diff -r 000000000000 -r 6474c204b198 tools/reorder/mcount.c
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/tools/reorder/mcount.c	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,38 @@
+/* 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/. */
+
+/*
+
+  A library that can be LD_PRELOAD-ed to dump a function's address on
+  function entry.
+
+ */
+
+#include <unistd.h>
+
+/**
+ * When compiled with `-pg' (or even just `-p'), gcc inserts a call to
+ * |mcount| in each function prologue. This implementation of |mcount|
+ * will grab the return address from the stack, and write it to stdout
+ * as a binary stream, creating a gross execution trace the
+ * instrumented program.
+ *
+ * For more info on gcc inline assembly, see:
+ *
+ *   <http://www.ibm.com/developerworks/linux/library/l-ia.html?dwzone=linux>
+ *
+ */
+void
+mcount()
+{
+    register unsigned int caller;
+    unsigned int buf[1];
+
+    // Grab the return address. 
+    asm("movl 4(%%esp),%0"
+        : "=r"(caller));
+
+    buf[0] = caller;
+    write(STDOUT_FILENO, buf, sizeof buf[0]);
+}