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diff --git a/js/public/experimental/JitInfo.h b/js/public/experimental/JitInfo.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 js_experimental_JitInfo_h
+#define js_experimental_JitInfo_h
+
+#include "mozilla/Assertions.h"  // MOZ_ASSERT
+
+#include <stddef.h>  // size_t
+#include <stdint.h>  // uint16_t, uint32_t
+
+#include "js/CallArgs.h"    // JS::CallArgs, JS::detail::CallArgsBase, JSNative
+#include "js/RootingAPI.h"  // JS::{,Mutable}Handle, JS::Rooted
+#include "js/Value.h"       // JS::Value, JSValueType
+
+namespace js {
+
+namespace jit {
+
+enum class InlinableNative : uint16_t;
+
+}  // namespace jit
+
+}  // namespace js
+
+/**
+ * A class, expected to be passed by value, which represents the CallArgs for a
+ * JSJitGetterOp.
+ */
+class JSJitGetterCallArgs : protected JS::MutableHandle<JS::Value> {
+ public:
+  explicit JSJitGetterCallArgs(const JS::CallArgs& args)
+      : JS::MutableHandle<JS::Value>(args.rval()) {}
+
+  explicit JSJitGetterCallArgs(JS::Rooted<JS::Value>* rooted)
+      : JS::MutableHandle<JS::Value>(rooted) {}
+
+  explicit JSJitGetterCallArgs(JS::MutableHandle<JS::Value> handle)
+      : JS::MutableHandle<JS::Value>(handle) {}
+
+  JS::MutableHandle<JS::Value> rval() { return *this; }
+};
+
+/**
+ * A class, expected to be passed by value, which represents the CallArgs for a
+ * JSJitSetterOp.
+ */
+class JSJitSetterCallArgs : protected JS::MutableHandle<JS::Value> {
+ public:
+  explicit JSJitSetterCallArgs(const JS::CallArgs& args)
+      : JS::MutableHandle<JS::Value>(args[0]) {}
+
+  explicit JSJitSetterCallArgs(JS::Rooted<JS::Value>* rooted)
+      : JS::MutableHandle<JS::Value>(rooted) {}
+
+  JS::MutableHandle<JS::Value> operator[](unsigned i) {
+    MOZ_ASSERT(i == 0);
+    return *this;
+  }
+
+  unsigned length() const { return 1; }
+
+  // Add get() or maybe hasDefined() as needed
+};
+
+struct JSJitMethodCallArgsTraits;
+
+/**
+ * A class, expected to be passed by reference, which represents the CallArgs
+ * for a JSJitMethodOp.
+ */
+class JSJitMethodCallArgs
+    : protected JS::detail::CallArgsBase<JS::detail::NoUsedRval> {
+ private:
+  using Base = JS::detail::CallArgsBase<JS::detail::NoUsedRval>;
+  friend struct JSJitMethodCallArgsTraits;
+
+ public:
+  explicit JSJitMethodCallArgs(const JS::CallArgs& args) {
+    argv_ = args.array();
+    argc_ = args.length();
+  }
+
+  JS::MutableHandle<JS::Value> rval() const { return Base::rval(); }
+
+  unsigned length() const { return Base::length(); }
+
+  JS::MutableHandle<JS::Value> operator[](unsigned i) const {
+    return Base::operator[](i);
+  }
+
+  bool hasDefined(unsigned i) const { return Base::hasDefined(i); }
+
+  JSObject& callee() const {
+    // We can't use Base::callee() because that will try to poke at
+    // this->usedRval_, which we don't have.
+    return argv_[-2].toObject();
+  }
+
+  JS::Handle<JS::Value> get(unsigned i) const { return Base::get(i); }
+
+  bool requireAtLeast(JSContext* cx, const char* fnname,
+                      unsigned required) const {
+    // Can just forward to Base, since it only needs the length and we
+    // forward that already.
+    return Base::requireAtLeast(cx, fnname, required);
+  }
+};
+
+struct JSJitMethodCallArgsTraits {
+  static constexpr size_t offsetOfArgv = offsetof(JSJitMethodCallArgs, argv_);
+  static constexpr size_t offsetOfArgc = offsetof(JSJitMethodCallArgs, argc_);
+};
+
+using JSJitGetterOp = bool (*)(JSContext*, JS::Handle<JSObject*>, void*,
+                               JSJitGetterCallArgs);
+using JSJitSetterOp = bool (*)(JSContext*, JS::Handle<JSObject*>, void*,
+                               JSJitSetterCallArgs);
+using JSJitMethodOp = bool (*)(JSContext*, JS::Handle<JSObject*>, void*,
+                               const JSJitMethodCallArgs&);
+
+/**
+ * This struct contains metadata passed from the DOM to the JS Engine for JIT
+ * optimizations on DOM property accessors.
+ *
+ * Eventually, this should be made available to general JSAPI users as *not*
+ * experimental and *not* a friend API, but we're not ready to do so yet.
+ */
+class JSJitInfo {
+ public:
+  enum OpType {
+    Getter,
+    Setter,
+    Method,
+    StaticMethod,
+    InlinableNative,
+    IgnoresReturnValueNative,
+    // Must be last
+    OpTypeCount
+  };
+
+  enum ArgType {
+    // Basic types
+    String = (1 << 0),
+    Integer = (1 << 1),  // Only 32-bit or less
+    Double = (1 << 2),   // Maybe we want to add Float sometime too
+    Boolean = (1 << 3),
+    Object = (1 << 4),
+    Null = (1 << 5),
+
+    // And derived types
+    Numeric = Integer | Double,
+    // Should "Primitive" use the WebIDL definition, which
+    // excludes string and null, or the typical JS one that includes them?
+    Primitive = Numeric | Boolean | Null | String,
+    ObjectOrNull = Object | Null,
+    Any = ObjectOrNull | Primitive,
+
+    // Our sentinel value.
+    ArgTypeListEnd = (1 << 31)
+  };
+
+  static_assert(Any & String, "Any must include String");
+  static_assert(Any & Integer, "Any must include Integer");
+  static_assert(Any & Double, "Any must include Double");
+  static_assert(Any & Boolean, "Any must include Boolean");
+  static_assert(Any & Object, "Any must include Object");
+  static_assert(Any & Null, "Any must include Null");
+
+  /**
+   * An enum that describes what this getter/setter/method aliases.  This
+   * determines what things can be hoisted past this call, and if this
+   * call is movable what it can be hoisted past.
+   */
+  enum AliasSet {
+    /**
+     * Alias nothing: a constant value, getting it can't affect any other
+     * values, nothing can affect it.
+     */
+    AliasNone,
+
+    /**
+     * Alias things that can modify the DOM but nothing else.  Doing the
+     * call can't affect the behavior of any other function.
+     */
+    AliasDOMSets,
+
+    /**
+     * Alias the world.  Calling this can change arbitrary values anywhere
+     * in the system.  Most things fall in this bucket.
+     */
+    AliasEverything,
+
+    /** Must be last. */
+    AliasSetCount
+  };
+
+  bool needsOuterizedThisObject() const {
+    return type() != Getter && type() != Setter;
+  }
+
+  bool isTypedMethodJitInfo() const { return isTypedMethod; }
+
+  OpType type() const { return OpType(type_); }
+
+  AliasSet aliasSet() const { return AliasSet(aliasSet_); }
+
+  JSValueType returnType() const { return JSValueType(returnType_); }
+
+  union {
+    JSJitGetterOp getter;
+    JSJitSetterOp setter;
+    JSJitMethodOp method;
+    /** A DOM static method, used for Promise wrappers */
+    JSNative staticMethod;
+    JSNative ignoresReturnValueMethod;
+  };
+
+  static unsigned offsetOfIgnoresReturnValueNative() {
+    return offsetof(JSJitInfo, ignoresReturnValueMethod);
+  }
+
+  union {
+    uint16_t protoID;
+    js::jit::InlinableNative inlinableNative;
+  };
+
+  union {
+    uint16_t depth;
+
+    // Additional opcode for some InlinableNative functions.
+    uint16_t nativeOp;
+  };
+
+  // These fields are carefully packed to take up 4 bytes.  If you need more
+  // bits for whatever reason, please see if you can steal bits from existing
+  // fields before adding more members to this structure.
+  static constexpr size_t OpTypeBits = 4;
+  static constexpr size_t AliasSetBits = 4;
+  static constexpr size_t ReturnTypeBits = 8;
+  static constexpr size_t SlotIndexBits = 10;
+
+  /** The OpType that says what sort of function we are. */
+  uint32_t type_ : OpTypeBits;
+
+  /**
+   * The alias set for this op.  This is a _minimal_ alias set; in
+   * particular for a method it does not include whatever argument
+   * conversions might do.  That's covered by argTypes and runtime
+   * analysis of the actual argument types being passed in.
+   */
+  uint32_t aliasSet_ : AliasSetBits;
+
+  /** The return type tag.  Might be JSVAL_TYPE_UNKNOWN. */
+  uint32_t returnType_ : ReturnTypeBits;
+
+  static_assert(OpTypeCount <= (1 << OpTypeBits),
+                "Not enough space for OpType");
+  static_assert(AliasSetCount <= (1 << AliasSetBits),
+                "Not enough space for AliasSet");
+  static_assert((sizeof(JSValueType) * 8) <= ReturnTypeBits,
+                "Not enough space for JSValueType");
+
+  /** Is op fallible? False in setters. */
+  uint32_t isInfallible : 1;
+
+  /**
+   * Is op movable?  To be movable the op must
+   * not AliasEverything, but even that might
+   * not be enough (e.g. in cases when it can
+   * throw or is explicitly not movable).
+   */
+  uint32_t isMovable : 1;
+
+  /**
+   * Can op be dead-code eliminated? Again, this
+   * depends on whether the op can throw, in
+   * addition to the alias set.
+   */
+  uint32_t isEliminatable : 1;
+
+  // XXXbz should we have a JSValueType for the type of the member?
+  /**
+   * True if this is a getter that can always
+   * get the value from a slot of the "this" object.
+   */
+  uint32_t isAlwaysInSlot : 1;
+
+  /**
+   * True if this is a getter that can sometimes (if the slot doesn't contain
+   * UndefinedValue()) get the value from a slot of the "this" object.
+   */
+  uint32_t isLazilyCachedInSlot : 1;
+
+  /** True if this is an instance of JSTypedMethodJitInfo. */
+  uint32_t isTypedMethod : 1;
+
+  /**
+   * If isAlwaysInSlot or isSometimesInSlot is true,
+   * the index of the slot to get the value from.
+   * Otherwise 0.
+   */
+  uint32_t slotIndex : SlotIndexBits;
+
+  static constexpr size_t maxSlotIndex = (1 << SlotIndexBits) - 1;
+};
+
+static_assert(sizeof(JSJitInfo) == (sizeof(void*) + 2 * sizeof(uint32_t)),
+              "There are several thousand instances of JSJitInfo stored in "
+              "a binary. Please don't increase its space requirements without "
+              "verifying that there is no other way forward (better packing, "
+              "smaller datatypes for fields, subclassing, etc.).");
+
+struct JSTypedMethodJitInfo {
+  // We use C-style inheritance here, rather than C++ style inheritance
+  // because not all compilers support brace-initialization for non-aggregate
+  // classes. Using C++ style inheritance and constructors instead of
+  // brace-initialization would also force the creation of static
+  // constructors (on some compilers) when JSJitInfo and JSTypedMethodJitInfo
+  // structures are declared. Since there can be several thousand of these
+  // structures present and we want to have roughly equivalent performance
+  // across a range of compilers, we do things manually.
+  JSJitInfo base;
+
+  const JSJitInfo::ArgType* const argTypes; /* For a method, a list of sets of
+                                               types that the function
+                                               expects.  This can be used,
+                                               for example, to figure out
+                                               when argument coercions can
+                                               have side-effects. */
+};
+
+#endif  // js_experimental_JitInfo_h