diff --git a/llvm/Makefile.rules b/llvm/Makefile.rules
index bf13b26200c0..3c115d2a52e0 100644
--- a/llvm/Makefile.rules
+++ b/llvm/Makefile.rules
@@ -519,6 +519,16 @@ ifdef UNIVERSAL
 
   # Building universal cannot compute dependencies automatically.
   DISABLE_AUTO_DEPENDENCIES=1
+else
+  ifeq ($(OS),Darwin)
+    ifeq ($(ARCH),x86_64)
+      CompileCommonOpts += -m64
+    else
+      ifeq ($(ARCH),x86)
+        CompileCommonOpts += -m32
+      endif
+    endif
+  endif
 endif
 
 ifeq ($(OS),SunOS)