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diff --git a/js/src/vm/HelperThreads.h b/js/src/vm/HelperThreads.h
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+/* -*- 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/. */
+
+/*
+ * API for managing off-thread work.
+ */
+
+#ifndef vm_HelperThreads_h
+#define vm_HelperThreads_h
+
+#include "mozilla/Variant.h"
+
+#include "js/OffThreadScriptCompilation.h"
+#include "js/shadow/Zone.h"
+#include "js/Transcoding.h"
+#include "js/UniquePtr.h"
+#include "threading/LockGuard.h"
+#include "threading/Mutex.h"
+#include "wasm/WasmConstants.h"
+
+namespace mozilla {
+union Utf8Unit;
+}
+
+namespace JS {
+class OffThreadToken {};
+class JS_PUBLIC_API ReadOnlyCompileOptions;
+class Zone;
+
+template <typename UnitT>
+class SourceText;
+}  // namespace JS
+
+namespace js {
+
+class AutoLockHelperThreadState;
+struct PromiseHelperTask;
+class SourceCompressionTask;
+
+namespace frontend {
+struct CompilationStencil;
+}
+
+namespace gc {
+class GCRuntime;
+}
+
+namespace jit {
+class IonCompileTask;
+class IonFreeTask;
+}  // namespace jit
+
+namespace wasm {
+struct CompileTask;
+struct CompileTaskState;
+struct Tier2GeneratorTask;
+using UniqueTier2GeneratorTask = UniquePtr<Tier2GeneratorTask>;
+}  // namespace wasm
+
+/*
+ * Lock protecting all mutable shared state accessed by helper threads, and used
+ * by all condition variables.
+ */
+extern Mutex gHelperThreadLock MOZ_UNANNOTATED;
+
+class MOZ_RAII AutoLockHelperThreadState : public LockGuard<Mutex> {
+  using Base = LockGuard<Mutex>;
+
+ public:
+  explicit AutoLockHelperThreadState() : Base(gHelperThreadLock) {}
+};
+
+class MOZ_RAII AutoUnlockHelperThreadState : public UnlockGuard<Mutex> {
+  using Base = UnlockGuard<Mutex>;
+
+ public:
+  explicit AutoUnlockHelperThreadState(AutoLockHelperThreadState& locked)
+      : Base(locked) {}
+};
+
+// Create data structures used by helper threads.
+bool CreateHelperThreadsState();
+
+// Destroy data structures used by helper threads.
+void DestroyHelperThreadsState();
+
+// Initialize helper threads unless already initialized.
+bool EnsureHelperThreadsInitialized();
+
+size_t GetHelperThreadCount();
+size_t GetHelperThreadCPUCount();
+size_t GetMaxWasmCompilationThreads();
+
+// This allows the JS shell to override GetCPUCount() when passed the
+// --thread-count=N option.
+bool SetFakeCPUCount(size_t count);
+
+// Enqueues a wasm compilation task.
+bool StartOffThreadWasmCompile(wasm::CompileTask* task, wasm::CompileMode mode);
+
+// Remove any pending wasm compilation tasks queued with
+// StartOffThreadWasmCompile that match the arguments. Return the number
+// removed.
+size_t RemovePendingWasmCompileTasks(const wasm::CompileTaskState& taskState,
+                                     wasm::CompileMode mode,
+                                     const AutoLockHelperThreadState& lock);
+
+// Enqueues a wasm compilation task.
+void StartOffThreadWasmTier2Generator(wasm::UniqueTier2GeneratorTask task);
+
+// Cancel all background Wasm Tier-2 compilations.
+void CancelOffThreadWasmTier2Generator();
+
+/*
+ * If helper threads are available, call execute() then dispatchResolve() on the
+ * given task in a helper thread. If no helper threads are available, the given
+ * task is executed and resolved synchronously.
+ *
+ * This function takes ownership of task unconditionally; if it fails, task is
+ * deleted.
+ */
+bool StartOffThreadPromiseHelperTask(JSContext* cx,
+                                     UniquePtr<PromiseHelperTask> task);
+
+/*
+ * Like the JSContext-accepting version, but only safe to use when helper
+ * threads are available, so we can be sure we'll never need to fall back on
+ * synchronous execution.
+ *
+ * This function can be called from any thread, but takes ownership of the task
+ * only on success. On OOM, it is the caller's responsibility to arrange for the
+ * task to be cleaned up properly.
+ */
+bool StartOffThreadPromiseHelperTask(PromiseHelperTask* task);
+
+/*
+ * Schedule an off-thread Ion compilation for a script, given a task.
+ */
+bool StartOffThreadIonCompile(jit::IonCompileTask* task,
+                              const AutoLockHelperThreadState& lock);
+
+/*
+ * Schedule deletion of Ion compilation data.
+ */
+bool StartOffThreadIonFree(jit::IonCompileTask* task,
+                           const AutoLockHelperThreadState& lock);
+
+void FinishOffThreadIonCompile(jit::IonCompileTask* task,
+                               const AutoLockHelperThreadState& lock);
+
+struct ZonesInState {
+  JSRuntime* runtime;
+  JS::shadow::Zone::GCState state;
+};
+
+using CompilationSelector = mozilla::Variant<JSScript*, JS::Realm*, JS::Zone*,
+                                             ZonesInState, JSRuntime*>;
+
+/*
+ * Cancel scheduled or in progress Ion compilations.
+ */
+void CancelOffThreadIonCompile(const CompilationSelector& selector);
+
+inline void CancelOffThreadIonCompile(JSScript* script) {
+  CancelOffThreadIonCompile(CompilationSelector(script));
+}
+
+inline void CancelOffThreadIonCompile(JS::Realm* realm) {
+  CancelOffThreadIonCompile(CompilationSelector(realm));
+}
+
+inline void CancelOffThreadIonCompile(JS::Zone* zone) {
+  CancelOffThreadIonCompile(CompilationSelector(zone));
+}
+
+inline void CancelOffThreadIonCompile(JSRuntime* runtime,
+                                      JS::shadow::Zone::GCState state) {
+  CancelOffThreadIonCompile(CompilationSelector(ZonesInState{runtime, state}));
+}
+
+inline void CancelOffThreadIonCompile(JSRuntime* runtime) {
+  CancelOffThreadIonCompile(CompilationSelector(runtime));
+}
+
+#ifdef DEBUG
+bool HasOffThreadIonCompile(JS::Realm* realm);
+#endif
+
+// True iff the current thread is a ParseTask or a DelazifyTask.
+bool CurrentThreadIsParseThread();
+
+/*
+ * Cancel all scheduled, in progress or finished parses for runtime.
+ *
+ * Parse tasks which have completed but for which JS::FinishOffThreadScript (or
+ * equivalent) has not been called are removed from the system. This is only
+ * safe to do during shutdown, or if you know that the main thread isn't waiting
+ * for tasks to complete.
+ */
+void CancelOffThreadParses(JSRuntime* runtime);
+
+/*
+ * Cancel all scheduled or in progress eager delazification phases for a
+ * runtime.
+ */
+void CancelOffThreadDelazify(JSRuntime* runtime);
+
+/*
+ * Wait for all delazification to complete.
+ */
+void WaitForAllDelazifyTasks(JSRuntime* rt);
+
+/*
+ * Start a parse/emit cycle for a stream of source. The characters must stay
+ * alive until the compilation finishes.
+ */
+
+JS::OffThreadToken* StartOffThreadCompileToStencil(
+    JSContext* cx, const JS::ReadOnlyCompileOptions& options,
+    JS::SourceText<char16_t>& srcBuf, JS::OffThreadCompileCallback callback,
+    void* callbackData);
+JS::OffThreadToken* StartOffThreadCompileToStencil(
+    JSContext* cx, const JS::ReadOnlyCompileOptions& options,
+    JS::SourceText<mozilla::Utf8Unit>& srcBuf,
+    JS::OffThreadCompileCallback callback, void* callbackData);
+
+JS::OffThreadToken* StartOffThreadCompileModuleToStencil(
+    JSContext* cx, const JS::ReadOnlyCompileOptions& options,
+    JS::SourceText<char16_t>& srcBuf, JS::OffThreadCompileCallback callback,
+    void* callbackData);
+JS::OffThreadToken* StartOffThreadCompileModuleToStencil(
+    JSContext* cx, const JS::ReadOnlyCompileOptions& options,
+    JS::SourceText<mozilla::Utf8Unit>& srcBuf,
+    JS::OffThreadCompileCallback callback, void* callbackData);
+
+JS::OffThreadToken* StartOffThreadDecodeStencil(
+    JSContext* cx, const JS::DecodeOptions& options,
+    const JS::TranscodeRange& range, JS::OffThreadCompileCallback callback,
+    void* callbackData);
+
+JS::OffThreadToken* StartOffThreadDecodeMultiStencils(
+    JSContext* cx, const JS::DecodeOptions& options,
+    JS::TranscodeSources& sources, JS::OffThreadCompileCallback callback,
+    void* callbackData);
+
+// Start off-thread delazification task, to race the delazification of inner
+// functions.
+void StartOffThreadDelazification(JSContext* cx,
+                                  const JS::ReadOnlyCompileOptions& options,
+                                  const frontend::CompilationStencil& stencil);
+
+// Drain the task queues and wait for all helper threads to finish running.
+//
+// Note that helper threads are shared between runtimes and it's possible that
+// another runtime could saturate the helper thread system and cause this to
+// never return.
+void WaitForAllHelperThreads();
+void WaitForAllHelperThreads(AutoLockHelperThreadState& lock);
+
+// Enqueue a compression job to be processed later. These are started at the
+// start of the major GC after the next one.
+bool EnqueueOffThreadCompression(JSContext* cx,
+                                 UniquePtr<SourceCompressionTask> task);
+
+// Start handling any compression tasks for this runtime. Called at the start of
+// major GC.
+void StartHandlingCompressionsOnGC(JSRuntime* rt);
+
+// Cancel all scheduled, in progress, or finished compression tasks for
+// runtime.
+void CancelOffThreadCompressions(JSRuntime* runtime);
+
+void AttachFinishedCompressions(JSRuntime* runtime,
+                                AutoLockHelperThreadState& lock);
+
+// Sweep pending tasks that are holding onto should-be-dead ScriptSources.
+void SweepPendingCompressions(AutoLockHelperThreadState& lock);
+
+// Run all pending source compression tasks synchronously, for testing purposes
+void RunPendingSourceCompressions(JSRuntime* runtime);
+
+// False if the off-thread source compression mechanism isn't being used. This
+// happens on low core count machines where we are concerned about blocking
+// main-thread execution.
+bool IsOffThreadSourceCompressionEnabled();
+
+}  // namespace js
+
+#endif /* vm_HelperThreads_h */