diff -r 000000000000 -r 6474c204b198 build/autoconf/llvm-pr8927.m4
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/build/autoconf/llvm-pr8927.m4	Wed Dec 31 06:09:35 2014 +0100
@@ -0,0 +1,52 @@
+dnl This Source Code Form is subject to the terms of the Mozilla Public
+dnl License, v. 2.0. If a copy of the MPL was not distributed with this
+dnl file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+dnl Check if the compiler suffers from http://llvm.org/pr8927. If so, ask the
+dnl user to upgrade.
+
+AC_DEFUN([MOZ_LLVM_PR8927],
+[
+AC_MSG_CHECKING(for llvm pr8927)
+ac_have_llvm_pr8927="no"
+AC_LANG_SAVE
+AC_LANG_C
+
+_SAVE_CFLAGS=$CFLAGS
+CFLAGS="-O2"
+AC_TRY_RUN([
+struct foobar {
+  int x;
+};
+static const struct foobar* foo() {
+  static const struct foobar d = { 0 };
+  return &d;
+}
+static const struct foobar* bar() {
+  static const struct foobar d = { 0 };
+  return &d;
+}
+__attribute__((noinline)) int zed(const struct foobar *a,
+                                  const struct foobar *b) {
+  return a == b;
+}
+int main() {
+  return zed(foo(), bar());
+}
+], true,
+   ac_have_llvm_pr8927="yes",
+   true)
+CFLAGS="$_SAVE_CFLAGS"
+
+AC_LANG_RESTORE
+
+if test "$ac_have_llvm_pr8927" = "yes"; then
+   AC_MSG_RESULT(yes)
+   echo This compiler would miscompile firefox, please upgrade.
+   echo see http://developer.mozilla.org/en-US/docs/Developer_Guide/Build_Instructions/Mac_OS_X_Prerequisites
+   echo for more information.
+   exit 1
+else
+   AC_MSG_RESULT(no)
+fi
+])