From 61c9683d712e5f168b34ca79d6e94a10bb30e59f Mon Sep 17 00:00:00 2001
From: "marja@chromium.org" <marja@chromium.org>
Date: Thu, 11 Sep 2014 11:06:26 +0000
Subject: [PATCH] Add script streaming API.

Blink will use this API to stream script data into V8 as the scripts
load. During loading, V8 can already parse the scripts. They will be then
compiled and executed when the loading is complete.

BUG=
R=jochen@chromium.org, rossberg@chromium.org, svenpanne@chromium.org, yangguo@chromium.org

Review URL: https://codereview.chromium.org/366153002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@23865 ce2b1a6d-e550-0410-aec6-3dcde31c8c00
---
 BUILD.gn                         |   2 +
 include/v8.h                     |  94 ++++++++++++
 src/api.cc                       |  98 +++++++++++++
 src/background-parsing-task.cc   |  62 ++++++++
 src/background-parsing-task.h    |  67 +++++++++
 src/compiler.cc                  |  95 ++++++++----
 src/compiler.h                   |  23 +++
 src/parser.cc                    | 100 +++++++++----
 src/parser.h                     |  16 ++-
 src/scanner-character-streams.cc | 182 ++++++++++++++++++++---
 src/scanner-character-streams.h  |  43 ++++++
 test/cctest/test-api.cc          | 304 +++++++++++++++++++++++++++++++++++++++
 tools/gyp/v8.gyp                 |   2 +
 13 files changed, 1010 insertions(+), 78 deletions(-)
 create mode 100644 src/background-parsing-task.cc
 create mode 100644 src/background-parsing-task.h

diff --git a/BUILD.gn b/BUILD.gn
index 76d48d1..b5e56e5 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -435,6 +435,8 @@ source_set("v8_base") {
     "src/ast-value-factory.h",
     "src/ast.cc",
     "src/ast.h",
+    "src/background-parsing-task.cc",
+    "src/background-parsing-task.h",
     "src/bignum-dtoa.cc",
     "src/bignum-dtoa.h",
     "src/bignum.cc",
diff --git a/include/v8.h b/include/v8.h
index d6fbf67..19b2778 100644
--- a/include/v8.h
+++ b/include/v8.h
@@ -130,6 +130,7 @@ class Heap;
 class HeapObject;
 class Isolate;
 class Object;
+class StreamedSource;
 template<typename T> class CustomArguments;
 class PropertyCallbackArguments;
 class FunctionCallbackArguments;
@@ -1088,6 +1089,73 @@ class V8_EXPORT ScriptCompiler {
     CachedData* cached_data;
   };
 
+  /**
+   * For streaming incomplete script data to V8. The embedder should implement a
+   * subclass of this class.
+   */
+  class ExternalSourceStream {
+   public:
+    virtual ~ExternalSourceStream() {}
+
+    /**
+     * V8 calls this to request the next chunk of data from the embedder. This
+     * function will be called on a background thread, so it's OK to block and
+     * wait for the data, if the embedder doesn't have data yet. Returns the
+     * length of the data returned. When the data ends, GetMoreData should
+     * return 0. Caller takes ownership of the data.
+     *
+     * When streaming UTF-8 data, V8 handles multi-byte characters split between
+     * two data chunks, but doesn't handle multi-byte characters split between
+     * more than two data chunks. The embedder can avoid this problem by always
+     * returning at least 2 bytes of data.
+     *
+     * If the embedder wants to cancel the streaming, they should make the next
+     * GetMoreData call return 0. V8 will interpret it as end of data (and most
+     * probably, parsing will fail). The streaming task will return as soon as
+     * V8 has parsed the data it received so far.
+     */
+    virtual size_t GetMoreData(const uint8_t** src) = 0;
+  };
+
+
+  /**
+   * Source code which can be streamed into V8 in pieces. It will be parsed
+   * while streaming. It can be compiled after the streaming is complete.
+   * StreamedSource must be kept alive while the streaming task is ran (see
+   * ScriptStreamingTask below).
+   */
+  class V8_EXPORT StreamedSource {
+   public:
+    enum Encoding { ONE_BYTE, TWO_BYTE, UTF8 };
+
+    StreamedSource(ExternalSourceStream* source_stream, Encoding encoding);
+    ~StreamedSource();
+
+    // Ownership of the CachedData or its buffers is *not* transferred to the
+    // caller. The CachedData object is alive as long as the StreamedSource
+    // object is alive.
+    const CachedData* GetCachedData() const;
+
+    internal::StreamedSource* impl() const { return impl_; }
+
+   private:
+    // Prevent copying. Not implemented.
+    StreamedSource(const StreamedSource&);
+    StreamedSource& operator=(const StreamedSource&);
+
+    internal::StreamedSource* impl_;
+  };
+
+  /**
+   * A streaming task which the embedder must run on a background thread to
+   * stream scripts into V8. Returned by ScriptCompiler::StartStreamingScript.
+   */
+  class ScriptStreamingTask {
+   public:
+    virtual ~ScriptStreamingTask() {}
+    virtual void Run() = 0;
+  };
+
   enum CompileOptions {
     kNoCompileOptions = 0,
     kProduceParserCache,
@@ -1130,6 +1198,32 @@ class V8_EXPORT ScriptCompiler {
   static Local<Script> Compile(
       Isolate* isolate, Source* source,
       CompileOptions options = kNoCompileOptions);
+
+  /**
+   * Returns a task which streams script data into V8, or NULL if the script
+   * cannot be streamed. The user is responsible for running the task on a
+   * background thread and deleting it. When ran, the task starts parsing the
+   * script, and it will request data from the StreamedSource as needed. When
+   * ScriptStreamingTask::Run exits, all data has been streamed and the script
+   * can be compiled (see Compile below).
+   *
+   * This API allows to start the streaming with as little data as possible, and
+   * the remaining data (for example, the ScriptOrigin) is passed to Compile.
+   */
+  static ScriptStreamingTask* StartStreamingScript(
+      Isolate* isolate, StreamedSource* source,
+      CompileOptions options = kNoCompileOptions);
+
+  /**
+   * Compiles a streamed script (bound to current context).
+   *
+   * This can only be called after the streaming has finished
+   * (ScriptStreamingTask has been run). V8 doesn't construct the source string
+   * during streaming, so the embedder needs to pass the full source here.
+   */
+  static Local<Script> Compile(Isolate* isolate, StreamedSource* source,
+                               Handle<String> full_source_string,
+                               const ScriptOrigin& origin);
 };
 
 
diff --git a/src/api.cc b/src/api.cc
index 4e10def..e4a0065 100644
--- a/src/api.cc
+++ b/src/api.cc
@@ -13,6 +13,7 @@
 #include "include/v8-profiler.h"
 #include "include/v8-testing.h"
 #include "src/assert-scope.h"
+#include "src/background-parsing-task.h"
 #include "src/base/platform/platform.h"
 #include "src/base/platform/time.h"
 #include "src/base/utils/random-number-generator.h"
@@ -1600,6 +1601,20 @@ ScriptCompiler::CachedData::~CachedData() {
 }
 
 
+ScriptCompiler::StreamedSource::StreamedSource(ExternalSourceStream* stream,
+                                               Encoding encoding)
+    : impl_(new i::StreamedSource(stream, encoding)) {}
+
+
+ScriptCompiler::StreamedSource::~StreamedSource() { delete impl_; }
+
+
+const ScriptCompiler::CachedData*
+ScriptCompiler::StreamedSource::GetCachedData() const {
+  return impl_->cached_data.get();
+}
+
+
 Local<Script> UnboundScript::BindToCurrentContext() {
   i::Handle<i::HeapObject> obj =
       i::Handle<i::HeapObject>::cast(Utils::OpenHandle(this));
@@ -1814,6 +1829,89 @@ Local<Script> ScriptCompiler::Compile(
 }
 
 
+ScriptCompiler::ScriptStreamingTask* ScriptCompiler::StartStreamingScript(
+    Isolate* v8_isolate, StreamedSource* source, CompileOptions options) {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
+  if (!isolate->global_context().is_null() &&
+      !isolate->global_context()->IsNativeContext()) {
+    // The context chain is non-trivial, and constructing the corresponding
+    // non-trivial Scope chain outside the V8 heap is not implemented. Don't
+    // stream the script. This will only occur if Harmony scoping is enabled and
+    // a previous script has introduced "let" or "const" variables. TODO(marja):
+    // Implement externalizing ScopeInfos and constructing non-trivial Scope
+    // chains independent of the V8 heap so that we can stream also in this
+    // case.
+    return NULL;
+  }
+  return new i::BackgroundParsingTask(source->impl(), options,
+                                      i::FLAG_stack_size, isolate);
+}
+
+
+Local<Script> ScriptCompiler::Compile(Isolate* v8_isolate,
+                                      StreamedSource* v8_source,
+                                      Handle<String> full_source_string,
+                                      const ScriptOrigin& origin) {
+  i::Isolate* isolate = reinterpret_cast<i::Isolate*>(v8_isolate);
+  i::StreamedSource* source = v8_source->impl();
+  ON_BAILOUT(isolate, "v8::ScriptCompiler::Compile()", return Local<Script>());
+  LOG_API(isolate, "ScriptCompiler::Compile()");
+  ENTER_V8(isolate);
+  i::SharedFunctionInfo* raw_result = NULL;
+
+  {
+    i::HandleScope scope(isolate);
+    i::Handle<i::String> str = Utils::OpenHandle(*(full_source_string));
+    i::Handle<i::Script> script = isolate->factory()->NewScript(str);
+    if (!origin.ResourceName().IsEmpty()) {
+      script->set_name(*Utils::OpenHandle(*(origin.ResourceName())));
+    }
+    if (!origin.ResourceLineOffset().IsEmpty()) {
+      script->set_line_offset(i::Smi::FromInt(
+          static_cast<int>(origin.ResourceLineOffset()->Value())));
+    }
+    if (!origin.ResourceColumnOffset().IsEmpty()) {
+      script->set_column_offset(i::Smi::FromInt(
+          static_cast<int>(origin.ResourceColumnOffset()->Value())));
+    }
+    if (!origin.ResourceIsSharedCrossOrigin().IsEmpty()) {
+      script->set_is_shared_cross_origin(origin.ResourceIsSharedCrossOrigin() ==
+                                         v8::True(v8_isolate));
+    }
+    source->info->set_script(script);
+    source->info->SetContext(isolate->global_context());
+
+    EXCEPTION_PREAMBLE(isolate);
+
+    // Do the parsing tasks which need to be done on the main thread. This will
+    // also handle parse errors.
+    source->parser->Internalize();
+
+    i::Handle<i::SharedFunctionInfo> result =
+        i::Handle<i::SharedFunctionInfo>::null();
+    if (source->info->function() != NULL) {
+      // Parsing has succeeded.
+      result =
+          i::Compiler::CompileStreamedScript(source->info.get(), str->length());
+    }
+    has_pending_exception = result.is_null();
+    if (has_pending_exception) isolate->ReportPendingMessages();
+    EXCEPTION_BAILOUT_CHECK(isolate, Local<Script>());
+
+    raw_result = *result;
+    // The Handle<Script> will go out of scope soon; make sure CompilationInfo
+    // doesn't point to it.
+    source->info->set_script(i::Handle<i::Script>());
+  }  // HandleScope goes out of scope.
+  i::Handle<i::SharedFunctionInfo> result(raw_result, isolate);
+  Local<UnboundScript> generic = ToApiHandle<UnboundScript>(result);
+  if (generic.IsEmpty()) {
+    return Local<Script>();
+  }
+  return generic->BindToCurrentContext();
+}
+
+
 Local<Script> Script::Compile(v8::Handle<String> source,
                               v8::ScriptOrigin* origin) {
   i::Handle<i::String> str = Utils::OpenHandle(*source);
diff --git a/src/background-parsing-task.cc b/src/background-parsing-task.cc
new file mode 100644
index 0000000..c7602a7
--- /dev/null
+++ b/src/background-parsing-task.cc
@@ -0,0 +1,62 @@
+// 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/background-parsing-task.h"
+
+namespace v8 {
+namespace internal {
+
+BackgroundParsingTask::BackgroundParsingTask(
+    StreamedSource* source, ScriptCompiler::CompileOptions options,
+    int stack_size, Isolate* isolate)
+    : source_(source), options_(options), stack_size_(stack_size) {
+  // Prepare the data for the internalization phase and compilation phase, which
+  // will happen in the main thread after parsing.
+  source->info.Reset(new i::CompilationInfoWithZone(source->source_stream.get(),
+                                                    source->encoding, isolate));
+  source->info->MarkAsGlobal();
+
+  // We don't set the context to the CompilationInfo yet, because the background
+  // thread cannot do anything with it anyway. We set it just before compilation
+  // on the foreground thread.
+  DCHECK(options == ScriptCompiler::kProduceParserCache ||
+         options == ScriptCompiler::kProduceCodeCache ||
+         options == ScriptCompiler::kNoCompileOptions);
+  source->allow_lazy =
+      !i::Compiler::DebuggerWantsEagerCompilation(source->info.get());
+  source->hash_seed = isolate->heap()->HashSeed();
+}
+
+
+void BackgroundParsingTask::Run() {
+  DisallowHeapAllocation no_allocation;
+  DisallowHandleAllocation no_handles;
+  DisallowHandleDereference no_deref;
+
+  ScriptData* script_data = NULL;
+  if (options_ == ScriptCompiler::kProduceParserCache ||
+      options_ == ScriptCompiler::kProduceCodeCache) {
+    source_->info->SetCachedData(&script_data, options_);
+  }
+
+  uintptr_t limit = reinterpret_cast<uintptr_t>(&limit) - stack_size_ * KB;
+  Parser::ParseInfo parse_info = {limit, source_->hash_seed,
+                                  &source_->unicode_cache};
+
+  // Parser needs to stay alive for finalizing the parsing on the main
+  // thread. Passing &parse_info is OK because Parser doesn't store it.
+  source_->parser.Reset(new Parser(source_->info.get(), &parse_info));
+  source_->parser->set_allow_lazy(source_->allow_lazy);
+  source_->parser->ParseOnBackground();
+
+  if (script_data != NULL) {
+    source_->cached_data.Reset(new ScriptCompiler::CachedData(
+        script_data->data(), script_data->length(),
+        ScriptCompiler::CachedData::BufferOwned));
+    script_data->ReleaseDataOwnership();
+    delete script_data;
+  }
+}
+}
+}  // namespace v8::internal
diff --git a/src/background-parsing-task.h b/src/background-parsing-task.h
new file mode 100644
index 0000000..19c93a8
--- /dev/null
+++ b/src/background-parsing-task.h
@@ -0,0 +1,67 @@
+// 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.
+
+#ifndef V8_BACKGROUND_PARSING_TASK_H_
+#define V8_BACKGROUND_PARSING_TASK_H_
+
+#include "src/base/platform/platform.h"
+#include "src/base/platform/semaphore.h"
+#include "src/compiler.h"
+#include "src/parser.h"
+#include "src/smart-pointers.h"
+
+namespace v8 {
+namespace internal {
+
+class Parser;
+
+// Internal representation of v8::ScriptCompiler::StreamedSource. Contains all
+// data which needs to be transmitted between threads for background parsing,
+// finalizing it on the main thread, and compiling on the main thread.
+struct StreamedSource {
+  StreamedSource(ScriptCompiler::ExternalSourceStream* source_stream,
+                 ScriptCompiler::StreamedSource::Encoding encoding)
+      : source_stream(source_stream),
+        encoding(encoding),
+        hash_seed(0),
+        allow_lazy(false) {}
+
+  // Internal implementation of v8::ScriptCompiler::StreamedSource.
+  SmartPointer<ScriptCompiler::ExternalSourceStream> source_stream;
+  ScriptCompiler::StreamedSource::Encoding encoding;
+  SmartPointer<ScriptCompiler::CachedData> cached_data;
+
+  // Data needed for parsing, and data needed to to be passed between thread
+  // between parsing and compilation. These need to be initialized before the
+  // compilation starts.
+  UnicodeCache unicode_cache;
+  SmartPointer<CompilationInfo> info;
+  uint32_t hash_seed;
+  bool allow_lazy;
+  SmartPointer<Parser> parser;
+
+ private:
+  // Prevent copying. Not implemented.
+  StreamedSource(const StreamedSource&);
+  StreamedSource& operator=(const StreamedSource&);
+};
+
+
+class BackgroundParsingTask : public ScriptCompiler::ScriptStreamingTask {
+ public:
+  BackgroundParsingTask(StreamedSource* source,
+                        ScriptCompiler::CompileOptions options, int stack_size,
+                        Isolate* isolate);
+
+  virtual void Run();
+
+ private:
+  StreamedSource* source_;  // Not owned.
+  ScriptCompiler::CompileOptions options_;
+  int stack_size_;
+};
+}
+}  // namespace v8::internal
+
+#endif  // V8_BACKGROUND_PARSING_TASK_H_
diff --git a/src/compiler.cc b/src/compiler.cc
index d43177e..70a2c95 100644
--- a/src/compiler.cc
+++ b/src/compiler.cc
@@ -47,6 +47,7 @@ ScriptData::ScriptData(const byte* data, int length)
 CompilationInfo::CompilationInfo(Handle<Script> script, Zone* zone)
     : flags_(kThisHasUses),
       script_(script),
+      source_stream_(NULL),
       osr_ast_id_(BailoutId::None()),
       parameter_count_(0),
       optimization_id_(-1),
@@ -59,6 +60,7 @@ CompilationInfo::CompilationInfo(Handle<Script> script, Zone* zone)
 CompilationInfo::CompilationInfo(Isolate* isolate, Zone* zone)
     : flags_(kThisHasUses),
       script_(Handle<Script>::null()),
+      source_stream_(NULL),
       osr_ast_id_(BailoutId::None()),
       parameter_count_(0),
       optimization_id_(-1),
@@ -73,6 +75,7 @@ CompilationInfo::CompilationInfo(Handle<SharedFunctionInfo> shared_info,
     : flags_(kLazy | kThisHasUses),
       shared_info_(shared_info),
       script_(Handle<Script>(Script::cast(shared_info->script()))),
+      source_stream_(NULL),
       osr_ast_id_(BailoutId::None()),
       parameter_count_(0),
       optimization_id_(-1),
@@ -87,6 +90,7 @@ CompilationInfo::CompilationInfo(Handle<JSFunction> closure, Zone* zone)
       closure_(closure),
       shared_info_(Handle<SharedFunctionInfo>(closure->shared())),
       script_(Handle<Script>(Script::cast(shared_info_->script()))),
+      source_stream_(NULL),
       context_(closure->context()),
       osr_ast_id_(BailoutId::None()),
       parameter_count_(0),
@@ -100,6 +104,7 @@ CompilationInfo::CompilationInfo(Handle<JSFunction> closure, Zone* zone)
 CompilationInfo::CompilationInfo(HydrogenCodeStub* stub, Isolate* isolate,
                                  Zone* zone)
     : flags_(kLazy | kThisHasUses),
+      source_stream_(NULL),
       osr_ast_id_(BailoutId::None()),
       parameter_count_(0),
       optimization_id_(-1),
@@ -110,6 +115,22 @@ CompilationInfo::CompilationInfo(HydrogenCodeStub* stub, Isolate* isolate,
 }
 
 
+CompilationInfo::CompilationInfo(
+    ScriptCompiler::ExternalSourceStream* stream,
+    ScriptCompiler::StreamedSource::Encoding encoding, Isolate* isolate,
+    Zone* zone)
+    : flags_(kThisHasUses),
+      source_stream_(stream),
+      source_stream_encoding_(encoding),
+      osr_ast_id_(BailoutId::None()),
+      parameter_count_(0),
+      optimization_id_(-1),
+      ast_value_factory_(NULL),
+      ast_value_factory_owned_(false) {
+  Initialize(isolate, BASE, zone);
+}
+
+
 void CompilationInfo::Initialize(Isolate* isolate,
                                  Mode mode,
                                  Zone* zone) {
@@ -136,7 +157,9 @@ void CompilationInfo::Initialize(Isolate* isolate,
   }
   mode_ = mode;
   abort_due_to_dependency_ = false;
-  if (script_->type()->value() == Script::TYPE_NATIVE) MarkAsNative();
+  if (!script_.is_null() && script_->type()->value() == Script::TYPE_NATIVE) {
+    MarkAsNative();
+  }
   if (isolate_->debug()->is_active()) MarkAsDebug();
   if (FLAG_context_specialization) MarkAsContextSpecializing();
   if (FLAG_turbo_inlining) MarkAsInliningEnabled();
@@ -810,13 +833,6 @@ void Compiler::CompileForLiveEdit(Handle<Script> script) {
 }
 
 
-static bool DebuggerWantsEagerCompilation(CompilationInfo* info,
-                                          bool allow_lazy_without_ctx = false) {
-  return LiveEditFunctionTracker::IsActive(info->isolate()) ||
-         (info->isolate()->DebuggerHasBreakPoints() && !allow_lazy_without_ctx);
-}
-
-
 static Handle<SharedFunctionInfo> CompileToplevel(CompilationInfo* info) {
   Isolate* isolate = info->isolate();
   PostponeInterruptsScope postpone(isolate);
@@ -831,28 +847,30 @@ static Handle<SharedFunctionInfo> CompileToplevel(CompilationInfo* info) {
 
   DCHECK(info->is_eval() || info->is_global());
 
-  bool parse_allow_lazy =
-      (info->compile_options() == ScriptCompiler::kConsumeParserCache ||
-       String::cast(script->source())->length() > FLAG_min_preparse_length) &&
-      !DebuggerWantsEagerCompilation(info);
-
-  if (!parse_allow_lazy &&
-      (info->compile_options() == ScriptCompiler::kProduceParserCache ||
-       info->compile_options() == ScriptCompiler::kConsumeParserCache)) {
-    // We are going to parse eagerly, but we either 1) have cached data produced
-    // by lazy parsing or 2) are asked to generate cached data. We cannot use
-    // the existing data, since it won't contain all the symbols we need for
-    // eager parsing. In addition, it doesn't make sense to produce the data
-    // when parsing eagerly. That data would contain all symbols, but no
-    // functions, so it cannot be used to aid lazy parsing later.
-    info->SetCachedData(NULL, ScriptCompiler::kNoCompileOptions);
-  }
-
   Handle<SharedFunctionInfo> result;
 
   { VMState<COMPILER> state(info->isolate());
-    if (!Parser::Parse(info, parse_allow_lazy)) {
-      return Handle<SharedFunctionInfo>::null();
+    if (info->function() == NULL) {
+      // Parse the script if needed (if it's already parsed, function() is
+      // non-NULL).
+      bool parse_allow_lazy =
+          (info->compile_options() == ScriptCompiler::kConsumeParserCache ||
+           String::cast(script->source())->length() >
+               FLAG_min_preparse_length) &&
+          !Compiler::DebuggerWantsEagerCompilation(info);
+
+      if (!parse_allow_lazy &&
+          (info->compile_options() == ScriptCompiler::kProduceParserCache ||
+           info->compile_options() == ScriptCompiler::kConsumeParserCache)) {
+        // We are going to parse eagerly, but we either 1) have cached data
+        // produced by lazy parsing or 2) are asked to generate cached data.
+        // Eager parsing cannot benefit from cached data, and producing cached
+        // data while parsing eagerly is not implemented.
+        info->SetCachedData(NULL, ScriptCompiler::kNoCompileOptions);
+      }
+      if (!Parser::Parse(info, parse_allow_lazy)) {
+        return Handle<SharedFunctionInfo>::null();
+      }
     }
 
     FunctionLiteral* lit = info->function();
@@ -898,7 +916,8 @@ static Handle<SharedFunctionInfo> CompileToplevel(CompilationInfo* info) {
     SetExpectedNofPropertiesFromEstimate(result,
                                          lit->expected_property_count());
 
-    script->set_compilation_state(Script::COMPILATION_STATE_COMPILED);
+    if (!script.is_null())
+      script->set_compilation_state(Script::COMPILATION_STATE_COMPILED);
 
     live_edit_tracker.RecordFunctionInfo(result, lit, info->zone());
   }
@@ -1055,6 +1074,19 @@ Handle<SharedFunctionInfo> Compiler::CompileScript(
 }
 
 
+Handle<SharedFunctionInfo> Compiler::CompileStreamedScript(
+    CompilationInfo* info, int source_length) {
+  Isolate* isolate = info->isolate();
+  isolate->counters()->total_load_size()->Increment(source_length);
+  isolate->counters()->total_compile_size()->Increment(source_length);
+
+  if (FLAG_use_strict) info->SetStrictMode(STRICT);
+  // TODO(marja): FLAG_serialize_toplevel is not honoured and won't be; when the
+  // real code caching lands, streaming needs to be adapted to use it.
+  return CompileToplevel(info);
+}
+
+
 Handle<SharedFunctionInfo> Compiler::BuildFunctionInfo(
     FunctionLiteral* literal, Handle<Script> script,
     CompilationInfo* outer_info) {
@@ -1362,6 +1394,13 @@ void Compiler::RecordFunctionCompilation(Logger::LogEventsAndTags tag,
 }
 
 
+bool Compiler::DebuggerWantsEagerCompilation(CompilationInfo* info,
+                                             bool allow_lazy_without_ctx) {
+  return LiveEditFunctionTracker::IsActive(info->isolate()) ||
+         (info->isolate()->DebuggerHasBreakPoints() && !allow_lazy_without_ctx);
+}
+
+
 CompilationPhase::CompilationPhase(const char* name, CompilationInfo* info)
     : name_(name), info_(info), zone_(info->isolate()) {
   if (FLAG_hydrogen_stats) {
diff --git a/src/compiler.h b/src/compiler.h
index 00cd0f4..ca64e40 100644
--- a/src/compiler.h
+++ b/src/compiler.h
@@ -107,12 +107,19 @@ class CompilationInfo {
   Handle<JSFunction> closure() const { return closure_; }
   Handle<SharedFunctionInfo> shared_info() const { return shared_info_; }
   Handle<Script> script() const { return script_; }
+  void set_script(Handle<Script> script) { script_ = script; }
   HydrogenCodeStub* code_stub() const {return code_stub_; }
   v8::Extension* extension() const { return extension_; }
   ScriptData** cached_data() const { return cached_data_; }
   ScriptCompiler::CompileOptions compile_options() const {
     return compile_options_;
   }
+  ScriptCompiler::ExternalSourceStream* source_stream() const {
+    return source_stream_;
+  }
+  ScriptCompiler::StreamedSource::Encoding source_stream_encoding() const {
+    return source_stream_encoding_;
+  }
   Handle<Context> context() const { return context_; }
   BailoutId osr_ast_id() const { return osr_ast_id_; }
   Handle<Code> unoptimized_code() const { return unoptimized_code_; }
@@ -378,6 +385,10 @@ class CompilationInfo {
   CompilationInfo(HydrogenCodeStub* stub,
                   Isolate* isolate,
                   Zone* zone);
+  CompilationInfo(ScriptCompiler::ExternalSourceStream* source_stream,
+                  ScriptCompiler::StreamedSource::Encoding encoding,
+                  Isolate* isolate, Zone* zone);
+
 
  private:
   Isolate* isolate_;
@@ -427,6 +438,8 @@ class CompilationInfo {
   Handle<JSFunction> closure_;
   Handle<SharedFunctionInfo> shared_info_;
   Handle<Script> script_;
+  ScriptCompiler::ExternalSourceStream* source_stream_;  // Not owned.
+  ScriptCompiler::StreamedSource::Encoding source_stream_encoding_;
 
   // Fields possibly needed for eager compilation, NULL by default.
   v8::Extension* extension_;
@@ -508,6 +521,10 @@ class CompilationInfoWithZone: public CompilationInfo {
   CompilationInfoWithZone(HydrogenCodeStub* stub, Isolate* isolate)
       : CompilationInfo(stub, isolate, &zone_),
         zone_(isolate) {}
+  CompilationInfoWithZone(ScriptCompiler::ExternalSourceStream* stream,
+                          ScriptCompiler::StreamedSource::Encoding encoding,
+                          Isolate* isolate)
+      : CompilationInfo(stream, encoding, isolate, &zone_), zone_(isolate) {}
 
   // Virtual destructor because a CompilationInfoWithZone has to exit the
   // zone scope and get rid of dependent maps even when the destructor is
@@ -667,6 +684,9 @@ class Compiler : public AllStatic {
       ScriptCompiler::CompileOptions compile_options,
       NativesFlag is_natives_code);
 
+  static Handle<SharedFunctionInfo> CompileStreamedScript(CompilationInfo* info,
+                                                          int source_length);
+
   // Create a shared function info object (the code may be lazily compiled).
   static Handle<SharedFunctionInfo> BuildFunctionInfo(FunctionLiteral* node,
                                                       Handle<Script> script,
@@ -690,6 +710,9 @@ class Compiler : public AllStatic {
   static void RecordFunctionCompilation(Logger::LogEventsAndTags tag,
                                         CompilationInfo* info,
                                         Handle<SharedFunctionInfo> shared);
+
+  static bool DebuggerWantsEagerCompilation(
+      CompilationInfo* info, bool allow_lazy_without_ctx = false);
 };
 
 
diff --git a/src/parser.cc b/src/parser.cc
index adb797d..0664e2a 100644
--- a/src/parser.cc
+++ b/src/parser.cc
@@ -750,7 +750,7 @@ Parser::Parser(CompilationInfo* info, ParseInfo* parse_info)
       pending_error_char_arg_(NULL),
       total_preparse_skipped_(0),
       pre_parse_timer_(NULL) {
-  DCHECK(!script().is_null());
+  DCHECK(!script().is_null() || info->source_stream() != NULL);
   set_allow_harmony_scoping(!info->is_native() && FLAG_harmony_scoping);
   set_allow_modules(!info->is_native() && FLAG_harmony_modules);
   set_allow_natives_syntax(FLAG_allow_natives_syntax || info->is_native());
@@ -798,6 +798,9 @@ FunctionLiteral* Parser::ParseProgram() {
 
   source = String::Flatten(source);
   FunctionLiteral* result;
+
+  Scope* top_scope = NULL;
+  Scope* eval_scope = NULL;
   if (source->IsExternalTwoByteString()) {
     // Notice that the stream is destroyed at the end of the branch block.
     // The last line of the blocks can't be moved outside, even though they're
@@ -805,11 +808,15 @@ FunctionLiteral* Parser::ParseProgram() {
     ExternalTwoByteStringUtf16CharacterStream stream(
         Handle<ExternalTwoByteString>::cast(source), 0, source->length());
     scanner_.Initialize(&stream);
-    result = DoParseProgram(info(), source);
+    result = DoParseProgram(info(), &top_scope, &eval_scope);
   } else {
     GenericStringUtf16CharacterStream stream(source, 0, source->length());
     scanner_.Initialize(&stream);
-    result = DoParseProgram(info(), source);
+    result = DoParseProgram(info(), &top_scope, &eval_scope);
+  }
+  top_scope->set_end_position(source->length());
+  if (eval_scope != NULL) {
+    eval_scope->set_end_position(source->length());
   }
   HandleSourceURLComments();
 
@@ -834,51 +841,52 @@ FunctionLiteral* Parser::ParseProgram() {
 }
 
 
-FunctionLiteral* Parser::DoParseProgram(CompilationInfo* info,
-                                        Handle<String> source) {
+FunctionLiteral* Parser::DoParseProgram(CompilationInfo* info, Scope** scope,
+                                        Scope** eval_scope) {
   DCHECK(scope_ == NULL);
   DCHECK(target_stack_ == NULL);
 
   FunctionLiteral* result = NULL;
-  { Scope* scope = NewScope(scope_, GLOBAL_SCOPE);
-    info->SetGlobalScope(scope);
+  {
+    *scope = NewScope(scope_, GLOBAL_SCOPE);
+    info->SetGlobalScope(*scope);
     if (!info->context().is_null() && !info->context()->IsNativeContext()) {
-      scope = Scope::DeserializeScopeChain(*info->context(), scope, zone());
+      *scope = Scope::DeserializeScopeChain(*info->context(), *scope, zone());
       // The Scope is backed up by ScopeInfo (which is in the V8 heap); this
       // means the Parser cannot operate independent of the V8 heap. Tell the
       // string table to internalize strings and values right after they're
       // created.
       ast_value_factory()->Internalize(isolate());
     }
-    original_scope_ = scope;
+    original_scope_ = *scope;
     if (info->is_eval()) {
-      if (!scope->is_global_scope() || info->strict_mode() == STRICT) {
-        scope = NewScope(scope, EVAL_SCOPE);
+      if (!(*scope)->is_global_scope() || info->strict_mode() == STRICT) {
+        *scope = NewScope(*scope, EVAL_SCOPE);
       }
     } else if (info->is_global()) {
-      scope = NewScope(scope, GLOBAL_SCOPE);
+      *scope = NewScope(*scope, GLOBAL_SCOPE);
     }
-    scope->set_start_position(0);
-    scope->set_end_position(source->length());
+    (*scope)->set_start_position(0);
+    // End position will be set by the caller.
 
     // Compute the parsing mode.
     Mode mode = (FLAG_lazy && allow_lazy()) ? PARSE_LAZILY : PARSE_EAGERLY;
-    if (allow_natives_syntax() ||
-        extension_ != NULL ||
-        scope->is_eval_scope()) {
+    if (allow_natives_syntax() || extension_ != NULL ||
+        (*scope)->is_eval_scope()) {
       mode = PARSE_EAGERLY;
     }
     ParsingModeScope parsing_mode(this, mode);
 
     // Enters 'scope'.
-    FunctionState function_state(&function_state_, &scope_, scope, zone(),
+    FunctionState function_state(&function_state_, &scope_, *scope, zone(),
                                  ast_value_factory(), info->ast_node_id_gen());
 
     scope_->SetStrictMode(info->strict_mode());
     ZoneList<Statement*>* body = new(zone()) ZoneList<Statement*>(16, zone());
     bool ok = true;
     int beg_pos = scanner()->location().beg_pos;
-    ParseSourceElements(body, Token::EOS, info->is_eval(), true, &ok);
+    ParseSourceElements(body, Token::EOS, info->is_eval(), true, eval_scope,
+                        &ok);
 
     if (ok && strict_mode() == STRICT) {
       CheckOctalLiteral(beg_pos, scanner()->location().end_pos, &ok);
@@ -1023,10 +1031,8 @@ FunctionLiteral* Parser::ParseLazy(Utf16CharacterStream* source) {
 
 
 void* Parser::ParseSourceElements(ZoneList<Statement*>* processor,
-                                  int end_token,
-                                  bool is_eval,
-                                  bool is_global,
-                                  bool* ok) {
+                                  int end_token, bool is_eval, bool is_global,
+                                  Scope** eval_scope, bool* ok) {
   // SourceElements ::
   //   (ModuleElement)* <end_token>
 
@@ -1082,6 +1088,10 @@ void* Parser::ParseSourceElements(ZoneList<Statement*>* processor,
             scope->set_start_position(scope_->start_position());
             scope->set_end_position(scope_->end_position());
             scope_ = scope;
+            if (eval_scope != NULL) {
+              // Caller will correct the positions of the ad hoc eval scope.
+              *eval_scope = scope;
+            }
             mode_ = PARSE_EAGERLY;
           }
           scope_->SetStrictMode(STRICT);
@@ -3722,7 +3732,7 @@ ZoneList<Statement*>* Parser::ParseEagerFunctionBody(
         yield, RelocInfo::kNoPosition), zone());
   }
 
-  ParseSourceElements(body, Token::RBRACE, false, false, CHECK_OK);
+  ParseSourceElements(body, Token::RBRACE, false, false, NULL, CHECK_OK);
 
   if (is_generator) {
     VariableProxy* get_proxy = factory()->NewVariableProxy(
@@ -4839,4 +4849,46 @@ bool Parser::Parse() {
   return (result != NULL);
 }
 
+
+void Parser::ParseOnBackground() {
+  DCHECK(info()->function() == NULL);
+  FunctionLiteral* result = NULL;
+  fni_ = new (zone()) FuncNameInferrer(ast_value_factory(), zone());
+
+  CompleteParserRecorder recorder;
+  if (compile_options() == ScriptCompiler::kProduceParserCache) {
+    log_ = &recorder;
+  }
+
+  DCHECK(info()->source_stream() != NULL);
+  ExternalStreamingStream stream(info()->source_stream(),
+                                 info()->source_stream_encoding());
+  scanner_.Initialize(&stream);
+  DCHECK(info()->context().is_null() || info()->context()->IsNativeContext());
+
+  // When streaming, we don't know the length of the source until we have parsed
+  // it. The raw data can be UTF-8, so we wouldn't know the source length until
+  // we have decoded it anyway even if we knew the raw data length (which we
+  // don't). We work around this by storing all the scopes which need their end
+  // position set at the end of the script (the top scope and possible eval
+  // scopes) and set their end position after we know the script length.
+  Scope* top_scope = NULL;
+  Scope* eval_scope = NULL;
+  result = DoParseProgram(info(), &top_scope, &eval_scope);
+
+  top_scope->set_end_position(scanner()->location().end_pos);
+  if (eval_scope != NULL) {
+    eval_scope->set_end_position(scanner()->location().end_pos);
+  }
+
+  info()->SetFunction(result);
+
+  // We cannot internalize on a background thread; a foreground task will take
+  // care of calling Parser::Internalize just before compilation.
+
+  if (compile_options() == ScriptCompiler::kProduceParserCache) {
+    if (result != NULL) *info_->cached_data() = recorder.GetScriptData();
+    log_ = NULL;
+  }
+}
 } }  // namespace v8::internal
diff --git a/src/parser.h b/src/parser.h
index bba3648..b5bf239 100644
--- a/src/parser.h
+++ b/src/parser.h
@@ -623,6 +623,11 @@ class Parser : public ParserBase<ParserTraits> {
     return parser.Parse();
   }
   bool Parse();
+  void ParseOnBackground();
+
+  // Handle errors detected during parsing, move statistics to Isolate,
+  // internalize strings (move them to the heap).
+  void Internalize();
 
  private:
   friend class ParserTraits;
@@ -663,8 +668,8 @@ class Parser : public ParserBase<ParserTraits> {
   }
 
   // Called by ParseProgram after setting up the scanner.
-  FunctionLiteral* DoParseProgram(CompilationInfo* info,
-                                  Handle<String> source);
+  FunctionLiteral* DoParseProgram(CompilationInfo* info, Scope** scope,
+                                  Scope** ad_hoc_eval_scope);
 
   void SetCachedData();
 
@@ -682,7 +687,8 @@ class Parser : public ParserBase<ParserTraits> {
   // By making the 'exception handling' explicit, we are forced to check
   // for failure at the call sites.
   void* ParseSourceElements(ZoneList<Statement*>* processor, int end_token,
-                            bool is_eval, bool is_global, bool* ok);
+                            bool is_eval, bool is_global,
+                            Scope** ad_hoc_eval_scope, bool* ok);
   Statement* ParseModuleElement(ZoneList<const AstRawString*>* labels,
                                 bool* ok);
   Statement* ParseModuleDeclaration(ZoneList<const AstRawString*>* names,
@@ -805,10 +811,6 @@ class Parser : public ParserBase<ParserTraits> {
 
   void ThrowPendingError();
 
-  // Handle errors detected during parsing, move statistics to Isolate,
-  // internalize strings (move them to the heap).
-  void Internalize();
-
   Scanner scanner_;
   PreParser* reusable_preparser_;
   Scope* original_scope_;  // for ES5 function declarations in sloppy eval
diff --git a/src/scanner-character-streams.cc b/src/scanner-character-streams.cc
index 9ec0ad1..74adda0 100644
--- a/src/scanner-character-streams.cc
+++ b/src/scanner-character-streams.cc
@@ -6,12 +6,40 @@
 
 #include "src/scanner-character-streams.h"
 
+#include "include/v8.h"
 #include "src/handles.h"
 #include "src/unicode-inl.h"
 
 namespace v8 {
 namespace internal {
 
+namespace {
+
+unsigned CopyCharsHelper(uint16_t* dest, unsigned length, const uint8_t* src,
+                         unsigned* src_pos, unsigned src_length,
+                         ScriptCompiler::StreamedSource::Encoding encoding) {
+  if (encoding == ScriptCompiler::StreamedSource::UTF8) {
+    return v8::internal::Utf8ToUtf16CharacterStream::CopyChars(
+        dest, length, src, src_pos, src_length);
+  }
+
+  unsigned to_fill = length;
+  if (to_fill > src_length - *src_pos) to_fill = src_length - *src_pos;
+
+  if (encoding == ScriptCompiler::StreamedSource::ONE_BYTE) {
+    v8::internal::CopyChars<uint8_t, uint16_t>(dest, src + *src_pos, to_fill);
+  } else {
+    DCHECK(encoding == ScriptCompiler::StreamedSource::TWO_BYTE);
+    v8::internal::CopyChars<uint16_t, uint16_t>(
+        dest, reinterpret_cast<const uint16_t*>(src + *src_pos), to_fill);
+  }
+  *src_pos += to_fill;
+  return to_fill;
+}
+
+}  // namespace
+
+
 // ----------------------------------------------------------------------------
 // BufferedUtf16CharacterStreams
 
@@ -145,6 +173,35 @@ Utf8ToUtf16CharacterStream::Utf8ToUtf16CharacterStream(const byte* data,
 Utf8ToUtf16CharacterStream::~Utf8ToUtf16CharacterStream() { }
 
 
+unsigned Utf8ToUtf16CharacterStream::CopyChars(uint16_t* dest, unsigned length,
+                                               const byte* src,
+                                               unsigned* src_pos,
+                                               unsigned src_length) {
+  static const unibrow::uchar kMaxUtf16Character = 0xffff;
+  unsigned i = 0;
+  // Because of the UTF-16 lead and trail surrogates, we stop filling the buffer
+  // one character early (in the normal case), because we need to have at least
+  // two free spaces in the buffer to be sure that the next character will fit.
+  while (i < length - 1) {
+    if (*src_pos == src_length) break;
+    unibrow::uchar c = src[*src_pos];
+    if (c <= unibrow::Utf8::kMaxOneByteChar) {
+      *src_pos = *src_pos + 1;
+    } else {
+      c = unibrow::Utf8::CalculateValue(src + *src_pos, src_length - *src_pos,
+                                        src_pos);
+    }
+    if (c > kMaxUtf16Character) {
+      dest[i++] = unibrow::Utf16::LeadSurrogate(c);
+      dest[i++] = unibrow::Utf16::TrailSurrogate(c);
+    } else {
+      dest[i++] = static_cast<uc16>(c);
+    }
+  }
+  return i;
+}
+
+
 unsigned Utf8ToUtf16CharacterStream::BufferSeekForward(unsigned delta) {
   unsigned old_pos = pos_;
   unsigned target_pos = pos_ + delta;
@@ -156,31 +213,14 @@ unsigned Utf8ToUtf16CharacterStream::BufferSeekForward(unsigned delta) {
 
 
 unsigned Utf8ToUtf16CharacterStream::FillBuffer(unsigned char_position) {
-  static const unibrow::uchar kMaxUtf16Character = 0xffff;
   SetRawPosition(char_position);
   if (raw_character_position_ != char_position) {
     // char_position was not a valid position in the stream (hit the end
     // while spooling to it).
     return 0u;
   }
-  unsigned i = 0;
-  while (i < kBufferSize - 1) {
-    if (raw_data_pos_ == raw_data_length_) break;
-    unibrow::uchar c = raw_data_[raw_data_pos_];
-    if (c <= unibrow::Utf8::kMaxOneByteChar) {
-      raw_data_pos_++;
-    } else {
-      c =  unibrow::Utf8::CalculateValue(raw_data_ + raw_data_pos_,
-                                         raw_data_length_ - raw_data_pos_,
-                                         &raw_data_pos_);
-    }
-    if (c > kMaxUtf16Character) {
-      buffer_[i++] = unibrow::Utf16::LeadSurrogate(c);
-      buffer_[i++] = unibrow::Utf16::TrailSurrogate(c);
-    } else {
-      buffer_[i++] = static_cast<uc16>(c);
-    }
-  }
+  unsigned i = CopyChars(buffer_, kBufferSize, raw_data_, &raw_data_pos_,
+                         raw_data_length_);
   raw_character_position_ = char_position + i;
   return i;
 }
@@ -276,6 +316,110 @@ void Utf8ToUtf16CharacterStream::SetRawPosition(unsigned target_position) {
 }
 
 
+unsigned ExternalStreamingStream::FillBuffer(unsigned position) {
+  // Ignore "position" which is the position in the decoded data. Instead,
+  // ExternalStreamingStream keeps track of the position in the raw data.
+  unsigned data_in_buffer = 0;
+  // Note that the UTF-8 decoder might not be able to fill the buffer
+  // completely; it will typically leave the last character empty (see
+  // Utf8ToUtf16CharacterStream::CopyChars).
+  while (data_in_buffer < kBufferSize - 1) {
+    if (current_data_ == NULL) {
+      // GetSomeData will wait until the embedder has enough data.
+      current_data_length_ = source_stream_->GetMoreData(&current_data_);
+      current_data_offset_ = 0;
+      bool data_ends = current_data_length_ == 0;
+
+      // A caveat: a data chunk might end with bytes from an incomplete UTF-8
+      // character (the rest of the bytes will be in the next chunk).
+      if (encoding_ == ScriptCompiler::StreamedSource::UTF8) {
+        HandleUtf8SplitCharacters(&data_in_buffer);
+        if (!data_ends && current_data_offset_ == current_data_length_) {
+          // The data stream didn't end, but we used all the data in the
+          // chunk. This will only happen when the chunk was really small. We
+          // don't handle the case where a UTF-8 character is split over several
+          // chunks; in that case V8 won't crash, but it will be a parse error.
+          delete[] current_data_;
+          current_data_ = NULL;
+          current_data_length_ = 0;
+          current_data_offset_ = 0;
+          continue;  // Request a new chunk.
+        }
+      }
+
+      // Did the data stream end?
+      if (data_ends) {
+        DCHECK(utf8_split_char_buffer_length_ == 0);
+        return data_in_buffer;
+      }
+    }
+
+    // Fill the buffer from current_data_.
+    unsigned new_offset = 0;
+    unsigned new_chars_in_buffer =
+        CopyCharsHelper(buffer_ + data_in_buffer, kBufferSize - data_in_buffer,
+                        current_data_ + current_data_offset_, &new_offset,
+                        current_data_length_ - current_data_offset_, encoding_);
+    data_in_buffer += new_chars_in_buffer;
+    current_data_offset_ += new_offset;
+    DCHECK(data_in_buffer <= kBufferSize);
+
+    // Did we use all the data in the data chunk?
+    if (current_data_offset_ == current_data_length_) {
+      delete[] current_data_;
+      current_data_ = NULL;
+      current_data_length_ = 0;
+      current_data_offset_ = 0;
+    }
+  }
+  return data_in_buffer;
+}
+
+void ExternalStreamingStream::HandleUtf8SplitCharacters(
+    unsigned* data_in_buffer) {
+  // First check if we have leftover data from the last chunk.
+  unibrow::uchar c;
+  if (utf8_split_char_buffer_length_ > 0) {
+    // Move the bytes which are part of the split character (which started in
+    // the previous chunk) into utf8_split_char_buffer_.
+    while (current_data_offset_ < current_data_length_ &&
+           utf8_split_char_buffer_length_ < 4 &&
+           (c = current_data_[current_data_offset_]) >
+               unibrow::Utf8::kMaxOneByteChar) {
+      utf8_split_char_buffer_[utf8_split_char_buffer_length_] = c;
+      ++utf8_split_char_buffer_length_;
+      ++current_data_offset_;
+    }
+
+    // Convert the data in utf8_split_char_buffer_.
+    unsigned new_offset = 0;
+    unsigned new_chars_in_buffer =
+        CopyCharsHelper(buffer_ + *data_in_buffer,
+                        kBufferSize - *data_in_buffer, utf8_split_char_buffer_,
+                        &new_offset, utf8_split_char_buffer_length_, encoding_);
+    *data_in_buffer += new_chars_in_buffer;
+    // Make sure we used all the data.
+    DCHECK(new_offset == utf8_split_char_buffer_length_);
+    DCHECK(*data_in_buffer <= kBufferSize);
+
+    utf8_split_char_buffer_length_ = 0;
+  }
+
+  // Move bytes which are part of an incomplete character from the end of the
+  // current chunk to utf8_split_char_buffer_. They will be converted when the
+  // next data chunk arrives.
+  while (current_data_length_ > current_data_offset_ &&
+         (c = current_data_[current_data_length_ - 1]) >
+             unibrow::Utf8::kMaxOneByteChar) {
+    --current_data_length_;
+    ++utf8_split_char_buffer_length_;
+  }
+  for (unsigned i = 0; i < utf8_split_char_buffer_length_; ++i) {
+    utf8_split_char_buffer_[i] = current_data_[current_data_length_ + i];
+  }
+}
+
+
 // ----------------------------------------------------------------------------
 // ExternalTwoByteStringUtf16CharacterStream
 
diff --git a/src/scanner-character-streams.h b/src/scanner-character-streams.h
index eeb40e2..afca13f 100644
--- a/src/scanner-character-streams.h
+++ b/src/scanner-character-streams.h
@@ -59,6 +59,9 @@ class Utf8ToUtf16CharacterStream: public BufferedUtf16CharacterStream {
   Utf8ToUtf16CharacterStream(const byte* data, unsigned length);
   virtual ~Utf8ToUtf16CharacterStream();
 
+  static unsigned CopyChars(uint16_t* dest, unsigned length, const byte* src,
+                            unsigned* src_pos, unsigned src_length);
+
  protected:
   virtual unsigned BufferSeekForward(unsigned delta);
   virtual unsigned FillBuffer(unsigned char_position);
@@ -73,6 +76,46 @@ class Utf8ToUtf16CharacterStream: public BufferedUtf16CharacterStream {
 };
 
 
+// ExternalStreamingStream is a wrapper around an ExternalSourceStream (see
+// include/v8.h) subclass implemented by the embedder.
+class ExternalStreamingStream : public BufferedUtf16CharacterStream {
+ public:
+  ExternalStreamingStream(ScriptCompiler::ExternalSourceStream* source_stream,
+                          v8::ScriptCompiler::StreamedSource::Encoding encoding)
+      : source_stream_(source_stream),
+        encoding_(encoding),
+        current_data_(NULL),
+        current_data_offset_(0),
+        current_data_length_(0),
+        utf8_split_char_buffer_length_(0) {}
+
+  virtual ~ExternalStreamingStream() { delete[] current_data_; }
+
+  virtual unsigned BufferSeekForward(unsigned delta) OVERRIDE {
+    // We never need to seek forward when streaming scripts. We only seek
+    // forward when we want to parse a function whose location we already know,
+    // and when streaming, we don't know the locations of anything we haven't
+    // seen yet.
+    UNREACHABLE();
+    return 0;
+  }
+
+  virtual unsigned FillBuffer(unsigned position);
+
+ private:
+  void HandleUtf8SplitCharacters(unsigned* data_in_buffer);
+
+  ScriptCompiler::ExternalSourceStream* source_stream_;
+  v8::ScriptCompiler::StreamedSource::Encoding encoding_;
+  const uint8_t* current_data_;
+  unsigned current_data_offset_;
+  unsigned current_data_length_;
+  // For converting UTF-8 characters which are split across two data chunks.
+  uint8_t utf8_split_char_buffer_[4];
+  unsigned utf8_split_char_buffer_length_;
+};
+
+
 // UTF16 buffer to read characters from an external string.
 class ExternalTwoByteStringUtf16CharacterStream: public Utf16CharacterStream {
  public:
diff --git a/test/cctest/test-api.cc b/test/cctest/test-api.cc
index 75c5b8e..e320893 100644
--- a/test/cctest/test-api.cc
+++ b/test/cctest/test-api.cc
@@ -23035,3 +23035,307 @@ TEST(GetHiddenPropertyTableAfterAccessCheck) {
 
   obj->SetHiddenValue(v8_str("hidden key 2"), v8_str("hidden value 2"));
 }
+
+
+class TestSourceStream : public v8::ScriptCompiler::ExternalSourceStream {
+ public:
+  explicit TestSourceStream(const char** chunks) : chunks_(chunks), index_(0) {}
+
+  virtual size_t GetMoreData(const uint8_t** src) {
+    // Unlike in real use cases, this function will never block.
+    if (chunks_[index_] == NULL) {
+      return 0;
+    }
+    // Copy the data, since the caller takes ownership of it.
+    size_t len = strlen(chunks_[index_]);
+    // We don't need to zero-terminate since we return the length.
+    uint8_t* copy = new uint8_t[len];
+    memcpy(copy, chunks_[index_], len);
+    *src = copy;
+    ++index_;
+    return len;
+  }
+
+  // Helper for constructing a string from chunks (the compilation needs it
+  // too).
+  static char* FullSourceString(const char** chunks) {
+    size_t total_len = 0;
+    for (size_t i = 0; chunks[i] != NULL; ++i) {
+      total_len += strlen(chunks[i]);
+    }
+    char* full_string = new char[total_len + 1];
+    size_t offset = 0;
+    for (size_t i = 0; chunks[i] != NULL; ++i) {
+      size_t len = strlen(chunks[i]);
+      memcpy(full_string + offset, chunks[i], len);
+      offset += len;
+    }
+    full_string[total_len] = 0;
+    return full_string;
+  }
+
+ private:
+  const char** chunks_;
+  unsigned index_;
+};
+
+
+// Helper function for running streaming tests.
+void RunStreamingTest(const char** chunks,
+                      v8::ScriptCompiler::StreamedSource::Encoding encoding =
+                          v8::ScriptCompiler::StreamedSource::ONE_BYTE,
+                      bool expected_success = true) {
+  LocalContext env;
+  v8::Isolate* isolate = env->GetIsolate();
+  v8::HandleScope scope(isolate);
+  v8::TryCatch try_catch;
+
+  v8::ScriptCompiler::StreamedSource source(new TestSourceStream(chunks),
+                                            encoding);
+  v8::ScriptCompiler::ScriptStreamingTask* task =
+      v8::ScriptCompiler::StartStreamingScript(isolate, &source);
+
+  // TestSourceStream::GetMoreData won't block, so it's OK to just run the
+  // task here in the main thread.
+  task->Run();
+  delete task;
+
+  v8::ScriptOrigin origin(v8_str("http://foo.com"));
+  char* full_source = TestSourceStream::FullSourceString(chunks);
+
+  // The possible errors are only produced while compiling.
+  CHECK_EQ(false, try_catch.HasCaught());
+
+  v8::Handle<Script> script = v8::ScriptCompiler::Compile(
+      isolate, &source, v8_str(full_source), origin);
+  if (expected_success) {
+    CHECK(!script.IsEmpty());
+    v8::Handle<Value> result(script->Run());
+    // All scripts are supposed to return the fixed value 13 when ran.
+    CHECK_EQ(13, result->Int32Value());
+  } else {
+    CHECK(script.IsEmpty());
+    CHECK(try_catch.HasCaught());
+  }
+  delete[] full_source;
+}
+
+
+TEST(StreamingSimpleScript) {
+  // This script is unrealistically small, since no one chunk is enough to fill
+  // the backing buffer of Scanner, let alone overflow it.
+  const char* chunks[] = {"function foo() { ret", "urn 13; } f", "oo(); ",
+                          NULL};
+  RunStreamingTest(chunks);
+}
+
+
+TEST(StreamingBiggerScript) {
+  const char* chunk1 =
+      "function foo() {\n"
+      "  // Make this chunk sufficiently long so that it will overflow the\n"
+      "  // backing buffer of the Scanner.\n"
+      "  var i = 0;\n"
+      "  var result = 0;\n"
+      "  for (i = 0; i < 13; ++i) { result = result + 1; }\n"
+      "  result = 0;\n"
+      "  for (i = 0; i < 13; ++i) { result = result + 1; }\n"
+      "  result = 0;\n"
+      "  for (i = 0; i < 13; ++i) { result = result + 1; }\n"
+      "  result = 0;\n"
+      "  for (i = 0; i < 13; ++i) { result = result + 1; }\n"
+      "  return result;\n"
+      "}\n";
+  const char* chunks[] = {chunk1, "foo(); ", NULL};
+  RunStreamingTest(chunks);
+}
+
+
+TEST(StreamingScriptWithParseError) {
+  // Test that parse errors from streamed scripts are propagated correctly.
+  {
+    char chunk1[] =
+        "  // This will result in a parse error.\n"
+        "  var if else then foo";
+    char chunk2[] = "  13\n";
+    const char* chunks[] = {chunk1, chunk2, "foo();", NULL};
+
+    RunStreamingTest(chunks, v8::ScriptCompiler::StreamedSource::ONE_BYTE,
+                     false);
+  }
+  // Test that the next script succeeds normally.
+  {
+    char chunk1[] =
+        "  // This will be parsed successfully.\n"
+        "  function foo() { return ";
+    char chunk2[] = "  13; }\n";
+    const char* chunks[] = {chunk1, chunk2, "foo();", NULL};
+
+    RunStreamingTest(chunks);
+  }
+}
+
+
+TEST(StreamingUtf8Script) {
+  const char* chunk1 =
+      "function foo() {\n"
+      "  // This function will contain an UTF-8 character which is not in\n"
+      "  // ASCII.\n"
+      "  var foob\uc481r = 13;\n"
+      "  return foob\uc481r;\n"
+      "}\n";
+  const char* chunks[] = {chunk1, "foo(); ", NULL};
+  RunStreamingTest(chunks, v8::ScriptCompiler::StreamedSource::UTF8);
+}
+
+
+TEST(StreamingUtf8ScriptWithSplitCharactersSanityCheck) {
+  // A sanity check to prove that the approach of splitting UTF-8
+  // characters is correct. Here is an UTF-8 character which will take three
+  // bytes.
+  const char* reference = "\uc481";
+  CHECK_EQ(3, strlen(reference));
+  char chunk1[] =
+      "function foo() {\n"
+      "  // This function will contain an UTF-8 character which is not in\n"
+      "  // ASCII.\n"
+      "  var foob";
+  char chunk2[] =
+      "XXXr = 13;\n"
+      "  return foob\uc481r;\n"
+      "}\n";
+  for (int i = 0; i < 3; ++i) {
+    chunk2[i] = reference[i];
+  }
+  const char* chunks[] = {chunk1, chunk2, "foo();", NULL};
+  RunStreamingTest(chunks, v8::ScriptCompiler::StreamedSource::UTF8);
+}
+
+
+TEST(StreamingUtf8ScriptWithSplitCharacters) {
+  // Stream data where a multi-byte UTF-8 character is split between two data
+  // chunks.
+  const char* reference = "\uc481";
+  char chunk1[] =
+      "function foo() {\n"
+      "  // This function will contain an UTF-8 character which is not in\n"
+      "  // ASCII.\n"
+      "  var foobX";
+  char chunk2[] =
+      "XXr = 13;\n"
+      "  return foob\uc481r;\n"
+      "}\n";
+  chunk1[strlen(chunk1) - 1] = reference[0];
+  chunk2[0] = reference[1];
+  chunk2[1] = reference[2];
+  const char* chunks[] = {chunk1, chunk2, "foo();", NULL};
+  RunStreamingTest(chunks, v8::ScriptCompiler::StreamedSource::UTF8);
+}
+
+
+TEST(StreamingUtf8ScriptWithSplitCharactersValidEdgeCases) {
+  // Tests edge cases which should still be decoded correctly.
+
+  // Case 1: a chunk contains only bytes for a split character (and no other
+  // data). This kind of a chunk would be exceptionally small, but we should
+  // still decode it correctly.
+  const char* reference = "\uc481";
+  fprintf(stderr, "%d %d %d\n", reference[0], reference[1], reference[2]);
+  // The small chunk is at the beginning of the split character
+  {
+    char chunk1[] =
+        "function foo() {\n"
+        "  // This function will contain an UTF-8 character which is not in\n"
+        "  // ASCII.\n"
+        "  var foob";
+    char chunk2[] = "XX";
+    char chunk3[] =
+        "Xr = 13;\n"
+        "  return foob\uc481r;\n"
+        "}\n";
+    chunk2[0] = reference[0];
+    chunk2[1] = reference[1];
+    chunk3[0] = reference[2];
+    const char* chunks[] = {chunk1, chunk2, chunk3, "foo();", NULL};
+    RunStreamingTest(chunks, v8::ScriptCompiler::StreamedSource::UTF8);
+  }
+  // The small chunk is at the end of a character
+  {
+    char chunk1[] =
+        "function foo() {\n"
+        "  // This function will contain an UTF-8 character which is not in\n"
+        "  // ASCII.\n"
+        "  var foobX";
+    char chunk2[] = "XX";
+    char chunk3[] =
+        "r = 13;\n"
+        "  return foob\uc481r;\n"
+        "}\n";
+    chunk1[strlen(chunk1) - 1] = reference[0];
+    chunk2[0] = reference[1];
+    chunk2[1] = reference[2];
+    const char* chunks[] = {chunk1, chunk2, chunk3, "foo();", NULL};
+    RunStreamingTest(chunks, v8::ScriptCompiler::StreamedSource::UTF8);
+  }
+  // Case 2: the script ends with a multi-byte character. Make sure that it's
+  // decoded correctly and not just ignored.
+  {
+    char chunk1[] =
+        "var foob\uc481 = 13;\n"
+        "foob\uc481";
+    const char* chunks[] = {chunk1, NULL};
+    RunStreamingTest(chunks, v8::ScriptCompiler::StreamedSource::UTF8);
+  }
+}
+
+
+TEST(StreamingUtf8ScriptWithSplitCharactersInvalidEdgeCases) {
+  // Test cases where a UTF-8 character is split over several chunks. Those
+  // cases are not supported (the embedder should give the data in big enough
+  // chunks), but we shouldn't crash, just produce a parse error.
+  const char* reference = "\uc481";
+  char chunk1[] =
+      "function foo() {\n"
+      "  // This function will contain an UTF-8 character which is not in\n"
+      "  // ASCII.\n"
+      "  var foobX";
+  char chunk2[] = "X";
+  char chunk3[] =
+      "Xr = 13;\n"
+      "  return foob\uc481r;\n"
+      "}\n";
+  chunk1[strlen(chunk1) - 1] = reference[0];
+  chunk2[0] = reference[1];
+  chunk3[0] = reference[2];
+  const char* chunks[] = {chunk1, chunk2, chunk3, "foo();", NULL};
+
+  RunStreamingTest(chunks, v8::ScriptCompiler::StreamedSource::UTF8, false);
+}
+
+
+TEST(StreamingProducesParserCache) {
+  i::FLAG_min_preparse_length = 0;
+  const char* chunks[] = {"function foo() { ret", "urn 13; } f", "oo(); ",
+                          NULL};
+
+  LocalContext env;
+  v8::Isolate* isolate = env->GetIsolate();
+  v8::HandleScope scope(isolate);
+
+  v8::ScriptCompiler::StreamedSource source(
+      new TestSourceStream(chunks),
+      v8::ScriptCompiler::StreamedSource::ONE_BYTE);
+  v8::ScriptCompiler::ScriptStreamingTask* task =
+      v8::ScriptCompiler::StartStreamingScript(
+          isolate, &source, v8::ScriptCompiler::kProduceParserCache);
+
+  // TestSourceStream::GetMoreData won't block, so it's OK to just run the
+  // task here in the main thread.
+  task->Run();
+  delete task;
+
+  const v8::ScriptCompiler::CachedData* cached_data = source.GetCachedData();
+  CHECK(cached_data != NULL);
+  CHECK(cached_data->data != NULL);
+  CHECK_GT(cached_data->length, 0);
+}
diff --git a/tools/gyp/v8.gyp b/tools/gyp/v8.gyp
index 1641138..79a1adf 100644
--- a/tools/gyp/v8.gyp
+++ b/tools/gyp/v8.gyp
@@ -347,6 +347,8 @@
         '../../src/ast-value-factory.h',
         '../../src/ast.cc',
         '../../src/ast.h',
+        '../../src/background-parsing-task.cc',
+        '../../src/background-parsing-task.h',
         '../../src/bignum-dtoa.cc',
         '../../src/bignum-dtoa.h',
         '../../src/bignum.cc',
-- 
2.7.4