diff --git a/include/ruby/ruby.h b/include/ruby/ruby.h
index a27fc5d637..bad0b3f1cc 100644
--- a/include/ruby/ruby.h
+++ b/include/ruby/ruby.h
@@ -41,8 +41,15 @@ extern "C" {
 #ifndef NOINLINE
 # define NOINLINE(x) x
 #endif
+#ifndef ASSUME
+# ifdef UNREACHABLE
+#   define ASSUME(x) (LIKELY(!!(x)) ? (void)0 : UNREACHABLE)
+# else
+#   define ASSUME(x) ((void)0)
+# endif
+#endif
 #ifndef UNREACHABLE
-# define UNREACHABLE		/* unreachable */
+# define UNREACHABLE ((void)0)	/* unreachable */
 #endif
 
 #ifdef __GNUC__
diff --git a/win32/Makefile.sub b/win32/Makefile.sub
index e07ed93b60..afcb29cd44 100644
--- a/win32/Makefile.sub
+++ b/win32/Makefile.sub
@@ -587,6 +587,7 @@ $(CONFIG_H): $(MKFILES) $(srcdir)/win32/Makefile.sub $(win_srcdir)/Makefile.sub
 #define NOINLINE(x) __declspec(noinline) x
 !endif
 #define UNREACHABLE __assume(0)
+#define ASSUME(x) __assume(!!(x))
 #define FUNC_STDCALL(x) __stdcall x
 #define FUNC_CDECL(x) __cdecl x
 #define FUNC_FASTCALL(x) __fastcall x