diff -r 000000000000 -r 6474c204b198 js/src/vm/StringBuffer.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/js/src/vm/StringBuffer.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,100 @@
+/* -*- 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/. */
+
+#include "vm/StringBuffer.h"
+
+#include "jsobjinlines.h"
+
+#include "vm/String-inl.h"
+
+using namespace js;
+
+jschar *
+StringBuffer::extractWellSized()
+{
+    size_t capacity = cb.capacity();
+    size_t length = cb.length();
+
+    jschar *buf = cb.extractRawBuffer();
+    if (!buf)
+        return nullptr;
+
+    /* For medium/big buffers, avoid wasting more than 1/4 of the memory. */
+    JS_ASSERT(capacity >= length);
+    if (length > CharBuffer::sMaxInlineStorage && capacity - length > length / 4) {
+        size_t bytes = sizeof(jschar) * (length + 1);
+        ExclusiveContext *cx = context();
+        jschar *tmp = (jschar *)cx->realloc_(buf, bytes);
+        if (!tmp) {
+            js_free(buf);
+            return nullptr;
+        }
+        buf = tmp;
+    }
+
+    return buf;
+}
+
+JSFlatString *
+StringBuffer::finishString()
+{
+    ExclusiveContext *cx = context();
+    if (cb.empty())
+        return cx->names().empty;
+
+    size_t length = cb.length();
+    if (!JSString::validateLength(cx, length))
+        return nullptr;
+
+    JS_STATIC_ASSERT(JSFatInlineString::MAX_FAT_INLINE_LENGTH < CharBuffer::InlineLength);
+    if (JSFatInlineString::lengthFits(length))
+        return NewFatInlineString<CanGC>(cx, TwoByteChars(cb.begin(), length));
+
+    if (!cb.append('\0'))
+        return nullptr;
+
+    jschar *buf = extractWellSized();
+    if (!buf)
+        return nullptr;
+
+    JSFlatString *str = js_NewString<CanGC>(cx, buf, length);
+    if (!str)
+        js_free(buf);
+    return str;
+}
+
+JSAtom *
+StringBuffer::finishAtom()
+{
+    ExclusiveContext *cx = context();
+
+    size_t length = cb.length();
+    if (length == 0)
+        return cx->names().empty;
+
+    JSAtom *atom = AtomizeChars(cx, cb.begin(), length);
+    cb.clear();
+    return atom;
+}
+
+bool
+js::ValueToStringBufferSlow(JSContext *cx, const Value &arg, StringBuffer &sb)
+{
+    RootedValue v(cx, arg);
+    if (!ToPrimitive(cx, JSTYPE_STRING, &v))
+        return false;
+
+    if (v.isString())
+        return sb.append(v.toString());
+    if (v.isNumber())
+        return NumberValueToStringBuffer(cx, v, sb);
+    if (v.isBoolean())
+        return BooleanToStringBuffer(v.toBoolean(), sb);
+    if (v.isNull())
+        return sb.append(cx->names().null);
+    JS_ASSERT(v.isUndefined());
+    return sb.append(cx->names().undefined);
+}