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diff --git a/js/src/vm/GeneratorObject.h b/js/src/vm/GeneratorObject.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/. */
+
+#ifndef vm_GeneratorObject_h
+#define vm_GeneratorObject_h
+
+#include "js/Class.h"
+#include "vm/ArgumentsObject.h"
+#include "vm/ArrayObject.h"
+#include "vm/BytecodeUtil.h"
+#include "vm/GeneratorResumeKind.h"  // GeneratorResumeKind
+#include "vm/JSContext.h"
+#include "vm/JSObject.h"
+#include "vm/Stack.h"
+
+namespace js {
+
+namespace frontend {
+class ParserAtom;
+}  // namespace frontend
+
+extern const JSClass GeneratorFunctionClass;
+
+class AbstractGeneratorObject : public NativeObject {
+ public:
+  // Magic value stored in the resumeIndex slot when the generator is
+  // running or closing. See the resumeIndex comment below.
+  static const int32_t RESUME_INDEX_RUNNING = INT32_MAX;
+
+  enum {
+    CALLEE_SLOT = 0,
+    ENV_CHAIN_SLOT,
+    ARGS_OBJ_SLOT,
+    STACK_STORAGE_SLOT,
+    RESUME_INDEX_SLOT,
+    RESERVED_SLOTS
+  };
+
+  // Maximum number of fixed stack slots in a generator or async function
+  // script. If a script would have more, we instead store some variables in
+  // heap EnvironmentObjects.
+  //
+  // This limit is a performance heuristic. Stack slots reduce allocations,
+  // and `Local` opcodes are a bit faster than `AliasedVar` ones; but at each
+  // `yield` or `await` the stack slots must be memcpy'd into a
+  // GeneratorObject. At some point the memcpy is too much. The limit is
+  // plenty for typical human-authored code.
+  static constexpr uint32_t FixedSlotLimit = 256;
+
+ private:
+  static JSObject* createModuleGenerator(JSContext* cx, AbstractFramePtr frame);
+
+ public:
+  static JSObject* createFromFrame(JSContext* cx, AbstractFramePtr frame);
+  static AbstractGeneratorObject* create(JSContext* cx, HandleFunction callee,
+                                         HandleScript script,
+                                         HandleObject environmentChain,
+                                         Handle<ArgumentsObject*> argsObject);
+
+  static bool resume(JSContext* cx, InterpreterActivation& activation,
+                     Handle<AbstractGeneratorObject*> genObj, HandleValue arg,
+                     HandleValue resumeKind);
+
+  static bool suspend(JSContext* cx, HandleObject obj, AbstractFramePtr frame,
+                      jsbytecode* pc, unsigned nvalues);
+
+  static void finalSuspend(HandleObject obj);
+
+  JSFunction& callee() const {
+    return getFixedSlot(CALLEE_SLOT).toObject().as<JSFunction>();
+  }
+  void setCallee(JSFunction& callee) {
+    setFixedSlot(CALLEE_SLOT, ObjectValue(callee));
+  }
+
+  JSObject& environmentChain() const {
+    return getFixedSlot(ENV_CHAIN_SLOT).toObject();
+  }
+  void setEnvironmentChain(JSObject& envChain) {
+    setFixedSlot(ENV_CHAIN_SLOT, ObjectValue(envChain));
+  }
+
+  bool hasArgsObj() const { return getFixedSlot(ARGS_OBJ_SLOT).isObject(); }
+  ArgumentsObject& argsObj() const {
+    return getFixedSlot(ARGS_OBJ_SLOT).toObject().as<ArgumentsObject>();
+  }
+  void setArgsObj(ArgumentsObject& argsObj) {
+    setFixedSlot(ARGS_OBJ_SLOT, ObjectValue(argsObj));
+  }
+
+  bool hasStackStorage() const {
+    return getFixedSlot(STACK_STORAGE_SLOT).isObject();
+  }
+  bool isStackStorageEmpty() const {
+    return stackStorage().getDenseInitializedLength() == 0;
+  }
+  ArrayObject& stackStorage() const {
+    return getFixedSlot(STACK_STORAGE_SLOT).toObject().as<ArrayObject>();
+  }
+  void setStackStorage(ArrayObject& stackStorage) {
+    setFixedSlot(STACK_STORAGE_SLOT, ObjectValue(stackStorage));
+  }
+
+  // Access stack storage. Requires `hasStackStorage() && isSuspended()`.
+  // `slot` is the index of the desired local in the stack frame when this
+  // generator is *not* suspended.
+  const Value& getUnaliasedLocal(uint32_t slot) const;
+  void setUnaliasedLocal(uint32_t slot, const Value& value);
+
+  // The resumeIndex slot is abused for a few purposes.  It's undefined if
+  // it hasn't been set yet (before the initial yield), and null if the
+  // generator is closed. If the generator is running, the resumeIndex is
+  // RESUME_INDEX_RUNNING.
+  //
+  // If the generator is suspended, it's the resumeIndex (stored as
+  // JSOp::InitialYield/JSOp::Yield/JSOp::Await operand) of the yield
+  // instruction that suspended the generator. The resumeIndex can be mapped to
+  // the bytecode offset (interpreter) or to the native code offset (JIT).
+
+  bool isBeforeInitialYield() const {
+    return getFixedSlot(RESUME_INDEX_SLOT).isUndefined();
+  }
+  bool isRunning() const {
+    return getFixedSlot(RESUME_INDEX_SLOT) == Int32Value(RESUME_INDEX_RUNNING);
+  }
+  bool isSuspended() const {
+    // Note: also update Baseline's IsSuspendedGenerator code if this
+    // changes.
+    Value resumeIndex = getFixedSlot(RESUME_INDEX_SLOT);
+    return resumeIndex.isInt32() &&
+           resumeIndex.toInt32() < RESUME_INDEX_RUNNING;
+  }
+  void setRunning() {
+    MOZ_ASSERT(isSuspended());
+    setFixedSlot(RESUME_INDEX_SLOT, Int32Value(RESUME_INDEX_RUNNING));
+  }
+  void setResumeIndex(jsbytecode* pc) {
+    MOZ_ASSERT(JSOp(*pc) == JSOp::InitialYield || JSOp(*pc) == JSOp::Yield ||
+               JSOp(*pc) == JSOp::Await);
+
+    MOZ_ASSERT_IF(JSOp(*pc) == JSOp::InitialYield,
+                  getFixedSlot(RESUME_INDEX_SLOT).isUndefined());
+    MOZ_ASSERT_IF(JSOp(*pc) != JSOp::InitialYield, isRunning());
+
+    uint32_t resumeIndex = GET_UINT24(pc);
+    MOZ_ASSERT(resumeIndex < uint32_t(RESUME_INDEX_RUNNING));
+
+    setFixedSlot(RESUME_INDEX_SLOT, Int32Value(resumeIndex));
+    MOZ_ASSERT(isSuspended());
+  }
+  void setResumeIndex(int32_t resumeIndex) {
+    setFixedSlot(RESUME_INDEX_SLOT, Int32Value(resumeIndex));
+  }
+  uint32_t resumeIndex() const {
+    MOZ_ASSERT(isSuspended());
+    return getFixedSlot(RESUME_INDEX_SLOT).toInt32();
+  }
+  bool isClosed() const { return getFixedSlot(CALLEE_SLOT).isNull(); }
+  void setClosed() {
+    setFixedSlot(CALLEE_SLOT, NullValue());
+    setFixedSlot(ENV_CHAIN_SLOT, NullValue());
+    setFixedSlot(ARGS_OBJ_SLOT, NullValue());
+    setFixedSlot(STACK_STORAGE_SLOT, NullValue());
+    setFixedSlot(RESUME_INDEX_SLOT, NullValue());
+  }
+
+  bool isAfterYield();
+  bool isAfterAwait();
+
+ private:
+  bool isAfterYieldOrAwait(JSOp op);
+
+ public:
+  void trace(JSTracer* trc);
+
+  static size_t offsetOfCalleeSlot() { return getFixedSlotOffset(CALLEE_SLOT); }
+  static size_t offsetOfEnvironmentChainSlot() {
+    return getFixedSlotOffset(ENV_CHAIN_SLOT);
+  }
+  static size_t offsetOfArgsObjSlot() {
+    return getFixedSlotOffset(ARGS_OBJ_SLOT);
+  }
+  static size_t offsetOfResumeIndexSlot() {
+    return getFixedSlotOffset(RESUME_INDEX_SLOT);
+  }
+  static size_t offsetOfStackStorageSlot() {
+    return getFixedSlotOffset(STACK_STORAGE_SLOT);
+  }
+
+  static size_t calleeSlot() { return CALLEE_SLOT; }
+  static size_t envChainSlot() { return ENV_CHAIN_SLOT; }
+  static size_t argsObjectSlot() { return ARGS_OBJ_SLOT; }
+  static size_t stackStorageSlot() { return STACK_STORAGE_SLOT; }
+  static size_t resumeIndexSlot() { return RESUME_INDEX_SLOT; }
+
+#ifdef DEBUG
+  void dump() const;
+#endif
+};
+
+class GeneratorObject : public AbstractGeneratorObject {
+ public:
+  enum { RESERVED_SLOTS = AbstractGeneratorObject::RESERVED_SLOTS };
+
+  static const JSClass class_;
+  static const JSClassOps classOps_;
+
+  static GeneratorObject* create(JSContext* cx, HandleFunction fun);
+};
+
+bool GeneratorThrowOrReturn(JSContext* cx, AbstractFramePtr frame,
+                            Handle<AbstractGeneratorObject*> obj,
+                            HandleValue val, GeneratorResumeKind resumeKind);
+
+/**
+ * Return the generator object associated with the given frame. The frame must
+ * be a call frame for a generator.
+ *
+ * This may return nullptr at certain points in the generator lifecycle:
+ *
+ * - While a generator call evaluates default argument values and performs
+ *   destructuring, which occurs before the generator object is created.
+ *
+ * - Between the `Generator` instruction and the `SetAliasedVar .generator`
+ *   instruction, at which point the generator object does exist, but is held
+ *   only on the stack, and not the `.generator` pseudo-variable this function
+ *   consults.
+ */
+AbstractGeneratorObject* GetGeneratorObjectForFrame(JSContext* cx,
+                                                    AbstractFramePtr frame);
+
+/**
+ * If `env` or any enclosing environment is a `CallObject` associated with a
+ * generator object, return the generator.
+ *
+ * Otherwise `env` is not in a generator or async function, or the generator
+ * object hasn't been created yet; return nullptr with no pending exception.
+ */
+AbstractGeneratorObject* GetGeneratorObjectForEnvironment(JSContext* cx,
+                                                          HandleObject env);
+
+GeneratorResumeKind ParserAtomToResumeKind(JSContext* cx,
+                                           const frontend::ParserAtom* atom);
+JSAtom* ResumeKindToAtom(JSContext* cx, GeneratorResumeKind kind);
+
+}  // namespace js
+
+template <>
+bool JSObject::is<js::AbstractGeneratorObject>() const;
+
+#endif /* vm_GeneratorObject_h */