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diff --git a/js/src/vm/FunctionFlags.h b/js/src/vm/FunctionFlags.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_FunctionFlags_h
+#define vm_FunctionFlags_h
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT, MOZ_ASSERT_IF
+#include "mozilla/Attributes.h"  // MOZ_IMPLICIT
+
+#include <stdint.h>  // uint8_t, uint16_t
+
+#include "jstypes.h"  // JS_PUBLIC_API
+
+class JS_PUBLIC_API JSAtom;
+
+namespace js {
+
+class FunctionFlags {
+ public:
+  enum FunctionKind : uint8_t {
+    NormalFunction = 0,
+    Arrow,   // ES6 '(args) => body' syntax
+    Method,  // ES6 MethodDefinition
+    ClassConstructor,
+    Getter,
+    Setter,
+    AsmJS,  // An asm.js module or exported function
+    Wasm,   // An exported WebAssembly function
+    FunctionKindLimit
+  };
+
+  enum Flags : uint16_t {
+    // The general kind of a function. This is used to describe characteristics
+    // of functions that do not merit a dedicated flag bit below.
+    FUNCTION_KIND_SHIFT = 0,
+    FUNCTION_KIND_MASK = 0x0007,
+
+    // The AllocKind used was FunctionExtended and extra slots were allocated.
+    // These slots may be used by the engine or the embedding so care must be
+    // taken to avoid conflicts.
+    EXTENDED = 1 << 3,
+
+    // Set if function is a self-hosted builtin or intrinsic. An 'intrinsic'
+    // here means a native function used inside self-hosted code. In general, a
+    // self-hosted function should appear to script as though it were a native
+    // builtin.
+    SELF_HOSTED = 1 << 4,
+
+    // An interpreted function has or may-have bytecode and an environment. Only
+    // one of these flags may be used at a time. As a memory optimization, the
+    // SELFHOSTLAZY flag indicates there is no js::BaseScript at all and we must
+    // clone from the self-hosted realm in order to get bytecode.
+    BASESCRIPT = 1 << 5,
+    SELFHOSTLAZY = 1 << 6,
+
+    // Function may be called as a constructor. This corresponds in the spec as
+    // having a [[Construct]] internal method.
+    CONSTRUCTOR = 1 << 7,
+
+    // (1 << 8) is unused.
+
+    // Function comes from a FunctionExpression, ArrowFunction, or Function()
+    // call (not a FunctionDeclaration or nonstandard function-statement).
+    LAMBDA = 1 << 9,
+
+    // The WASM function has a JIT entry which emulates the
+    // js::BaseScript::jitCodeRaw mechanism.
+    WASM_JIT_ENTRY = 1 << 10,
+
+    // Function had no explicit name, but a name was set by SetFunctionName at
+    // compile time or SetFunctionName at runtime.
+    HAS_INFERRED_NAME = 1 << 11,
+
+    // Function had no explicit name, but a name was guessed for it anyway.
+    HAS_GUESSED_ATOM = 1 << 12,
+
+    // The 'length' or 'name property has been resolved. See fun_resolve.
+    RESOLVED_NAME = 1 << 13,
+    RESOLVED_LENGTH = 1 << 14,
+
+    // This function is kept only for skipping it over during delazification.
+    //
+    // This function is inside arrow function's parameter expression, and
+    // parsed twice, once before finding "=>" token, and once after finding
+    // "=>" and rewinding to the beginning of the parameters.
+    // ScriptStencil is created for both case, and the first one is kept only
+    // for delazification, to make sure delazification sees the same sequence
+    // of inner function to skip over.
+    //
+    // We call the first one "ghost".
+    // It should be kept lazy, and shouldn't be exposed to debugger.
+    GHOST_FUNCTION = 1 << 15,
+
+    // Shifted form of FunctionKinds.
+    NORMAL_KIND = NormalFunction << FUNCTION_KIND_SHIFT,
+    ASMJS_KIND = AsmJS << FUNCTION_KIND_SHIFT,
+    WASM_KIND = Wasm << FUNCTION_KIND_SHIFT,
+    ARROW_KIND = Arrow << FUNCTION_KIND_SHIFT,
+    METHOD_KIND = Method << FUNCTION_KIND_SHIFT,
+    CLASSCONSTRUCTOR_KIND = ClassConstructor << FUNCTION_KIND_SHIFT,
+    GETTER_KIND = Getter << FUNCTION_KIND_SHIFT,
+    SETTER_KIND = Setter << FUNCTION_KIND_SHIFT,
+
+    // Derived Flags combinations to use when creating functions.
+    NATIVE_FUN = NORMAL_KIND,
+    NATIVE_CTOR = CONSTRUCTOR | NORMAL_KIND,
+    ASMJS_CTOR = CONSTRUCTOR | ASMJS_KIND,
+    ASMJS_LAMBDA_CTOR = CONSTRUCTOR | LAMBDA | ASMJS_KIND,
+    WASM = WASM_KIND,
+    INTERPRETED_NORMAL = BASESCRIPT | CONSTRUCTOR | NORMAL_KIND,
+    INTERPRETED_CLASS_CTOR = BASESCRIPT | CONSTRUCTOR | CLASSCONSTRUCTOR_KIND,
+    INTERPRETED_GENERATOR_OR_ASYNC = BASESCRIPT | NORMAL_KIND,
+    INTERPRETED_LAMBDA = BASESCRIPT | LAMBDA | CONSTRUCTOR | NORMAL_KIND,
+    INTERPRETED_LAMBDA_ARROW = BASESCRIPT | LAMBDA | ARROW_KIND,
+    INTERPRETED_LAMBDA_GENERATOR_OR_ASYNC = BASESCRIPT | LAMBDA | NORMAL_KIND,
+    INTERPRETED_GETTER = BASESCRIPT | GETTER_KIND,
+    INTERPRETED_SETTER = BASESCRIPT | SETTER_KIND,
+    INTERPRETED_METHOD = BASESCRIPT | METHOD_KIND,
+
+    // Flags that XDR ignores. See also: js::BaseScript::MutableFlags.
+    MUTABLE_FLAGS = RESOLVED_NAME | RESOLVED_LENGTH,
+
+    // Flags preserved when cloning a function.
+    STABLE_ACROSS_CLONES =
+        CONSTRUCTOR | LAMBDA | SELF_HOSTED | FUNCTION_KIND_MASK | GHOST_FUNCTION
+  };
+
+  uint16_t flags_;
+
+ public:
+  FunctionFlags() : flags_() {
+    static_assert(sizeof(FunctionFlags) == sizeof(flags_),
+                  "No extra members allowed is it'll grow JSFunction");
+    static_assert(offsetof(FunctionFlags, flags_) == 0,
+                  "Required for JIT flag access");
+  }
+
+  explicit FunctionFlags(uint16_t flags) : flags_(flags) {}
+  MOZ_IMPLICIT FunctionFlags(Flags f) : flags_(f) {}
+
+  static_assert(((FunctionKindLimit - 1) << FUNCTION_KIND_SHIFT) <=
+                    FUNCTION_KIND_MASK,
+                "FunctionKind doesn't fit into flags_");
+
+  uint16_t toRaw() const { return flags_; }
+
+  uint16_t stableAcrossClones() const { return flags_ & STABLE_ACROSS_CLONES; }
+
+  // For flag combinations the type is int.
+  bool hasFlags(uint16_t flags) const { return flags_ & flags; }
+  FunctionFlags& setFlags(uint16_t flags) {
+    flags_ |= flags;
+    return *this;
+  }
+  FunctionFlags& clearFlags(uint16_t flags) {
+    flags_ &= ~flags;
+    return *this;
+  }
+  FunctionFlags& setFlags(uint16_t flags, bool set) {
+    if (set) {
+      setFlags(flags);
+    } else {
+      clearFlags(flags);
+    }
+    return *this;
+  }
+
+  FunctionKind kind() const {
+    return static_cast<FunctionKind>((flags_ & FUNCTION_KIND_MASK) >>
+                                     FUNCTION_KIND_SHIFT);
+  }
+
+  /* A function can be classified as either native (C++) or interpreted (JS): */
+  bool isInterpreted() const {
+    return hasFlags(BASESCRIPT) || hasFlags(SELFHOSTLAZY);
+  }
+  bool isNativeFun() const { return !isInterpreted(); }
+
+  bool isConstructor() const { return hasFlags(CONSTRUCTOR); }
+
+  bool isNonBuiltinConstructor() const {
+    // Note: keep this in sync with branchIfNotFunctionIsNonBuiltinCtor in
+    // MacroAssembler.cpp.
+    return hasFlags(BASESCRIPT) && hasFlags(CONSTRUCTOR) &&
+           !hasFlags(SELF_HOSTED);
+  }
+
+  /* Possible attributes of a native function: */
+  bool isAsmJSNative() const {
+    MOZ_ASSERT_IF(kind() == AsmJS, isNativeFun());
+    return kind() == AsmJS;
+  }
+  bool isWasm() const {
+    MOZ_ASSERT_IF(kind() == Wasm, isNativeFun());
+    return kind() == Wasm;
+  }
+  bool isWasmWithJitEntry() const {
+    MOZ_ASSERT_IF(hasFlags(WASM_JIT_ENTRY), isWasm());
+    return hasFlags(WASM_JIT_ENTRY);
+  }
+  bool isNativeWithoutJitEntry() const {
+    MOZ_ASSERT_IF(!hasJitEntry(), isNativeFun());
+    return !hasJitEntry();
+  }
+  bool isBuiltinNative() const {
+    return isNativeFun() && !isAsmJSNative() && !isWasm();
+  }
+  bool hasJitEntry() const {
+    return hasBaseScript() || hasSelfHostedLazyScript() || isWasmWithJitEntry();
+  }
+
+  /* Possible attributes of an interpreted function: */
+  bool hasInferredName() const { return hasFlags(HAS_INFERRED_NAME); }
+  bool hasGuessedAtom() const { return hasFlags(HAS_GUESSED_ATOM); }
+  bool isLambda() const { return hasFlags(LAMBDA); }
+
+  bool isNamedLambda(bool hasName) const {
+    return hasName && isLambda() && !hasInferredName() && !hasGuessedAtom();
+  }
+
+  // These methods determine which of the u.scripted.s union arms are active.
+  // For live JSFunctions the pointer values will always be non-null, but due
+  // to partial initialization the GC (and other features that scan the heap
+  // directly) may still return a null pointer.
+  bool hasBaseScript() const { return hasFlags(BASESCRIPT); }
+  bool hasSelfHostedLazyScript() const { return hasFlags(SELFHOSTLAZY); }
+
+  // Arrow functions store their lexical new.target in the first extended slot.
+  bool isArrow() const { return kind() == Arrow; }
+  // Every class-constructor is also a method.
+  bool isMethod() const {
+    return kind() == Method || kind() == ClassConstructor;
+  }
+  bool isClassConstructor() const { return kind() == ClassConstructor; }
+
+  bool isGetter() const { return kind() == Getter; }
+  bool isSetter() const { return kind() == Setter; }
+
+  bool allowSuperProperty() const {
+    return isMethod() || isGetter() || isSetter();
+  }
+
+  bool hasResolvedLength() const { return hasFlags(RESOLVED_LENGTH); }
+  bool hasResolvedName() const { return hasFlags(RESOLVED_NAME); }
+
+  bool isSelfHostedOrIntrinsic() const { return hasFlags(SELF_HOSTED); }
+  bool isSelfHostedBuiltin() const {
+    return isSelfHostedOrIntrinsic() && !isNativeFun();
+  }
+  bool isIntrinsic() const {
+    return isSelfHostedOrIntrinsic() && isNativeFun();
+  }
+
+  FunctionFlags& setKind(FunctionKind kind) {
+    this->flags_ &= ~FUNCTION_KIND_MASK;
+    this->flags_ |= static_cast<uint16_t>(kind) << FUNCTION_KIND_SHIFT;
+    return *this;
+  }
+
+  // Make the function constructible.
+  FunctionFlags& setIsConstructor() {
+    MOZ_ASSERT(!isConstructor());
+    MOZ_ASSERT(isSelfHostedBuiltin());
+    return setFlags(CONSTRUCTOR);
+  }
+
+  FunctionFlags& setIsSelfHostedBuiltin() {
+    MOZ_ASSERT(isInterpreted());
+    MOZ_ASSERT(!isSelfHostedBuiltin());
+    setFlags(SELF_HOSTED);
+    // Self-hosted functions should not be constructable.
+    return clearFlags(CONSTRUCTOR);
+  }
+  FunctionFlags& setIsIntrinsic() {
+    MOZ_ASSERT(isNativeFun());
+    MOZ_ASSERT(!isIntrinsic());
+    return setFlags(SELF_HOSTED);
+  }
+
+  FunctionFlags& setResolvedLength() { return setFlags(RESOLVED_LENGTH); }
+  FunctionFlags& setResolvedName() { return setFlags(RESOLVED_NAME); }
+
+  FunctionFlags& setInferredName() { return setFlags(HAS_INFERRED_NAME); }
+
+  FunctionFlags& setGuessedAtom() { return setFlags(HAS_GUESSED_ATOM); }
+
+  FunctionFlags& setSelfHostedLazy() { return setFlags(SELFHOSTLAZY); }
+  FunctionFlags& clearSelfHostedLazy() { return clearFlags(SELFHOSTLAZY); }
+  FunctionFlags& setBaseScript() { return setFlags(BASESCRIPT); }
+  FunctionFlags& clearBaseScript() { return clearFlags(BASESCRIPT); }
+
+  FunctionFlags& setWasmJitEntry() { return setFlags(WASM_JIT_ENTRY); }
+
+  bool isExtended() const { return hasFlags(EXTENDED); }
+  FunctionFlags& setIsExtended() { return setFlags(EXTENDED); }
+
+  bool isNativeConstructor() const { return hasFlags(NATIVE_CTOR); }
+
+  FunctionFlags& setIsGhost() { return setFlags(GHOST_FUNCTION); }
+  bool isGhost() const { return hasFlags(GHOST_FUNCTION); }
+
+  static uint16_t HasJitEntryFlags(bool isConstructing) {
+    uint16_t flags = BASESCRIPT | SELFHOSTLAZY;
+    if (!isConstructing) {
+      flags |= WASM_JIT_ENTRY;
+    }
+    return flags;
+  }
+
+  static FunctionFlags clearMutableflags(FunctionFlags flags) {
+    return FunctionFlags(flags.toRaw() & ~FunctionFlags::MUTABLE_FLAGS);
+  }
+};
+
+} /* namespace js */
+
+#endif /* vm_FunctionFlags_h */