Adding upstream version 86.0.1.upstream/86.0.1upstream

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>

1 files changed, 677 insertions, 0 deletions

diff --git a/js/src/vm/HelperThreadState.h b/js/src/vm/HelperThreadState.h
new file mode 100644
index 0000000000..0ed8921b9f
--- /dev/null
+++ b/js/src/vm/HelperThreadState.h
@@ -0,0 +1,677 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
+ * vim: set ts=8 sts=2 et sw=2 tw=80:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Definitions for managing off-thread work using a process wide list of
+ * worklist items and pool of threads. Worklist items are engine internal, and
+ * are distinct from e.g. web workers.
+ */
+
+#ifndef vm_HelperThreadState_h
+#define vm_HelperThreadState_h
+
+#include "mozilla/Attributes.h"
+#include "mozilla/EnumeratedArray.h"
+#include "mozilla/TimeStamp.h"
+
+#include "jsapi.h"
+
+#include "ds/Fifo.h"
+#include "frontend/CompilationInfo.h"  // CompilationStencil, CompilationStencilSet, CompilationGCOutput
+#include "js/CompileOptions.h"
+#include "js/TypeDecls.h"
+#include "threading/ConditionVariable.h"
+#include "threading/Thread.h"
+#include "vm/HelperThreads.h"
+#include "vm/HelperThreadTask.h"
+#include "vm/JSContext.h"
+#include "vm/OffThreadPromiseRuntimeState.h"  // js::OffThreadPromiseTask
+
+namespace js {
+
+class AutoLockHelperThreadState;
+class AutoUnlockHelperThreadState;
+class CompileError;
+struct ParseTask;
+struct PromiseHelperTask;
+class PromiseObject;
+
+namespace jit {
+class IonCompileTask;
+class IonFreeTask;
+}  // namespace jit
+
+namespace wasm {
+struct Tier2GeneratorTask;
+}  // namespace wasm
+
+enum class ParseTaskKind { Script, Module, ScriptDecode, MultiScriptsDecode };
+enum class StartEncoding { No, Yes };
+
+namespace wasm {
+
+struct CompileTask;
+typedef Fifo<CompileTask*, 0, SystemAllocPolicy> CompileTaskPtrFifo;
+
+struct Tier2GeneratorTask : public HelperThreadTask {
+  virtual ~Tier2GeneratorTask() = default;
+  virtual void cancel() = 0;
+};
+
+using UniqueTier2GeneratorTask = UniquePtr<Tier2GeneratorTask>;
+typedef Vector<Tier2GeneratorTask*, 0, SystemAllocPolicy>
+    Tier2GeneratorTaskPtrVector;
+
+}  // namespace wasm
+
+// Per-process state for off thread work items.
+class GlobalHelperThreadState {
+  friend class AutoLockHelperThreadState;
+  friend class AutoUnlockHelperThreadState;
+
+ public:
+  // A single tier-2 ModuleGenerator job spawns many compilation jobs, and we
+  // do not want to allow more than one such ModuleGenerator to run at a time.
+  static const size_t MaxTier2GeneratorTasks = 1;
+
+  // Number of CPUs to treat this machine as having when creating threads.
+  // May be accessed without locking.
+  size_t cpuCount;
+
+  // Number of threads to create. May be accessed without locking.
+  size_t threadCount;
+
+  typedef Vector<jit::IonCompileTask*, 0, SystemAllocPolicy>
+      IonCompileTaskVector;
+  using IonFreeTaskVector =
+      Vector<js::UniquePtr<jit::IonFreeTask>, 0, SystemAllocPolicy>;
+  typedef Vector<UniquePtr<ParseTask>, 0, SystemAllocPolicy> ParseTaskVector;
+  using ParseTaskList = mozilla::LinkedList<ParseTask>;
+  typedef Vector<UniquePtr<SourceCompressionTask>, 0, SystemAllocPolicy>
+      SourceCompressionTaskVector;
+  using GCParallelTaskList = mozilla::LinkedList<GCParallelTask>;
+  typedef Vector<PromiseHelperTask*, 0, SystemAllocPolicy>
+      PromiseHelperTaskVector;
+  typedef Vector<JSContext*, 0, SystemAllocPolicy> ContextVector;
+  using HelperThreadVector =
+      Vector<UniquePtr<HelperThread>, 0, SystemAllocPolicy>;
+
+  // Count of running task by each threadType.
+  mozilla::EnumeratedArray<ThreadType, ThreadType::THREAD_TYPE_MAX, size_t>
+      runningTaskCount;
+  size_t totalCountRunningTasks;
+
+  WriteOnceData<JS::RegisterThreadCallback> registerThread;
+  WriteOnceData<JS::UnregisterThreadCallback> unregisterThread;
+
+ private:
+  // The lists below are all protected by |lock|.
+
+  // List of available helper threads.
+  HelperThreadVector threads_;
+
+  // Ion compilation worklist and finished jobs.
+  IonCompileTaskVector ionWorklist_, ionFinishedList_;
+  IonFreeTaskVector ionFreeList_;
+
+  // wasm worklists.
+  wasm::CompileTaskPtrFifo wasmWorklist_tier1_;
+  wasm::CompileTaskPtrFifo wasmWorklist_tier2_;
+  wasm::Tier2GeneratorTaskPtrVector wasmTier2GeneratorWorklist_;
+
+  // Count of finished Tier2Generator tasks.
+  uint32_t wasmTier2GeneratorsFinished_;
+
+  // Async tasks that, upon completion, are dispatched back to the JSContext's
+  // owner thread via embedding callbacks instead of a finished list.
+  PromiseHelperTaskVector promiseHelperTasks_;
+
+  // Script parsing/emitting worklist and finished jobs.
+  ParseTaskVector parseWorklist_;
+  ParseTaskList parseFinishedList_;
+
+  // Parse tasks waiting for an atoms-zone GC to complete.
+  ParseTaskVector parseWaitingOnGC_;
+
+  // Source compression worklist of tasks that we do not yet know can start.
+  SourceCompressionTaskVector compressionPendingList_;
+
+  // Source compression worklist of tasks that can start.
+  SourceCompressionTaskVector compressionWorklist_;
+
+  // Finished source compression tasks.
+  SourceCompressionTaskVector compressionFinishedList_;
+
+  // GC tasks needing to be done in parallel.
+  GCParallelTaskList gcParallelWorklist_;
+  size_t gcParallelThreadCount;
+
+  // Global list of JSContext for GlobalHelperThreadState to use.
+  ContextVector helperContexts_;
+
+  using HelperThreadTaskVector =
+      Vector<HelperThreadTask*, 0, SystemAllocPolicy>;
+  // Vector of running HelperThreadTask.
+  // This is used to get the HelperThreadTask that are currently running.
+  HelperThreadTaskVector helperTasks_;
+
+  ParseTask* removeFinishedParseTask(JSContext* cx, ParseTaskKind kind,
+                                     JS::OffThreadToken* token);
+
+ public:
+  void addSizeOfIncludingThis(JS::GlobalStats* stats,
+                              AutoLockHelperThreadState& lock) const;
+
+  size_t maxIonCompilationThreads() const;
+  size_t maxWasmCompilationThreads() const;
+  size_t maxWasmTier2GeneratorThreads() const;
+  size_t maxPromiseHelperThreads() const;
+  size_t maxParseThreads() const;
+  size_t maxCompressionThreads() const;
+  size_t maxGCParallelThreads(const AutoLockHelperThreadState& lock) const;
+
+  GlobalHelperThreadState();
+
+  HelperThreadVector& threads(const AutoLockHelperThreadState& lock) {
+    return threads_;
+  }
+  const HelperThreadVector& threads(
+      const AutoLockHelperThreadState& lock) const {
+    return threads_;
+  }
+
+  bool ensureInitialized();
+  bool ensureThreadCount(size_t count);
+  void finish();
+  void finishThreads();
+
+  MOZ_MUST_USE bool ensureContextList(size_t count);
+  JSContext* getFirstUnusedContext(AutoLockHelperThreadState& locked);
+  void destroyHelperContexts(AutoLockHelperThreadState& lock);
+
+#ifdef DEBUG
+  void assertIsLockedByCurrentThread() const;
+#endif
+
+  enum CondVar {
+    // For notifying threads waiting for work that they may be able to make
+    // progress, ie, a work item has been completed by a helper thread and
+    // the thread that created the work item can now consume it.
+    CONSUMER,
+
+    // For notifying helper threads doing the work that they may be able to
+    // make progress, ie, a work item has been enqueued and an idle helper
+    // thread may pick up up the work item and perform it.
+    PRODUCER,
+  };
+
+  void wait(AutoLockHelperThreadState& locked, CondVar which,
+            mozilla::TimeDuration timeout = mozilla::TimeDuration::Forever());
+  void notifyAll(CondVar which, const AutoLockHelperThreadState&);
+
+ private:
+  void notifyOne(CondVar which, const AutoLockHelperThreadState&);
+
+ public:
+  // Helper method for removing items from the vectors below while iterating
+  // over them.
+  template <typename T>
+  void remove(T& vector, size_t* index) {
+    // Self-moving is undefined behavior.
+    if (*index != vector.length() - 1) {
+      vector[*index] = std::move(vector.back());
+    }
+    (*index)--;
+    vector.popBack();
+  }
+
+  IonCompileTaskVector& ionWorklist(const AutoLockHelperThreadState&) {
+    return ionWorklist_;
+  }
+  IonCompileTaskVector& ionFinishedList(const AutoLockHelperThreadState&) {
+    return ionFinishedList_;
+  }
+  IonFreeTaskVector& ionFreeList(const AutoLockHelperThreadState&) {
+    return ionFreeList_;
+  }
+
+  wasm::CompileTaskPtrFifo& wasmWorklist(const AutoLockHelperThreadState&,
+                                         wasm::CompileMode m) {
+    switch (m) {
+      case wasm::CompileMode::Once:
+      case wasm::CompileMode::Tier1:
+        return wasmWorklist_tier1_;
+      case wasm::CompileMode::Tier2:
+        return wasmWorklist_tier2_;
+      default:
+        MOZ_CRASH();
+    }
+  }
+
+  wasm::Tier2GeneratorTaskPtrVector& wasmTier2GeneratorWorklist(
+      const AutoLockHelperThreadState&) {
+    return wasmTier2GeneratorWorklist_;
+  }
+
+  void incWasmTier2GeneratorsFinished(const AutoLockHelperThreadState&) {
+    wasmTier2GeneratorsFinished_++;
+  }
+
+  uint32_t wasmTier2GeneratorsFinished(const AutoLockHelperThreadState&) const {
+    return wasmTier2GeneratorsFinished_;
+  }
+
+  PromiseHelperTaskVector& promiseHelperTasks(
+      const AutoLockHelperThreadState&) {
+    return promiseHelperTasks_;
+  }
+
+  ParseTaskVector& parseWorklist(const AutoLockHelperThreadState&) {
+    return parseWorklist_;
+  }
+  ParseTaskList& parseFinishedList(const AutoLockHelperThreadState&) {
+    return parseFinishedList_;
+  }
+  ParseTaskVector& parseWaitingOnGC(const AutoLockHelperThreadState&) {
+    return parseWaitingOnGC_;
+  }
+
+  SourceCompressionTaskVector& compressionPendingList(
+      const AutoLockHelperThreadState&) {
+    return compressionPendingList_;
+  }
+
+  SourceCompressionTaskVector& compressionWorklist(
+      const AutoLockHelperThreadState&) {
+    return compressionWorklist_;
+  }
+
+  SourceCompressionTaskVector& compressionFinishedList(
+      const AutoLockHelperThreadState&) {
+    return compressionFinishedList_;
+  }
+
+  GCParallelTaskList& gcParallelWorklist(const AutoLockHelperThreadState&) {
+    return gcParallelWorklist_;
+  }
+
+  void setGCParallelThreadCount(size_t count,
+                                const AutoLockHelperThreadState&) {
+    MOZ_ASSERT(count >= 1);
+    MOZ_ASSERT(count <= threadCount);
+    gcParallelThreadCount = count;
+  }
+
+  HelperThreadTaskVector& helperTasks(const AutoLockHelperThreadState&) {
+    return helperTasks_;
+  }
+
+  HelperThreadTask* maybeGetWasmCompile(const AutoLockHelperThreadState& lock,
+                                        wasm::CompileMode mode);
+
+  HelperThreadTask* maybeGetWasmTier1CompileTask(
+      const AutoLockHelperThreadState& lock);
+  HelperThreadTask* maybeGetWasmTier2CompileTask(
+      const AutoLockHelperThreadState& lock);
+  HelperThreadTask* maybeGetWasmTier2GeneratorTask(
+      const AutoLockHelperThreadState& lock);
+  HelperThreadTask* maybeGetPromiseHelperTask(
+      const AutoLockHelperThreadState& lock);
+  HelperThreadTask* maybeGetIonCompileTask(
+      const AutoLockHelperThreadState& lock);
+  HelperThreadTask* maybeGetIonFreeTask(const AutoLockHelperThreadState& lock);
+  HelperThreadTask* maybeGetParseTask(const AutoLockHelperThreadState& lock);
+  HelperThreadTask* maybeGetCompressionTask(
+      const AutoLockHelperThreadState& lock);
+  HelperThreadTask* maybeGetGCParallelTask(
+      const AutoLockHelperThreadState& lock);
+
+  enum class ScheduleCompressionTask { GC, API };
+
+  // Used by a major GC to signal processing enqueued compression tasks.
+  void startHandlingCompressionTasks(ScheduleCompressionTask schedule,
+                                     JSRuntime* maybeRuntime,
+                                     const AutoLockHelperThreadState& lock);
+
+  jit::IonCompileTask* highestPriorityPendingIonCompile(
+      const AutoLockHelperThreadState& lock);
+
+ private:
+  UniquePtr<ParseTask> finishParseTaskCommon(JSContext* cx, ParseTaskKind kind,
+                                             JS::OffThreadToken* token);
+
+  JSScript* finishSingleParseTask(
+      JSContext* cx, ParseTaskKind kind, JS::OffThreadToken* token,
+      StartEncoding startEncoding = StartEncoding::No);
+  bool generateLCovSources(JSContext* cx, ParseTask* parseTask);
+  bool finishMultiParseTask(JSContext* cx, ParseTaskKind kind,
+                            JS::OffThreadToken* token,
+                            MutableHandle<ScriptVector> scripts);
+
+  void mergeParseTaskRealm(JSContext* cx, ParseTask* parseTask,
+                           JS::Realm* dest);
+
+ public:
+  void cancelParseTask(JSRuntime* rt, ParseTaskKind kind,
+                       JS::OffThreadToken* token);
+  void destroyParseTask(JSRuntime* rt, ParseTask* parseTask);
+
+  void trace(JSTracer* trc);
+
+  JSScript* finishScriptParseTask(
+      JSContext* cx, JS::OffThreadToken* token,
+      StartEncoding startEncoding = StartEncoding::No);
+  JSScript* finishScriptDecodeTask(JSContext* cx, JS::OffThreadToken* token);
+  bool finishMultiScriptsDecodeTask(JSContext* cx, JS::OffThreadToken* token,
+                                    MutableHandle<ScriptVector> scripts);
+  JSObject* finishModuleParseTask(JSContext* cx, JS::OffThreadToken* token);
+
+  bool hasActiveThreads(const AutoLockHelperThreadState&);
+  bool hasQueuedTasks(const AutoLockHelperThreadState& locked);
+  void waitForAllThreads();
+  void waitForAllThreadsLocked(AutoLockHelperThreadState&);
+
+  bool checkTaskThreadLimit(ThreadType threadType, size_t maxThreads,
+                            bool isMaster,
+                            const AutoLockHelperThreadState& lock) const;
+  bool checkTaskThreadLimit(ThreadType threadType, size_t maxThreads,
+                            const AutoLockHelperThreadState& lock) const {
+    return checkTaskThreadLimit(threadType, maxThreads, /* isMaster */ false,
+                                lock);
+  }
+
+  void triggerFreeUnusedMemory();
+
+ private:
+  /* Condvars for threads waiting/notifying each other. */
+  js::ConditionVariable consumerWakeup;
+  js::ConditionVariable producerWakeup;
+
+  js::ConditionVariable& whichWakeup(CondVar which) {
+    switch (which) {
+      case CONSUMER:
+        return consumerWakeup;
+      case PRODUCER:
+        return producerWakeup;
+      default:
+        MOZ_CRASH("Invalid CondVar in |whichWakeup|");
+    }
+  }
+
+  void dispatch(const AutoLockHelperThreadState& locked);
+
+ public:
+  bool submitTask(wasm::UniqueTier2GeneratorTask task);
+  bool submitTask(wasm::CompileTask* task, wasm::CompileMode mode);
+  bool submitTask(UniquePtr<jit::IonFreeTask> task,
+                  const AutoLockHelperThreadState& lock);
+  bool submitTask(jit::IonCompileTask* task,
+                  const AutoLockHelperThreadState& locked);
+  bool submitTask(UniquePtr<SourceCompressionTask> task,
+                  const AutoLockHelperThreadState& locked);
+  bool submitTask(JSRuntime* rt, UniquePtr<ParseTask> task,
+                  const AutoLockHelperThreadState& locked);
+  bool submitTask(PromiseHelperTask* task);
+  bool submitTask(GCParallelTask* task,
+                  const AutoLockHelperThreadState& locked);
+  void runTaskLocked(HelperThreadTask* task, AutoLockHelperThreadState& lock);
+};
+
+static inline GlobalHelperThreadState& HelperThreadState() {
+  extern GlobalHelperThreadState* gHelperThreadState;
+
+  MOZ_ASSERT(gHelperThreadState);
+  return *gHelperThreadState;
+}
+
+/* Individual helper thread, one allocated per core. */
+class HelperThread {
+  Thread thread;
+
+  /*
+   * The profiling thread for this helper thread, which can be used to push
+   * and pop label frames.
+   * This field being non-null indicates that this thread has been registered
+   * and needs to be unregistered at shutdown.
+   */
+  ProfilingStack* profilingStack = nullptr;
+
+  /*
+   * Indicate to a thread that it should terminate itself. This is only read
+   * or written with the helper thread state lock held.
+   */
+  bool terminate = false;
+
+ public:
+  HelperThread();
+  MOZ_MUST_USE bool init();
+
+  ThreadId threadId() { return thread.get_id(); }
+
+  void setTerminate(const AutoLockHelperThreadState& lock);
+  void join();
+
+  static void ThreadMain(void* arg);
+  void threadLoop();
+
+  void ensureRegisteredWithProfiler();
+  void unregisterWithProfilerIfNeeded();
+
+ private:
+  struct AutoProfilerLabel {
+    AutoProfilerLabel(HelperThread* helperThread, const char* label,
+                      JS::ProfilingCategoryPair categoryPair);
+    ~AutoProfilerLabel();
+
+   private:
+    ProfilingStack* profilingStack;
+  };
+
+  using Selector = HelperThreadTask* (
+      GlobalHelperThreadState::*)(const AutoLockHelperThreadState&);
+  static const Selector selectors[];
+
+  HelperThreadTask* findHighestPriorityTask(
+      const AutoLockHelperThreadState& locked);
+};
+
+class MOZ_RAII AutoSetHelperThreadContext {
+  JSContext* cx;
+  AutoLockHelperThreadState& lock;
+
+ public:
+  explicit AutoSetHelperThreadContext(AutoLockHelperThreadState& lock);
+  ~AutoSetHelperThreadContext();
+};
+
+struct MOZ_RAII AutoSetContextRuntime {
+  explicit AutoSetContextRuntime(JSRuntime* rt) {
+    TlsContext.get()->setRuntime(rt);
+  }
+  ~AutoSetContextRuntime() { TlsContext.get()->setRuntime(nullptr); }
+};
+
+struct ParseTask : public mozilla::LinkedListElement<ParseTask>,
+                   public JS::OffThreadToken,
+                   public HelperThreadTask {
+  ParseTaskKind kind;
+  JS::OwningCompileOptions options;
+
+  // HelperThreads are shared between all runtimes in the process so explicitly
+  // track which one we are associated with.
+  JSRuntime* runtime = nullptr;
+
+  // The global object to use while parsing.
+  JSObject* parseGlobal;
+
+  // Callback invoked off thread when the parse finishes.
+  JS::OffThreadCompileCallback callback;
+  void* callbackData;
+
+  // Holds the final scripts between the invocation of the callback and the
+  // point where FinishOffThreadScript is called, which will destroy the
+  // ParseTask.
+  GCVector<JSScript*, 1, SystemAllocPolicy> scripts;
+
+  // Holds the ScriptSourceObjects generated for the script compilation.
+  GCVector<ScriptSourceObject*, 1, SystemAllocPolicy> sourceObjects;
+
+  // Holds the CompilationStencil generated for the script compilation.
+  UniquePtr<frontend::CompilationStencil> stencil_;
+
+  // Holds the CompilationStencilSet generated by decoding task.
+  UniquePtr<frontend::CompilationStencilSet> stencilSet_;
+
+  frontend::CompilationGCOutput gcOutput_;
+
+  frontend::CompilationGCOutput gcOutputForDelazification_;
+
+  // Any errors or warnings produced during compilation. These are reported
+  // when finishing the script.
+  Vector<UniquePtr<CompileError>, 0, SystemAllocPolicy> errors;
+  bool overRecursed;
+  bool outOfMemory;
+
+  ParseTask(ParseTaskKind kind, JSContext* cx,
+            JS::OffThreadCompileCallback callback, void* callbackData);
+  virtual ~ParseTask();
+
+  bool init(JSContext* cx, const JS::ReadOnlyCompileOptions& options,
+            JSObject* global);
+
+  void activate(JSRuntime* rt);
+  virtual void parse(JSContext* cx) = 0;
+  bool instantiateStencils(JSContext* cx);
+
+  bool runtimeMatches(JSRuntime* rt) { return runtime == rt; }
+
+  void trace(JSTracer* trc);
+
+  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const;
+  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
+    return mallocSizeOf(this) + sizeOfExcludingThis(mallocSizeOf);
+  }
+
+  void runHelperThreadTask(AutoLockHelperThreadState& locked) override;
+  void runTask(AutoLockHelperThreadState& lock);
+  ThreadType threadType() override { return ThreadType::THREAD_TYPE_PARSE; }
+};
+
+struct ScriptDecodeTask : public ParseTask {
+  const JS::TranscodeRange range;
+
+  ScriptDecodeTask(JSContext* cx, const JS::TranscodeRange& range,
+                   JS::OffThreadCompileCallback callback, void* callbackData);
+  void parse(JSContext* cx) override;
+};
+
+struct MultiScriptsDecodeTask : public ParseTask {
+  JS::TranscodeSources* sources;
+
+  MultiScriptsDecodeTask(JSContext* cx, JS::TranscodeSources& sources,
+                         JS::OffThreadCompileCallback callback,
+                         void* callbackData);
+  void parse(JSContext* cx) override;
+};
+
+// It is not desirable to eagerly compress: if lazy functions that are tied to
+// the ScriptSource were to be executed relatively soon after parsing, they
+// would need to block on decompression, which hurts responsiveness.
+//
+// To this end, compression tasks are heap allocated and enqueued in a pending
+// list by ScriptSource::setSourceCopy. When a major GC occurs, we schedule
+// pending compression tasks and move the ones that are ready to be compressed
+// to the worklist. Currently, a compression task is considered ready 2 major
+// GCs after being enqueued. Completed tasks are handled during the sweeping
+// phase by AttachCompressedSourcesTask, which runs in parallel with other GC
+// sweeping tasks.
+class SourceCompressionTask : public HelperThreadTask {
+  friend class HelperThread;
+  friend class ScriptSource;
+
+  // The runtime that the ScriptSource is associated with, in the sense that
+  // it uses the runtime's immutable string cache.
+  JSRuntime* runtime_;
+
+  // The major GC number of the runtime when the task was enqueued.
+  uint64_t majorGCNumber_;
+
+  // The source to be compressed.
+  ScriptSourceHolder sourceHolder_;
+
+  // The resultant compressed string. If the compressed string is larger
+  // than the original, or we OOM'd during compression, or nothing else
+  // except the task is holding the ScriptSource alive when scheduled to
+  // compress, this will remain None upon completion.
+  mozilla::Maybe<SharedImmutableString> resultString_;
+
+ public:
+  // The majorGCNumber is used for scheduling tasks.
+  SourceCompressionTask(JSRuntime* rt, ScriptSource* source)
+      : runtime_(rt),
+        majorGCNumber_(rt->gc.majorGCCount()),
+        sourceHolder_(source) {}
+  virtual ~SourceCompressionTask() = default;
+
+  bool runtimeMatches(JSRuntime* runtime) const { return runtime == runtime_; }
+  bool shouldStart() const {
+    // We wait 2 major GCs to start compressing, in order to avoid
+    // immediate compression.
+    return runtime_->gc.majorGCCount() > majorGCNumber_ + 1;
+  }
+
+  bool shouldCancel() const {
+    // If the refcount is exactly 1, then nothing else is holding on to the
+    // ScriptSource, so no reason to compress it and we should cancel the task.
+    return sourceHolder_.get()->refs == 1;
+  }
+
+  void runTask();
+  void runHelperThreadTask(AutoLockHelperThreadState& locked) override;
+  void complete();
+
+  ThreadType threadType() override { return ThreadType::THREAD_TYPE_COMPRESS; }
+
+ private:
+  struct PerformTaskWork;
+  friend struct PerformTaskWork;
+
+  // The work algorithm, aware whether it's compressing one-byte UTF-8 source
+  // text or UTF-16, for CharT either Utf8Unit or char16_t.  Invoked by
+  // work() after doing a type-test of the ScriptSource*.
+  template <typename CharT>
+  void workEncodingSpecific();
+};
+
+// A PromiseHelperTask is an OffThreadPromiseTask that executes a single job on
+// a helper thread. Call js::StartOffThreadPromiseHelperTask to submit a
+// PromiseHelperTask for execution.
+//
+// Concrete subclasses must implement execute and OffThreadPromiseTask::resolve.
+// The helper thread will call execute() to do the main work. Then, the thread
+// of the JSContext used to create the PromiseHelperTask will call resolve() to
+// resolve promise according to those results.
+struct PromiseHelperTask : OffThreadPromiseTask, public HelperThreadTask {
+  PromiseHelperTask(JSContext* cx, Handle<PromiseObject*> promise)
+      : OffThreadPromiseTask(cx, promise) {}
+
+  // To be called on a helper thread and implemented by the derived class.
+  virtual void execute() = 0;
+
+  // May be called in the absence of helper threads or off-thread promise
+  // support to synchronously execute and resolve a PromiseTask.
+  //
+  // Warning: After this function returns, 'this' can be deleted at any time, so
+  // the caller must immediately return from the stream callback.
+  void executeAndResolveAndDestroy(JSContext* cx);
+
+  void runHelperThreadTask(AutoLockHelperThreadState& locked) override;
+  ThreadType threadType() override { return THREAD_TYPE_PROMISE_TASK; }
+};
+
+} /* namespace js */
+
+#endif /* vm_HelperThreadState_h */