diff --git a/xpcom/glue/nsMemory.h b/xpcom/glue/nsMemory.h
index 3cbb60500d2e..b4210be3b0f0 100644
--- a/xpcom/glue/nsMemory.h
+++ b/xpcom/glue/nsMemory.h
@@ -156,6 +156,22 @@ public:
 #define NS_ARRAY_LENGTH(array_) \
   (sizeof(array_)/sizeof(array_[0]))
 
+/**
+ * A macro, NS_ALIGNMENT_OF(t_) that determines the alignment
+ * requirements of a type.
+ */
+namespace mozilla {
+  template <class T>
+  struct AlignmentTestStruct
+  {
+    char c;
+    T t;
+  };
+}
+
+#define NS_ALIGNMENT_OF(t_) \
+  (sizeof(mozilla::AlignmentTestStruct<t_>) - sizeof(t_))
+
 /**
  * An enumeration type used to represent a method of assignment.
  */
diff --git a/xpcom/tests/Makefile.in b/xpcom/tests/Makefile.in
index ec586a805688..f23f5437bbf2 100644
--- a/xpcom/tests/Makefile.in
+++ b/xpcom/tests/Makefile.in
@@ -80,6 +80,7 @@ endif
 SIMPLE_PROGRAMS	:= $(CPPSRCS:.cpp=$(BIN_SUFFIX))
 
 CPP_UNIT_TESTS = \
+                 ShowAlignments.cpp \
                  ShowSSEConfig.cpp \
                  TestAutoPtr.cpp \
                  TestCOMArray.cpp \
diff --git a/xpcom/tests/ShowAlignments.cpp b/xpcom/tests/ShowAlignments.cpp
new file mode 100644
index 000000000000..921cadea082c
--- /dev/null
+++ b/xpcom/tests/ShowAlignments.cpp
@@ -0,0 +1,71 @@
+/* vim: set shiftwidth=2 tabstop=8 autoindent cindent expandtab: */
+/* ***** BEGIN LICENSE BLOCK *****
+ * Version: MPL 1.1/GPL 2.0/LGPL 2.1
+ *
+ * The contents of this file are subject to the Mozilla Public License Version
+ * 1.1 (the "License"); you may not use this file except in compliance with
+ * the License. You may obtain a copy of the License at
+ * http://www.mozilla.org/MPL/
+ *
+ * Software distributed under the License is distributed on an "AS IS" basis,
+ * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
+ * for the specific language governing rights and limitations under the
+ * License.
+ *
+ * The Original Code is ShowAlignments.cpp.
+ *
+ * The Initial Developer of the Original Code is the Mozilla Foundation.
+ * Portions created by the Initial Developer are Copyright (C) 2010
+ * the Initial Developer. All Rights Reserved.
+ *
+ * Contributor(s):
+ *   L. David Baron <dbaron@dbaron.org>, Mozilla Corporation (original author)
+ *
+ * Alternatively, the contents of this file may be used under the terms of
+ * either the GNU General Public License Version 2 or later (the "GPL"), or
+ * the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
+ * in which case the provisions of the GPL or the LGPL are applicable instead
+ * of those above. If you wish to allow use of your version of this file only
+ * under the terms of either the GPL or the LGPL, and not to allow others to
+ * use your version of this file under the terms of the MPL, indicate your
+ * decision by deleting the provisions above and replace them with the notice
+ * and other provisions required by the GPL or the LGPL. If you do not delete
+ * the provisions above, a recipient may use your version of this file under
+ * the terms of any one of the MPL, the GPL or the LGPL.
+ *
+ * ***** END LICENSE BLOCK ***** */
+
+/* show the size and alignment requirements of various types */
+
+#include "nsMemory.h"
+#include <stdio.h>
+
+struct S {
+  double d;
+  char c;
+  short s;
+};
+
+int main()
+{
+  static const char str[] =
+    "Type %s has size %u and alignment requirement %u\n";
+  #define SHOW_TYPE(t_) \
+    printf(str, #t_, unsigned(sizeof(t_)), unsigned(NS_ALIGNMENT_OF(t_)))
+
+  SHOW_TYPE(char);
+  SHOW_TYPE(unsigned short);
+  SHOW_TYPE(int);
+  SHOW_TYPE(long);
+  SHOW_TYPE(PRUint8);
+  SHOW_TYPE(PRInt16);
+  SHOW_TYPE(PRUint32);
+  SHOW_TYPE(PRFloat64);
+  SHOW_TYPE(void*);
+  SHOW_TYPE(double);
+  SHOW_TYPE(short[7]);
+  SHOW_TYPE(S);
+  SHOW_TYPE(double S::*);
+
+  return 0;
+}