--- a/js/src/methodjit/Logging.h
+++ b/js/src/methodjit/Logging.h
@@ -70,17 +70,21 @@ enum JaegerSpewChannel {
 # define JS_METHODJIT_SPEW
 #endif
 
 #if defined(JS_METHODJIT_SPEW)
 
 void JMCheckLogging();
 
 bool IsJaegerSpewChannelActive(JaegerSpewChannel channel);
+#ifdef __GNUC__
+void JaegerSpew(JaegerSpewChannel channel, const char *fmt, ...) __attribute__ ((format (printf, 2, 3)));
+#else
 void JaegerSpew(JaegerSpewChannel channel, const char *fmt, ...);
+#endif
 
 struct Profiler {
     JSInt64 t_start;
     JSInt64 t_stop;
 
     static inline JSInt64 now() {
         return PRMJ_Now();
     }