diff --git a/common.gypi b/common.gypi
index def3eaf76..ac572ed17 100644
--- a/common.gypi
+++ b/common.gypi
@@ -2313,6 +2313,14 @@
           ],
         },
       }],
+      ['clang==1 and OS!="win"', {
+        # This is here so that all files get recompiled after a clang roll and
+        # when turning clang on or off.
+        # (defines are passed via the command line, and build systems rebuild
+        # things when their commandline changes). Nothing should ever read this
+        # define.
+        'defines': ['CR_CLANG_REVISION=<!(<(DEPTH)/tools/clang/scripts/update.sh --print-revision)'],
+      }],
       ['enable_rlz==1', {
         'defines': ['ENABLE_RLZ'],
       }],