diff --git a/ChangeLog b/ChangeLog
index a55f95636a..9b8870d664 100644
--- a/ChangeLog
+++ b/ChangeLog
@@ -1,3 +1,12 @@
+Fri Feb 27 17:06:44 2015  Koichi Sasada  <ko1@atdot.net>
+
+	* vm_core.h: define vm_svar_index.
+
+	* vm_insnhelper.c, vm.c, compile.c: use vm_svar_index names.
+
+	* iseq.h: remove DEFAULT_SPECIAL_VAR_COUNT.
+	  use VM_SVAR_FLIPFLOP_START instead.
+
 Fri Feb 27 13:57:48 2015  Nobuyoshi Nakada  <nobu@ruby-lang.org>
 
 	* io.c (setup_narg): wipe away expanded part of buffer to get rid
diff --git a/compile.c b/compile.c
index b9e8f08912..e459bb90ab 100644
--- a/compile.c
+++ b/compile.c
@@ -5288,7 +5288,7 @@ iseq_compile_each(rb_iseq_t *iseq, LINK_ANCHOR *ret, NODE * node, int poped)
 	rb_num_t cnt;
 	VALUE key;
 
-	cnt = local_iseq->flip_cnt++ + DEFAULT_SPECIAL_VAR_COUNT;
+	cnt = local_iseq->flip_cnt++ + VM_SVAR_FLIPFLOP_START;
 	key = INT2FIX(cnt);
 
 	ADD_INSN2(ret, line, getspecial, key, INT2FIX(0));
diff --git a/iseq.h b/iseq.h
index 2dec51585d..f9fb62e1f3 100644
--- a/iseq.h
+++ b/iseq.h
@@ -151,8 +151,6 @@ enum defined_type {
 
 VALUE rb_iseq_defined_string(enum defined_type type);
 
-#define DEFAULT_SPECIAL_VAR_COUNT 2
-
 RUBY_SYMBOL_EXPORT_END
 
 #endif /* RUBY_COMPILE_H */
diff --git a/vm.c b/vm.c
index 3aaf1ea483..b84ba788c2 100644
--- a/vm.c
+++ b/vm.c
@@ -943,25 +943,25 @@ vm_svar_set(VALUE key, VALUE val)
 VALUE
 rb_backref_get(void)
 {
-    return vm_svar_get(1);
+    return vm_svar_get(VM_SVAR_BACKREF);
 }
 
 void
 rb_backref_set(VALUE val)
 {
-    vm_svar_set(1, val);
+    vm_svar_set(VM_SVAR_BACKREF, val);
 }
 
 VALUE
 rb_lastline_get(void)
 {
-    return vm_svar_get(0);
+    return vm_svar_get(VM_SVAR_LASTLINE);
 }
 
 void
 rb_lastline_set(VALUE val)
 {
-    vm_svar_set(0, val);
+    vm_svar_set(VM_SVAR_LASTLINE, val);
 }
 
 /* misc */
diff --git a/vm_core.h b/vm_core.h
index 6ab17e0dfc..1f17dfd56a 100644
--- a/vm_core.h
+++ b/vm_core.h
@@ -838,6 +838,14 @@ enum vm_special_object_type {
     VM_SPECIAL_OBJECT_CONST_BASE
 };
 
+enum vm_svar_index {
+    VM_SVAR_LASTLINE = 0,      /* $_ */
+    VM_SVAR_BACKREF = 1,       /* $~ */
+
+    VM_SVAR_EXTRA_START = 2,
+    VM_SVAR_FLIPFLOP_START = 2 /* flipflop */
+};
+
 #define VM_FRAME_MAGIC_METHOD 0x11
 #define VM_FRAME_MAGIC_BLOCK  0x21
 #define VM_FRAME_MAGIC_CLASS  0x31
diff --git a/vm_insnhelper.c b/vm_insnhelper.c
index 1f477650ce..815cf4e30b 100644
--- a/vm_insnhelper.c
+++ b/vm_insnhelper.c
@@ -140,11 +140,11 @@ rb_error_arity(int argc, int min, int max)
 /* svar */
 
 static inline NODE **
-lep_svar_place(rb_thread_t *th, VALUE *lep)
+lep_svar_place(rb_thread_t *th, const VALUE *lep)
 {
-    VALUE *svar;
+    const VALUE *svar;
 
-    if (lep && th->root_lep != lep) {
+    if (lep && (th == NULL || th->root_lep != lep)) {
 	svar = &lep[-1];
     }
     else {
@@ -155,17 +155,17 @@ lep_svar_place(rb_thread_t *th, VALUE *lep)
 }
 
 static VALUE
-lep_svar_get(rb_thread_t *th, VALUE *lep, rb_num_t key)
+lep_svar_get(rb_thread_t *th, const VALUE *lep, rb_num_t key)
 {
-    NODE **svar_place = lep_svar_place(th, lep);
-    NODE *svar = *svar_place;
+    NODE ** const svar_place = lep_svar_place(th, lep);
+    const NODE * const svar = *svar_place;
 
     if (NIL_P((VALUE)svar)) return Qnil;
 
     switch (key) {
-      case 0:
+      case VM_SVAR_LASTLINE:
 	return svar->u1.value;
-      case 1:
+      case VM_SVAR_BACKREF:
 	return svar->u2.value;
       default: {
 	const VALUE ary = svar->u3.value;
@@ -174,7 +174,7 @@ lep_svar_get(rb_thread_t *th, VALUE *lep, rb_num_t key)
 	    return Qnil;
 	}
 	else {
-	    return rb_ary_entry(ary, key - DEFAULT_SPECIAL_VAR_COUNT);
+	    return rb_ary_entry(ary, key - VM_SVAR_EXTRA_START);
 	}
       }
     }
@@ -183,7 +183,7 @@ lep_svar_get(rb_thread_t *th, VALUE *lep, rb_num_t key)
 static void
 lep_svar_set(rb_thread_t *th, VALUE *lep, rb_num_t key, VALUE val)
 {
-    NODE **svar_place = lep_svar_place(th, lep);
+    NODE **svar_place = (NODE **)lep_svar_place(th, lep);
     NODE *svar = *svar_place;
 
     if (NIL_P((VALUE)svar)) {
@@ -191,10 +191,10 @@ lep_svar_set(rb_thread_t *th, VALUE *lep, rb_num_t key, VALUE val)
     }
 
     switch (key) {
-      case 0:
+      case VM_SVAR_LASTLINE:
 	svar->u1.value = val;
 	return;
-      case 1:
+      case VM_SVAR_BACKREF:
 	svar->u2.value = val;
 	return;
       default: {
@@ -203,7 +203,7 @@ lep_svar_set(rb_thread_t *th, VALUE *lep, rb_num_t key, VALUE val)
 	if (NIL_P(ary)) {
 	    svar->u3.value = ary = rb_ary_new();
 	}
-	rb_ary_store(ary, key - DEFAULT_SPECIAL_VAR_COUNT, val);
+	rb_ary_store(ary, key - VM_SVAR_EXTRA_START, val);
       }
     }
 }
@@ -217,7 +217,7 @@ vm_getspecial(rb_thread_t *th, VALUE *lep, rb_num_t key, rb_num_t type)
 	val = lep_svar_get(th, lep, key);
     }
     else {
-	VALUE backref = lep_svar_get(th, lep, 1);
+	VALUE backref = lep_svar_get(th, lep, VM_SVAR_BACKREF);
 
 	if (type & 0x01) {
 	    switch (type >> 1) {