diff --git a/yjit.c b/yjit.c
index 53f6e55608..8b2de85b08 100644
--- a/yjit.c
+++ b/yjit.c
@@ -12,6 +12,25 @@
 
 #include "yjit_asm.c"
 
+#ifndef YJIT_CHECK_MODE
+# define YJIT_CHECK_MODE 0
+#endif
+
+// >= 1: print when output code invalidation happens
+// >= 2: dump list of instructions when regions compile
+#ifndef YJIT_DUMP_MODE
+# define YJIT_DUMP_MODE 0
+#endif
+
+#ifdef _WIN32
+# define PLATFORM_SUPPORTED_P 0
+#else
+# define PLATFORM_SUPPORTED_P 1
+#endif
+
+// USE_MJIT comes from configure options
+#define JIT_ENABLED USE_MJIT
+
 // Code block into which we write machine code
 static codeblock_t block;
 static codeblock_t *cb = NULL;
diff --git a/yjit.h b/yjit.h
index af0ebd0179..a3d8594a5d 100644
--- a/yjit.h
+++ b/yjit.h
@@ -1,40 +1,18 @@
+#ifndef YJIT_H
+#define YJIT_H 1
 //
 // This file contains definitions YJIT exposes to the CRuby codebase
 //
 
-#ifndef YJIT_H
-#define YJIT_H 1
-
 #include "ruby/internal/config.h"
 #include "ruby_assert.h" // for RUBY_DEBUG
 #include "vm_core.h"
 #include "method.h"
 
-#ifdef _WIN32
-#define PLATFORM_SUPPORTED_P 0
-#else
-#define PLATFORM_SUPPORTED_P 1
-#endif
-
-#define JIT_ENABLED USE_MJIT
-
-#ifndef YJIT_CHECK_MODE
-#define YJIT_CHECK_MODE 0
-#endif
-
-// >= 1: print when output code invalidation happens
-// >= 2: dump list of instructions when regions compile
-#ifndef YJIT_DUMP_MODE
-#define YJIT_DUMP_MODE 0
-#endif
-
+// YJIT_STATS controls whether to support runtime counters in generated code
+// and in the interpreter.
 #ifndef YJIT_STATS
 # define YJIT_STATS RUBY_DEBUG
-#endif // ifndef YJIT_STATS
-
-#ifndef rb_iseq_t
-typedef struct rb_iseq_struct rb_iseq_t;
-#define rb_iseq_t rb_iseq_t
 #endif
 
 struct rb_yjit_options {