diff -r 000000000000 -r 6474c204b198 xpcom/tests/TestJemalloc.cpp
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/xpcom/tests/TestJemalloc.cpp	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,78 @@
+/* -*-  Mode: C++; tab-width: 2; indent-tabs-mode: nil; c-basic-offset: 2; -*- */
+/* 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/. */
+
+/*
+ * Ideally, this test would be in memory/test/.  But I couldn't get it to build
+ * there (couldn't find TestHarness.h).  I think memory/ is processed too early
+ * in the build.  So it's here.
+ */
+
+#include "TestHarness.h"
+#include "mozmemory.h"
+
+static inline bool
+TestOne(size_t size)
+{
+    size_t req = size;
+    size_t adv = malloc_good_size(req);
+    char* p = (char*)malloc(req);
+    size_t usable = moz_malloc_usable_size(p);
+    if (adv != usable) {
+      fail("malloc_good_size(%d) --> %d; "
+           "malloc_usable_size(%d) --> %d",
+           req, adv, req, usable);
+      return false;
+    }
+    free(p);
+    return true;
+}
+
+static inline bool
+TestThree(size_t size)
+{
+    return TestOne(size - 1) && TestOne(size) && TestOne(size + 1);
+}
+
+static nsresult
+TestJemallocUsableSizeInAdvance()
+{
+  #define K   * 1024
+  #define M   * 1024 * 1024
+
+  /*
+   * Test every size up to a certain point, then (N-1, N, N+1) triplets for a
+   * various sizes beyond that.
+   */
+
+  for (size_t n = 0; n < 16 K; n++)
+    if (!TestOne(n))
+      return NS_ERROR_UNEXPECTED;
+
+  for (size_t n = 16 K; n < 1 M; n += 4 K)
+    if (!TestThree(n))
+      return NS_ERROR_UNEXPECTED;
+
+  for (size_t n = 1 M; n < 8 M; n += 128 K)
+    if (!TestThree(n))
+      return NS_ERROR_UNEXPECTED;
+
+  passed("malloc_good_size");
+
+  return NS_OK;
+}
+
+int main(int argc, char** argv)
+{
+  int rv = 0;
+  ScopedXPCOM xpcom("jemalloc");
+  if (xpcom.failed())
+      return 1;
+
+  if (NS_FAILED(TestJemallocUsableSizeInAdvance()))
+    rv = 1;
+
+  return rv;
+}
+