diff --git a/common.mk b/common.mk
index 0449d24dd5..a65877aa9e 100644
--- a/common.mk
+++ b/common.mk
@@ -211,6 +211,8 @@ mjit-headers: $(TIMESTAMPDIR)/$(MJIT_HEADER:.h=)$(MJIT_HEADER_SUFFIX).time $(MJI
 mjit.$(OBJEXT): mjit_config.h
 mjit_config.h: Makefile
 
+# Other `-Dxxx`s preceding `-DMJIT_HEADER` will be removed in transform_mjit_header.rb.
+# So `-DMJIT_HEADER` should be passed first when rb_mjit_header.h is generated.
 $(TIMESTAMPDIR)/$(MJIT_HEADER:.h=)$(MJIT_HEADER_SUFFIX).time: probes.h vm.$(OBJEXT)
 	$(ECHO) building $(@F:.time=.h)
 	$(Q) $(CPP) -DMJIT_HEADER $(MJIT_HEADER_FLAGS) $(CFLAGS) $(XCFLAGS) $(CPPFLAGS) $(srcdir)/vm.c $(CPPOUTFLAG)$(@F:.time=.h).new
diff --git a/tool/transform_mjit_header.rb b/tool/transform_mjit_header.rb
index c291004e7e..a2ebba0fb2 100644
--- a/tool/transform_mjit_header.rb
+++ b/tool/transform_mjit_header.rb
@@ -109,6 +109,7 @@ module MJITHeader
   end
 
   # -dD outputs those macros, and it produces redefinition warnings or errors
+  # This assumes common.mk passes `-DMJIT_HEADER` first when it creates rb_mjit_header.h.
   def self.remove_predefined_macros!(code)
     code.sub!(/\A(#define [^\n]+|\n)*(#define MJIT_HEADER 1\n)/, '\2')
   end