diff -r 000000000000 -r 6474c204b198 js/src/jsfuninlines.h
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/jsfuninlines.h	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,90 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * 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 jsfuninlines_h
+#define jsfuninlines_h
+
+#include "jsfun.h"
+
+#include "vm/ScopeObject.h"
+
+namespace js {
+
+inline const char *
+GetFunctionNameBytes(JSContext *cx, JSFunction *fun, JSAutoByteString *bytes)
+{
+    JSAtom *atom = fun->atom();
+    if (atom)
+        return bytes->encodeLatin1(cx, atom);
+    return js_anonymous_str;
+}
+
+static inline JSObject *
+SkipScopeParent(JSObject *parent)
+{
+    if (!parent)
+        return nullptr;
+    while (parent->is<ScopeObject>())
+        parent = &parent->as<ScopeObject>().enclosingScope();
+    return parent;
+}
+
+inline bool
+CanReuseFunctionForClone(JSContext *cx, HandleFunction fun)
+{
+    if (!fun->hasSingletonType())
+        return false;
+    if (fun->isInterpretedLazy()) {
+        LazyScript *lazy = fun->lazyScript();
+        if (lazy->hasBeenCloned())
+            return false;
+        lazy->setHasBeenCloned();
+    } else {
+        JSScript *script = fun->nonLazyScript();
+        if (script->hasBeenCloned())
+            return false;
+        script->setHasBeenCloned();
+    }
+    return true;
+}
+
+inline JSFunction *
+CloneFunctionObjectIfNotSingleton(JSContext *cx, HandleFunction fun, HandleObject parent,
+                                  NewObjectKind newKind = GenericObject)
+{
+    /*
+     * For attempts to clone functions at a function definition opcode,
+     * try to avoid the the clone if the function has singleton type. This
+     * was called pessimistically, and we need to preserve the type's
+     * property that if it is singleton there is only a single object
+     * with its type in existence.
+     *
+     * For functions inner to run once lambda, it may be possible that
+     * the lambda runs multiple times and we repeatedly clone it. In these
+     * cases, fall through to CloneFunctionObject, which will deep clone
+     * the function's script.
+     */
+    if (CanReuseFunctionForClone(cx, fun)) {
+        RootedObject obj(cx, SkipScopeParent(parent));
+        if (!JSObject::setParent(cx, fun, obj))
+            return nullptr;
+        fun->setEnvironment(parent);
+        return fun;
+    }
+
+    // These intermediate variables are needed to avoid link errors on some
+    // platforms.  Sigh.
+    gc::AllocKind finalizeKind = JSFunction::FinalizeKind;
+    gc::AllocKind extendedFinalizeKind = JSFunction::ExtendedFinalizeKind;
+    gc::AllocKind kind = fun->isExtended()
+                         ? extendedFinalizeKind
+                         : finalizeKind;
+    return CloneFunctionObject(cx, fun, parent, kind, newKind);
+}
+
+} /* namespace js */
+
+#endif /* jsfuninlines_h */