures over
// JSArray object. This is an adhoc method of passing structures from C++
// to JavaScript.
template<typename S>
class JSArrayBasedStruct {
static S Create(Isolate* isolate) {
Factory* factory = isolate->factory();
Handle<JSArray> array = factory->NewJSArray(S::kSize_);
return S(array);
static S cast(Object* object) {
JSArray* array = JSArray::cast(object);
Handle<JSArray> array_handle(array);
return S(array_handle);
explicit JSArrayBasedStruct(Handle<JSArray> array) : array_(array) {
Handle<JSArray> GetJSArray() {
return array_;
Isolate* isolate() const {
return array_->GetIsolate();
void SetField(int field_position, Handle<Object> value) {
Object::SetElement(isolate(), array_, field_position, value,
void SetSmiValueField(int field_position, int value) {
SetField(field_position, Handle<Smi>(Smi::FromInt(value), isolate()));
Handle<Object> GetField(int field_position) {
return JSReceiver::GetElement(isolate(), array_, field_position)
int GetSmiValueField(int field_position) {
Handle<Object> res = GetField(field_position);
return Handle<Smi>::cast(res)->value();
Handle<JSArray> array_;
// Represents some function compilation details. This structure will be used
// from JavaScript. It contains Code object, which is kept wrapped
// into a BlindReference for sanitizing reasons.
class FunctionInfoWrapper : public JSArrayBasedStruct<FunctionInfoWrapper> {
explicit FunctionInfoWrapper(Handle<JSArray> array)
: JSArrayBasedStruct<FunctionInfoWrapper>(array) {
void SetInitialProperties(Handle<String> name, int start_position,
int end_position, int param_num, int parent_index,
int function_literal_id);
void SetFunctionScopeInfo(Handle<Object> scope_info_array) {
this->SetField(kFunctionScopeInfoOffset_, scope_info_array);
void SetSharedFunctionInfo(Handle<SharedFunctionInfo> info);
Handle<SharedFunctionInfo> GetSharedFunctionInfo();
int GetParentIndex() {
return this->GetSmiValueField(kParentIndexOffset_);
int GetStartPosition() {
return this->GetSmiValueField(kStartPositionOffset_);
int GetEndPosition() { return this->GetSmiValueField(kEndPositionOffset_); }
static const int kFunctionNameOffset_ = 0;
static const int kStartPositionOffset_ = 1;
static const int kEndPositionOffset_ = 2;
static const int kParamNumOffset_ = 3;
static const int kFunctionScopeInfoOffset_ = 4;
static const int kParentIndexOffset_ = 5;
static const int kSharedFunctionInfoOffset_ = 6;
static const int kFunctionLiteralIdOffset_ = 7;
static const int kSize_ = 8;
friend class JSArrayBasedStruct<FunctionInfoWrapper>;
// Wraps SharedFunctionInfo along with some of its fields for passing it
// back to JavaScript. SharedFunctionInfo object itself is additionally
// wrapped into BlindReference for sanitizing reasons.
class SharedInfoWrapper : public JSArrayBasedStruct<SharedInfoWrapper> {
static bool IsInstance(Handle<JSArray> array) {
if (array->length() != Smi::FromInt(kSize_)) return false;
Handle<Object> element(
JSReceiver::GetElement(array->GetIsolate(), array, kSharedInfoOffset_)
if (!element->IsJSValue()) return false;
return Handle<JSValue>::cast(element)->value()->IsSharedFunctionInfo();
explicit SharedInfoWrapper(Handle<JSArray> array)
: JSArrayBasedStruct<SharedInfoWrapper>(array) {
void SetProperties(Handle<String> name,
int start_position,
int end_position,
Handle<SharedFunctionInfo> info);
Handle<SharedFunctionInfo> GetInfo();
static const int kFunctionNameOffset_ = 0;
static const int kStartPositionOffset_ = 1;
static const int kEndPositionOffset_ = 2;
static const int kSharedInfoOffset_ = 3;
static const int kSize_ = 4;
friend class JSArrayBasedStruct<SharedInfoWrapper>;
} // namespace internal
} // namespace v8
#endif /* V8_DEBUG_LIVEEDIT_H_ */