primary mruby fiber implementation

16 files changed, 508 insertions, 228 deletions

diff --git a/include/mruby.h b/include/mruby.h
index 9fe70e5b8..73c38cf1d 100644
--- a/include/mruby.h
+++ b/include/mruby.h
@@ -61,16 +61,8 @@ typedef struct {
   struct REnv *env;
 } mrb_callinfo;
 
-enum gc_state {
-  GC_STATE_NONE = 0,
-  GC_STATE_MARK,
-  GC_STATE_SWEEP
-};
-
-typedef struct mrb_state {
-  void *jmp;
-
-  mrb_allocf allocf;
+struct mrb_context {
+  struct mrb_context *prev;
 
   mrb_value *stack;
   mrb_value *stbase, *stend;
@@ -82,10 +74,24 @@ typedef struct mrb_state {
   int rsize;
   struct RProc **ensure;
   int esize;
+};
+
+enum gc_state {
+  GC_STATE_NONE = 0,
+  GC_STATE_MARK,
+  GC_STATE_SWEEP
+};
+
+typedef struct mrb_state {
+  void *jmp;
+
+  mrb_allocf allocf;
+
+  struct mrb_context *c;
+  struct mrb_context *root_c;
 
   struct RObject *exc;
   struct iv_tbl *globals;
-
   struct mrb_irep **irep;
   size_t irep_len, irep_capa;
 
@@ -140,7 +146,6 @@ typedef struct mrb_state {
   struct RClass *eStandardError_class;
 
   void *ud; /* auxiliary data */
-
 } mrb_state;
 
 typedef mrb_value (*mrb_func_t)(mrb_state *mrb, mrb_value);
diff --git a/include/mruby/gc.h b/include/mruby/gc.h
index 2f7dc73cf..7afa24b48 100644
--- a/include/mruby/gc.h
+++ b/include/mruby/gc.h
@@ -12,5 +12,6 @@
 
 typedef void (each_object_callback)(mrb_state *mrb, struct RBasic* obj, void *data);
 void mrb_objspace_each_objects(mrb_state *mrb, each_object_callback* callback, void *data);
+void mrb_free_context(mrb_state *mrb, struct mrb_context *c);
 
 #endif  /* MRUBY_GC_H */
diff --git a/include/mruby/value.h b/include/mruby/value.h
index 0ce59e06a..fa2a68ac8 100644
--- a/include/mruby/value.h
+++ b/include/mruby/value.h
@@ -34,7 +34,8 @@ enum mrb_vtype {
   MRB_TT_FILE,        /*  19 */
   MRB_TT_ENV,         /*  20 */
   MRB_TT_DATA,        /*  21 */
-  MRB_TT_MAXDEFINE    /*  22 */
+  MRB_TT_FIBER,       /*  22 */
+  MRB_TT_MAXDEFINE    /*  23 */
 };
 
 typedef struct mrb_value {
@@ -92,7 +93,8 @@ enum mrb_vtype {
   MRB_TT_FILE,        /*  20 */
   MRB_TT_ENV,         /*  21 */
   MRB_TT_DATA,        /*  22 */
-  MRB_TT_MAXDEFINE    /*  23 */
+  MRB_TT_FIBER,       /*  23 */
+  MRB_TT_MAXDEFINE    /*  24 */
 };
 
 #ifdef MRB_ENDIAN_BIG
@@ -202,6 +204,11 @@ struct RObject {
 #define mrb_immediate_p(x) (mrb_type(x) <= MRB_TT_VOIDP)
 #define mrb_special_const_p(x) mrb_immediate_p(x)
 
+struct RFiber {
+  MRB_OBJECT_HEADER;
+  struct mrb_context *cxt;
+};
+
 static inline mrb_value
 mrb_fixnum_value(mrb_int i)
 {
diff --git a/mrbgems/default.gembox b/mrbgems/default.gembox
index a53b07d8a..1c6732d48 100644
--- a/mrbgems/default.gembox
+++ b/mrbgems/default.gembox
@@ -44,6 +44,9 @@ MRuby::GemBox.new do |conf|
   # Use ObjectSpace class
   conf.gem :core => "mruby-objectspace"
 
+  # Use Fiber class
+  conf.gem :core => "mruby-fiber"
+
   # Generate mirb command
   conf.gem :core => "mruby-bin-mirb"
 
diff --git a/mrbgems/mruby-fiber/mrbgem.rake b/mrbgems/mruby-fiber/mrbgem.rake
new file mode 100644
index 000000000..cb258adcb
--- /dev/null
+++ b/mrbgems/mruby-fiber/mrbgem.rake
@@ -0,0 +1,4 @@
+MRuby::Gem::Specification.new('mruby-fiber') do |spec|
+  spec.license = 'MIT'
+  spec.authors = 'mruby developers'
+end
diff --git a/mrbgems/mruby-fiber/src/fiber.c b/mrbgems/mruby-fiber/src/fiber.c
new file mode 100644
index 000000000..01410e4b3
--- /dev/null
+++ b/mrbgems/mruby-fiber/src/fiber.c
@@ -0,0 +1,179 @@
+#include "mruby.h"
+#include "mruby/array.h"
+#include "mruby/class.h"
+#include "mruby/proc.h"
+
+#define FIBER_STACK_INIT_SIZE 64
+#define FIBER_CI_INIT_SIZE 8
+
+/*
+ *  call-seq:
+ *     Fiber.new{...} -> obj
+ *
+ *  Creates an fiber, whose execution is suspend until it explicitly
+ *  resumed using <code>Fibder#resume</code> method.  
+ *  <code>resume</code>. Arguments passed to resume will be the value of
+ *  the <code>Fiber.yield</code> expression or will be passed as block
+ *  parameters to the fiber's block if this is the first <code>resume</code>.
+ *
+ *  Alternatively, when resume is called it evaluates to the arguments passed
+ *  to the next <code>Fiber.yield</code> statement inside the fiber's block
+ *  or to the block value if it runs to completion without any
+ *  <code>Fiber.yield</code>
+ */
+static mrb_value
+fiber_init(mrb_state *mrb, mrb_value self)
+{
+  static const struct mrb_context mrb_context_zero = { 0 };
+  struct RFiber *f = (struct RFiber*)self.value.p;
+  struct mrb_context *c;
+  struct RProc *p;
+  mrb_callinfo *ci;
+  mrb_value blk;
+
+  mrb_get_args(mrb, "&", &blk);
+  
+  if (mrb_nil_p(blk)) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "tried to create Fiber object without a block");
+  }
+  p = mrb_proc_ptr(blk);
+  if (MRB_PROC_CFUNC_P(p)) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "tried to create Fiber from C defined method");
+  }
+
+  f->cxt = (struct mrb_context*)mrb_malloc(mrb, sizeof(struct mrb_context));
+  *f->cxt = mrb_context_zero;
+
+  /* initialize VM stack */
+  c = f->cxt;
+  c->stbase = (mrb_value *)mrb_calloc(mrb, FIBER_STACK_INIT_SIZE, sizeof(mrb_value));
+  c->stend = c->stbase + FIBER_STACK_INIT_SIZE;
+  c->stack = c->stbase;
+
+  /* copy receiver from a block */
+  c->stack[0] = mrb->c->stack[0];
+
+  /* initialize callinfo stack */
+  c->cibase = (mrb_callinfo *)mrb_calloc(mrb, FIBER_CI_INIT_SIZE, sizeof(mrb_callinfo));
+  c->ciend = c->cibase + FIBER_CI_INIT_SIZE;
+  c->ci = c->cibase;
+
+  /* adjust return callinfo */
+  ci = c->ci;
+  ci->target_class = p->target_class;
+  ci->proc = p;
+  ci->pc = p->body.irep->iseq;
+  ci->nregs = p->body.irep->nregs;
+  ci[1] = ci[0];
+  c->ci++;                      /* push dummy callinfo */
+
+  return self;
+}
+
+static struct mrb_context*
+fiber_check(mrb_state *mrb, mrb_value fib)
+{
+  struct RFiber *f = (struct RFiber*)fib.value.p;
+
+  if (!f->cxt) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "uninitialized Fiber");
+  }
+  return f->cxt;
+}
+
+static mrb_value
+fiber_result(mrb_state *mrb, mrb_value *a, int len)
+{
+  if (len == 0) return mrb_nil_value();
+  if (len == 1) return a[0];
+  return mrb_ary_new_from_values(mrb, len, a);
+}
+
+/*
+ *  call-seq:
+ *     fiber.resume(args, ...) -> obj
+ *
+ *  Resumes the fiber from the point at which the last <code>Fiber.yield</code>
+ *  was called, or starts running it if it is the first call to
+ *  <code>resume</code>. Arguments passed to resume will be the value of
+ *  the <code>Fiber.yield</code> expression or will be passed as block
+ *  parameters to the fiber's block if this is the first <code>resume</code>.
+ *
+ *  Alternatively, when resume is called it evaluates to the arguments passed
+ *  to the next <code>Fiber.yield</code> statement inside the fiber's block
+ *  or to the block value if it runs to completion without any
+ *  <code>Fiber.yield</code>
+ */
+static mrb_value
+fiber_resume(mrb_state *mrb, mrb_value self)
+{
+  struct mrb_context *c = fiber_check(mrb, self);
+  mrb_value *a;
+  int len;
+
+  mrb_get_args(mrb, "*", &a, &len);
+  if (!c->prev) {               /* first call */
+    mrb_value *b = c->stack+1;
+    mrb_value *e = b + len;
+
+    while (b<e) {
+      *b++ = *a++;
+    }
+    c->ci->argc = len;
+    c->prev = mrb->c;
+    mrb->c = c;
+
+    return c->ci->proc->env->stack[0];
+  }
+  if (c->ci == c->cibase) {
+    mrb_raise(mrb, E_RUNTIME_ERROR, "resuming dead Fiber");
+  }
+  c->prev = mrb->c;
+  mrb->c = c;
+  return fiber_result(mrb, a, len);
+}
+
+/*
+ *  call-seq:
+ *     Fiber.yield(args, ...) -> obj
+ *
+ *  Yields control back to the context that resumed the fiber, passing
+ *  along any arguments that were passed to it. The fiber will resume
+ *  processing at this point when <code>resume</code> is called next.
+ *  Any arguments passed to the next <code>resume</code> will be the
+ *  value that this <code>Fiber.yield</code> expression evaluates to.
+ */
+static mrb_value
+fiber_yield(mrb_state *mrb, mrb_value self)
+{
+  struct mrb_context *c = mrb->c;
+  mrb_value *a;
+  int len;
+
+  if (!c->prev) {
+    mrb_raise(mrb, E_ARGUMENT_ERROR, "can't yield from root Fiber");
+  }
+  mrb_get_args(mrb, "*", &a, &len);
+
+  mrb->c = c->prev;
+  return fiber_result(mrb, a, len);
+}
+
+void
+mrb_mruby_fiber_gem_init(mrb_state* mrb)
+{
+  struct RClass *c;
+
+  c = mrb_define_class(mrb, "Fiber", mrb->object_class);
+  MRB_SET_INSTANCE_TT(c, MRB_TT_FIBER);
+
+  mrb_define_method(mrb, c, "initialize", fiber_init, MRB_ARGS_NONE());
+  mrb_define_method(mrb, c, "resume", fiber_resume, MRB_ARGS_ANY());
+
+  mrb_define_class_method(mrb, c, "yield", fiber_yield, MRB_ARGS_ANY());
+}
+
+void
+mrb_mruby_fiber_gem_final(mrb_state* mrb)
+{
+}
diff --git a/mrbgems/mruby-struct/src/struct.c b/mrbgems/mruby-struct/src/struct.c
index 2775471fe..a2731ca54 100644
--- a/mrbgems/mruby-struct/src/struct.c
+++ b/mrbgems/mruby-struct/src/struct.c
@@ -132,7 +132,7 @@ mrb_struct_getmember(mrb_state *mrb, mrb_value obj, mrb_sym id)
 static mrb_value
 mrb_struct_ref(mrb_state *mrb, mrb_value obj)
 {
-  return mrb_struct_getmember(mrb, obj, mrb->ci->mid);
+  return mrb_struct_getmember(mrb, obj, mrb->c->ci->mid);
 }
 
 static mrb_value mrb_struct_ref0(mrb_state* mrb, mrb_value obj) {return RSTRUCT_PTR(obj)[0];}
@@ -191,7 +191,7 @@ mrb_struct_set(mrb_state *mrb, mrb_value obj, mrb_value val)
   mrb_value members, slot, *ptr, *ptr_members;
 
   /* get base id */
-  name = mrb_sym2name_len(mrb, mrb->ci->mid, &len);
+  name = mrb_sym2name_len(mrb, mrb->c->ci->mid, &len);
   mid = mrb_intern2(mrb, name, len-1); /* omit last "=" */
 
   members = mrb_struct_members(mrb, obj);
diff --git a/src/backtrace.c b/src/backtrace.c
index 3fadb9552..f6e3e2eb0 100644
--- a/src/backtrace.c
+++ b/src/backtrace.c
@@ -19,11 +19,11 @@ mrb_print_backtrace(mrb_state *mrb)
 
   printf("trace:\n");
   ciidx = mrb_fixnum(mrb_obj_iv_get(mrb, mrb->exc, mrb_intern(mrb, "ciidx")));
-  if (ciidx >= mrb->ciend - mrb->cibase)
+  if (ciidx >= mrb->c->ciend - mrb->c->cibase)
     ciidx = 10; /* ciidx is broken... */
 
   for (i = ciidx; i >= 0; i--) {
-    ci = &mrb->cibase[i];
+    ci = &mrb->c->cibase[i];
     filename = "(unknown)";
     line = -1;
 
@@ -38,7 +38,7 @@ mrb_print_backtrace(mrb_state *mrb)
         mrb_code *pc;
 
         if (i+1 <= ciidx) {
-          pc = mrb->cibase[i+1].pc;
+          pc = mrb->c->cibase[i+1].pc;
         }
         else {
           pc = (mrb_code*)mrb_voidp(mrb_obj_iv_get(mrb, mrb->exc, mrb_intern(mrb, "lastpc")));
diff --git a/src/class.c b/src/class.c
index e756cf562..4264129b6 100644
--- a/src/class.c
+++ b/src/class.c
@@ -391,14 +391,14 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
 {
   char c;
   int i = 0;
-  mrb_value *sp = mrb->stack + 1;
+  mrb_value *sp = mrb->c->stack + 1;
   va_list ap;
-  int argc = mrb->ci->argc;
+  int argc = mrb->c->ci->argc;
   int opt = 0;
 
   va_start(ap, format);
   if (argc < 0) {
-    struct RArray *a = mrb_ary_ptr(mrb->stack[1]);
+    struct RArray *a = mrb_ary_ptr(mrb->c->stack[1]);
 
     argc = a->len;
     sp = a->ptr;
@@ -620,11 +620,11 @@ mrb_get_args(mrb_state *mrb, const char *format, ...)
         mrb_value *p, *bp;
 
         p = va_arg(ap, mrb_value*);
-        if (mrb->ci->argc < 0) {
-          bp = mrb->stack + 2;
+        if (mrb->c->ci->argc < 0) {
+          bp = mrb->c->stack + 2;
         }
         else {
-          bp = mrb->stack + mrb->ci->argc + 1;
+          bp = mrb->c->stack + mrb->c->ci->argc + 1;
         }
         *p = *bp;
       }
diff --git a/src/error.c b/src/error.c
index 1ac42455c..bd3afd2e1 100644
--- a/src/error.c
+++ b/src/error.c
@@ -189,12 +189,12 @@ exc_equal(mrb_state *mrb, mrb_value exc)
 static void
 exc_debug_info(mrb_state *mrb, struct RObject *exc)
 {
-  mrb_callinfo *ci = mrb->ci;
+  mrb_callinfo *ci = mrb->c->ci;
   mrb_code *pc = ci->pc;
 
-  mrb_obj_iv_set(mrb, exc, mrb_intern2(mrb, "ciidx", 5), mrb_fixnum_value(ci - mrb->cibase));
+  mrb_obj_iv_set(mrb, exc, mrb_intern2(mrb, "ciidx", 5), mrb_fixnum_value(ci - mrb->c->cibase));
   ci--;
-  while (ci >= mrb->cibase) {
+  while (ci >= mrb->c->cibase) {
     if (ci->proc && !MRB_PROC_CFUNC_P(ci->proc)) {
       mrb_irep *irep = ci->proc->body.irep;
 
diff --git a/src/gc.c b/src/gc.c
index 2e5678f7d..4837d5207 100644
--- a/src/gc.c
+++ b/src/gc.c
@@ -387,6 +387,35 @@ add_gray_list(mrb_state *mrb, struct RBasic *obj)
 }
 
 static void
+mark_context(mrb_state *mrb, struct mrb_context *c)
+{
+  size_t i;
+  size_t e;
+  mrb_callinfo *ci;
+
+  /* mark stack */
+  e = c->stack - c->stbase;
+  if (c->ci) e += c->ci->nregs;
+  if (c->stbase + e > c->stend) e = c->stend - c->stbase;
+  for (i=0; i<e; i++) {
+    mrb_gc_mark_value(mrb, c->stbase[i]);
+  }
+  /* mark ensure stack */
+  e = (c->ci) ? c->ci->eidx : 0;
+  for (i=0; i<e; i++) {
+    mrb_gc_mark(mrb, (struct RBasic*)c->ensure[i]);
+  }
+  /* mark closure */
+  for (ci = c->cibase; ci <= c->ci; ci++) {
+    if (!ci) continue;
+    mrb_gc_mark(mrb, (struct RBasic*)ci->env);
+    mrb_gc_mark(mrb, (struct RBasic*)ci->proc);
+    mrb_gc_mark(mrb, (struct RBasic*)ci->target_class);
+  }
+  if (c->prev) mark_context(mrb, c->prev);
+}
+
+static void
 gc_mark_children(mrb_state *mrb, struct RBasic *obj)
 {
   gc_assert(is_gray(obj));
@@ -438,6 +467,15 @@ gc_mark_children(mrb_state *mrb, struct RBasic *obj)
     }
     break;
 
+  case MRB_TT_FIBER:
+    {
+      struct mrb_context *c = ((struct RFiber*)obj)->cxt;
+
+      mark_context(mrb, c);
+      ((struct RFiber*)obj)->cxt = NULL;
+    }
+    break;
+
   case MRB_TT_ARRAY:
     {
       struct RArray *a = (struct RArray*)obj;
@@ -517,6 +555,14 @@ obj_free(mrb_state *mrb, struct RBasic *obj)
     }
     break;
 
+  case MRB_TT_FIBER:
+    {
+      struct mrb_context *c = ((struct RFiber*)obj)->cxt;
+
+      mrb_free_context(mrb, c);
+    }
+    break;
+
   case MRB_TT_ARRAY:
     if (obj->flags & MRB_ARY_SHARED)
       mrb_ary_decref(mrb, ((struct RArray*)obj)->aux.shared);
@@ -557,9 +603,7 @@ static void
 root_scan_phase(mrb_state *mrb)
 {
   int j;
-  size_t i;
-  size_t e;
-  mrb_callinfo *ci;
+  size_t i, e;
 
   if (!is_minor_gc(mrb)) {
     mrb->gray_list = 0;
@@ -577,25 +621,8 @@ root_scan_phase(mrb_state *mrb)
   mrb_gc_mark(mrb, (struct RBasic*)mrb->top_self);
   /* mark exception */
   mrb_gc_mark(mrb, (struct RBasic*)mrb->exc);
-  /* mark stack */
-  e = mrb->stack - mrb->stbase;
-  if (mrb->ci) e += mrb->ci->nregs;
-  if (mrb->stbase + e > mrb->stend) e = mrb->stend - mrb->stbase;
-  for (i=0; i<e; i++) {
-    mrb_gc_mark_value(mrb, mrb->stbase[i]);
-  }
-  /* mark ensure stack */
-  e = (mrb->ci) ? mrb->ci->eidx : 0;
-  for (i=0; i<e; i++) {
-    mrb_gc_mark(mrb, (struct RBasic*)mrb->ensure[i]);
-  }
-  /* mark closure */
-  for (ci = mrb->cibase; ci <= mrb->ci; ci++) {
-    if (!ci) continue;
-    mrb_gc_mark(mrb, (struct RBasic*)ci->env);
-    mrb_gc_mark(mrb, (struct RBasic*)ci->proc);
-    mrb_gc_mark(mrb, (struct RBasic*)ci->target_class);
-  }
+
+  mark_context(mrb, mrb->c);
   /* mark irep pool */
   if (mrb->irep) {
     size_t len = mrb->irep_len;
@@ -643,6 +670,30 @@ gc_gray_mark(mrb_state *mrb, struct RBasic *obj)
     children += (int)obj->flags;
     break;
 
+  case MRB_TT_FIBER:
+    {
+      struct mrb_context *c = ((struct RFiber*)obj)->cxt;
+      size_t i;
+      mrb_callinfo *ci;
+
+      /* mark stack */
+      i = c->stack - c->stbase;
+      if (c->ci) i += c->ci->nregs;
+      if (c->stbase + i > c->stend) i = c->stend - c->stbase;
+      children += i;
+
+      /* mark ensure stack */
+      children += (c->ci) ? c->ci->eidx : 0;
+
+      /* mark closure */
+      if (c->cibase) {
+        for (i=0, ci = c->cibase; ci <= c->ci; i++, ci++)
+          ;
+      }
+      children += i;
+    }
+    break;
+
   case MRB_TT_ARRAY:
     {
       struct RArray *a = (struct RArray*)obj;
diff --git a/src/kernel.c b/src/kernel.c
index 033ce283a..af4806378 100644
--- a/src/kernel.c
+++ b/src/kernel.c
@@ -214,19 +214,19 @@ mrb_f_send(mrb_state *mrb, mrb_value self)
 static mrb_value
 mrb_f_block_given_p_m(mrb_state *mrb, mrb_value self)
 {
-  mrb_callinfo *ci = mrb->ci;
+  mrb_callinfo *ci = mrb->c->ci;
   mrb_value *bp;
   mrb_bool given_p;
 
-  bp = mrb->stbase + ci->stackidx + 1;
+  bp = mrb->c->stbase + ci->stackidx + 1;
   ci--;
-  if (ci <= mrb->cibase) {
+  if (ci <= mrb->c->cibase) {
     given_p = 0;
   }
   else {
     /* block_given? called within block; check upper scope */
     if (ci->proc->env && ci->proc->env->stack) {
-      given_p = !(ci->proc->env->stack == mrb->stbase ||
+      given_p = !(ci->proc->env->stack == mrb->c->stbase ||
                   mrb_nil_p(ci->proc->env->stack[1]));
     }
     else {
@@ -894,7 +894,7 @@ mrb_f_raise(mrb_state *mrb, mrb_value self)
     /* fall through */
   default:
     exc = mrb_make_exception(mrb, argc, a);
-    mrb_obj_iv_set(mrb, mrb_obj_ptr(exc), mrb_intern2(mrb, "lastpc", 6), mrb_voidp_value(mrb->ci->pc));
+    mrb_obj_iv_set(mrb, mrb_obj_ptr(exc), mrb_intern2(mrb, "lastpc", 6), mrb_voidp_value(mrb->c->ci->pc));
     mrb_exc_raise(mrb, exc);
     break;
   }
diff --git a/src/proc.c b/src/proc.c
index ce2728779..e21a37736 100644
--- a/src/proc.c
+++ b/src/proc.c
@@ -19,7 +19,7 @@ mrb_proc_new(mrb_state *mrb, mrb_irep *irep)
   struct RProc *p;
 
   p = (struct RProc*)mrb_obj_alloc(mrb, MRB_TT_PROC, mrb->proc_class);
-  p->target_class = (mrb->ci) ? mrb->ci->target_class : 0;
+  p->target_class = (mrb->c->ci) ? mrb->c->ci->target_class : 0;
   p->body.irep = irep;
   p->env = 0;
 
@@ -31,16 +31,16 @@ closure_setup(mrb_state *mrb, struct RProc *p, int nlocals)
 {
   struct REnv *e;
 
-  if (!mrb->ci->env) {
-    e = (struct REnv*)mrb_obj_alloc(mrb, MRB_TT_ENV, (struct RClass*)mrb->ci->proc->env);
+  if (!mrb->c->ci->env) {
+    e = (struct REnv*)mrb_obj_alloc(mrb, MRB_TT_ENV, (struct RClass*)mrb->c->ci->proc->env);
     e->flags= (unsigned int)nlocals;
-    e->mid = mrb->ci->mid;
-    e->cioff = mrb->ci - mrb->cibase;
-    e->stack = mrb->stack;
-    mrb->ci->env = e;
+    e->mid = mrb->c->ci->mid;
+    e->cioff = mrb->c->ci - mrb->c->cibase;
+    e->stack = mrb->c->stack;
+    mrb->c->ci->env = e;
   }
   else {
-    e = mrb->ci->env;
+    e = mrb->c->ci->env;
   }
   p->env = e;
 }
@@ -50,7 +50,7 @@ mrb_closure_new(mrb_state *mrb, mrb_irep *irep)
 {
   struct RProc *p = mrb_proc_new(mrb, irep);
 
-  closure_setup(mrb, p, mrb->ci->proc->body.irep->nlocals);
+  closure_setup(mrb, p, mrb->c->ci->proc->body.irep->nlocals);
   return p;
 }
 
diff --git a/src/state.c b/src/state.c
index b34fbd025..76b506d3e 100644
--- a/src/state.c
+++ b/src/state.c
@@ -25,6 +25,7 @@ mrb_state*
 mrb_open_allocf(mrb_allocf f, void *ud)
 {
   static const mrb_state mrb_state_zero = { 0 };
+  static const struct mrb_context mrb_context_zero = { 0 };
   mrb_state *mrb = (mrb_state *)(f)(NULL, NULL, sizeof(mrb_state), ud);
   if (mrb == NULL) return NULL;
 
@@ -34,7 +35,11 @@ mrb_open_allocf(mrb_allocf f, void *ud)
   mrb->current_white_part = MRB_GC_WHITE_A;
 
   mrb_init_heap(mrb);
+  mrb->c = (struct mrb_context*)mrb_malloc(mrb, sizeof(struct mrb_context));
+  *mrb->c = mrb_context_zero;
+  mrb->root_c = mrb->c;
   mrb_init_core(mrb);
+
   return mrb;
 }
 
@@ -107,6 +112,17 @@ mrb_irep_free(mrb_state *mrb, struct mrb_irep *irep)
 }
 
 void
+mrb_free_context(mrb_state *mrb, struct mrb_context *c)
+{
+  if (!c) return;
+  mrb_free(mrb, c->stbase);
+  mrb_free(mrb, c->cibase);
+  mrb_free(mrb, c->rescue);
+  mrb_free(mrb, c->ensure);
+  mrb_free(mrb, c);
+}
+
+void
 mrb_close(mrb_state *mrb)
 {
   size_t i;
@@ -115,14 +131,11 @@ mrb_close(mrb_state *mrb)
 
   /* free */
   mrb_gc_free_gv(mrb);
-  mrb_free(mrb, mrb->stbase);
-  mrb_free(mrb, mrb->cibase);
   for (i=0; i<mrb->irep_len; i++) {
     mrb_irep_free(mrb, mrb->irep[i]);
   }
   mrb_free(mrb, mrb->irep);
-  mrb_free(mrb, mrb->rescue);
-  mrb_free(mrb, mrb->ensure);
+  mrb_free_context(mrb, mrb->root_c);
   mrb_free_symtbl(mrb);
   mrb_free_heap(mrb);
   mrb_alloca_free(mrb);
diff --git a/src/variable.c b/src/variable.c
index 9ffc2ac92..2610c5d7a 100644
--- a/src/variable.c
+++ b/src/variable.c
@@ -579,14 +579,14 @@ mrb_value
 mrb_vm_iv_get(mrb_state *mrb, mrb_sym sym)
 {
   /* get self */
-  return mrb_iv_get(mrb, mrb->stack[0], sym);
+  return mrb_iv_get(mrb, mrb->c->stack[0], sym);
 }
 
 void
 mrb_vm_iv_set(mrb_state *mrb, mrb_sym sym, mrb_value v)
 {
   /* get self */
-  mrb_iv_set(mrb, mrb->stack[0], sym, v);
+  mrb_iv_set(mrb, mrb->c->stack[0], sym, v);
 }
 
 static int
@@ -763,9 +763,9 @@ mrb_cv_defined(mrb_state *mrb, mrb_value mod, mrb_sym sym)
 mrb_value
 mrb_vm_cv_get(mrb_state *mrb, mrb_sym sym)
 {
-  struct RClass *c = mrb->ci->proc->target_class;
+  struct RClass *c = mrb->c->ci->proc->target_class;
 
-  if (!c) c = mrb->ci->target_class;
+  if (!c) c = mrb->c->ci->target_class;
 
   return mrb_mod_cv_get(mrb, c, sym);
 }
@@ -773,9 +773,9 @@ mrb_vm_cv_get(mrb_state *mrb, mrb_sym sym)
 void
 mrb_vm_cv_set(mrb_state *mrb, mrb_sym sym, mrb_value v)
 {
-  struct RClass *c = mrb->ci->proc->target_class;
+  struct RClass *c = mrb->c->ci->proc->target_class;
 
-  if (!c) c = mrb->ci->target_class;
+  if (!c) c = mrb->c->ci->target_class;
   while (c) {
     if (c->iv) {
       iv_tbl *t = c->iv;
@@ -788,7 +788,7 @@ mrb_vm_cv_set(mrb_state *mrb, mrb_sym sym, mrb_value v)
     }
     c = c->super;
   }
-  c = mrb->ci->target_class;
+  c = mrb->c->ci->target_class;
   if (!c->iv) {
     c->iv = iv_new(mrb);
   }
@@ -868,9 +868,9 @@ mrb_const_get(mrb_state *mrb, mrb_value mod, mrb_sym sym)
 mrb_value
 mrb_vm_const_get(mrb_state *mrb, mrb_sym sym)
 {
-  struct RClass *c = mrb->ci->proc->target_class;
+  struct RClass *c = mrb->c->ci->proc->target_class;
 
-  if (!c) c = mrb->ci->target_class;
+  if (!c) c = mrb->c->ci->target_class;
   if (c) {
     struct RClass *c2;
     mrb_value v;
@@ -900,9 +900,9 @@ mrb_const_set(mrb_state *mrb, mrb_value mod, mrb_sym sym, mrb_value v)
  void
 mrb_vm_const_set(mrb_state *mrb, mrb_sym sym, mrb_value v)
 {
-  struct RClass *c = mrb->ci->proc->target_class;
+  struct RClass *c = mrb->c->ci->proc->target_class;
 
-  if (!c) c = mrb->ci->target_class;
+  if (!c) c = mrb->c->ci->target_class;
   mrb_obj_iv_set(mrb, (struct RObject*)c, sym, v);
 }
 
diff --git a/src/vm.c b/src/vm.c
index 4794323c7..31fdfe816 100644
--- a/src/vm.c
+++ b/src/vm.c
@@ -88,24 +88,26 @@ stack_copy(mrb_value *dst, const mrb_value *src, size_t size)
 static void
 stack_init(mrb_state *mrb)
 {
+  struct mrb_context *c = mrb->c;
+
   /* assert(mrb->stack == NULL); */
-  mrb->stbase = (mrb_value *)mrb_calloc(mrb, STACK_INIT_SIZE,   sizeof(mrb_value));
-  mrb->stend = mrb->stbase + STACK_INIT_SIZE;
-  mrb->stack = mrb->stbase;
-
-  /* assert(mrb->ci == NULL); */
-  mrb->cibase = (mrb_callinfo *)mrb_calloc(mrb, CALLINFO_INIT_SIZE, sizeof(mrb_callinfo));
-  mrb->ciend = mrb->cibase + CALLINFO_INIT_SIZE;
-  mrb->ci = mrb->cibase;
-  mrb->ci->target_class = mrb->object_class;
+  c->stbase = (mrb_value *)mrb_calloc(mrb, STACK_INIT_SIZE, sizeof(mrb_value));
+  c->stend = c->stbase + STACK_INIT_SIZE;
+  c->stack = c->stbase;
+
+  /* assert(ci == NULL); */
+  c->cibase = (mrb_callinfo *)mrb_calloc(mrb, CALLINFO_INIT_SIZE, sizeof(mrb_callinfo));
+  c->ciend = c->cibase + CALLINFO_INIT_SIZE;
+  c->ci = c->cibase;
+  c->ci->target_class = mrb->object_class;
 }
 
 static inline void
 envadjust(mrb_state *mrb, mrb_value *oldbase, mrb_value *newbase)
 {
-  mrb_callinfo *ci = mrb->cibase;
+  mrb_callinfo *ci = mrb->c->cibase;
 
-  while (ci <= mrb->ci) {
+  while (ci <= mrb->c->ci) {
     struct REnv *e = ci->env;
     if (e && e->cioff >= 0) {
       ptrdiff_t off = e->stack - oldbase;
@@ -121,13 +123,13 @@ envadjust(mrb_state *mrb, mrb_value *oldbase, mrb_value *newbase)
 static void
 stack_extend(mrb_state *mrb, int room, int keep)
 {
-  if (mrb->stack + room >= mrb->stend) {
+  if (mrb->c->stack + room >= mrb->c->stend) {
     int size, off;
 
-    mrb_value *oldbase = mrb->stbase;
+    mrb_value *oldbase = mrb->c->stbase;
 
-    size = mrb->stend - mrb->stbase;
-    off = mrb->stack - mrb->stbase;
+    size = mrb->c->stend - mrb->c->stbase;
+    off = mrb->c->stack - mrb->c->stbase;
 
     /* do not leave uninitialized malloc region */
     if (keep > size) keep = size;
@@ -140,10 +142,10 @@ stack_extend(mrb_state *mrb, int room, int keep)
     else
       size += room;
 
-    mrb->stbase = (mrb_value *)mrb_realloc(mrb, mrb->stbase, sizeof(mrb_value) * size);
-    mrb->stack = mrb->stbase + off;
-    mrb->stend = mrb->stbase + size;
-    envadjust(mrb, oldbase, mrb->stbase);
+    mrb->c->stbase = (mrb_value *)mrb_realloc(mrb, mrb->c->stbase, sizeof(mrb_value) * size);
+    mrb->c->stack = mrb->c->stbase + off;
+    mrb->c->stend = mrb->c->stbase + size;
+    envadjust(mrb, oldbase, mrb->c->stbase);
     /* Raise an exception if the new stack size will be too large,
     to prevent infinite recursion. However, do this only after resizing the stack, so mrb_raise has stack space to work with. */
     if (size > MRB_STACK_MAX) {
@@ -153,7 +155,7 @@ stack_extend(mrb_state *mrb, int room, int keep)
 
   if (room > keep) {
 #ifndef MRB_NAN_BOXING
-    stack_clear(&(mrb->stack[keep]), room - keep);
+    stack_clear(&(mrb->c->stack[keep]), room - keep);
 #else
     int i;
     for (i=keep; i<room; i++) {
@@ -166,7 +168,7 @@ stack_extend(mrb_state *mrb, int room, int keep)
 static inline struct REnv*
 uvenv(mrb_state *mrb, int up)
 {
-  struct REnv *e = mrb->ci->proc->env;
+  struct REnv *e = mrb->c->ci->proc->env;
 
   while (up--) {
     if (!e) return 0;
@@ -180,8 +182,8 @@ is_strict(mrb_state *mrb, struct REnv *e)
 {
   int cioff = e->cioff;
 
-  if (cioff >= 0 && mrb->cibase[cioff].proc &&
-      MRB_PROC_STRICT_P(mrb->cibase[cioff].proc)) {
+  if (cioff >= 0 && mrb->c->cibase[cioff].proc &&
+      MRB_PROC_STRICT_P(mrb->c->cibase[cioff].proc)) {
     return 1;
   }
   return 0;
@@ -203,29 +205,34 @@ top_env(mrb_state *mrb, struct RProc *proc)
 static mrb_callinfo*
 cipush(mrb_state *mrb)
 {
-  int eidx = mrb->ci->eidx;
-  int ridx = mrb->ci->ridx;
+  struct mrb_context *c = mrb->c;
+  mrb_callinfo *ci = c->ci;
+
+  int eidx = ci->eidx;
+  int ridx = ci->ridx;
 
-  if (mrb->ci + 1 == mrb->ciend) {
-    size_t size = mrb->ci - mrb->cibase;
+  if (ci + 1 == c->ciend) {
+    size_t size = ci - c->cibase;
 
-    mrb->cibase = (mrb_callinfo *)mrb_realloc(mrb, mrb->cibase, sizeof(mrb_callinfo)*size*2);
-    mrb->ci = mrb->cibase + size;
-    mrb->ciend = mrb->cibase + size * 2;
+    c->cibase = (mrb_callinfo *)mrb_realloc(mrb, c->cibase, sizeof(mrb_callinfo)*size*2);
+    ci = c->cibase + size;
+    c->ciend = c->cibase + size * 2;
   }
-  mrb->ci++;
-  mrb->ci->nregs = 2;   /* protect method_missing arg and block */
-  mrb->ci->eidx = eidx;
-  mrb->ci->ridx = ridx;
-  mrb->ci->env = 0;
-  return mrb->ci;
+  ci = ++c->ci;
+  ci->nregs = 2;   /* protect method_missing arg and block */
+  ci->eidx = eidx;
+  ci->ridx = ridx;
+  ci->env = 0;
+  return ci;
 }
 
 static void
 cipop(mrb_state *mrb)
 {
-  if (mrb->ci->env) {
-    struct REnv *e = mrb->ci->env;
+  struct mrb_context *c = mrb->c;
+
+  if (c->ci->env) {
+    struct REnv *e = c->ci->env;
     size_t len = (size_t)e->flags;
     mrb_value *p = (mrb_value *)mrb_malloc(mrb, sizeof(mrb_value)*len);
 
@@ -234,7 +241,7 @@ cipop(mrb_state *mrb)
     e->stack = p;
   }
 
-  mrb->ci--;
+  c->ci--;
 }
 
 static void
@@ -242,23 +249,23 @@ ecall(mrb_state *mrb, int i)
 {
   struct RProc *p;
   mrb_callinfo *ci;
-  mrb_value *self = mrb->stack;
+  mrb_value *self = mrb->c->stack;
   struct RObject *exc;
 
-  p = mrb->ensure[i];
+  p = mrb->c->ensure[i];
   if (!p) return;
   ci = cipush(mrb);
-  ci->stackidx = mrb->stack - mrb->stbase;
+  ci->stackidx = mrb->c->stack - mrb->c->stbase;
   ci->mid = ci[-1].mid;
   ci->acc = -1;
   ci->argc = 0;
   ci->proc = p;
   ci->nregs = p->body.irep->nregs;
   ci->target_class = p->target_class;
-  mrb->stack = mrb->stack + ci[-1].nregs;
+  mrb->c->stack = mrb->c->stack + ci[-1].nregs;
   exc = mrb->exc; mrb->exc = 0;
   mrb_run(mrb, p, *self);
-  mrb->ensure[i] = NULL;
+  mrb->c->ensure[i] = NULL;
   if (!mrb->exc) mrb->exc = exc;
 }
 
@@ -308,11 +315,11 @@ mrb_funcall_with_block(mrb_state *mrb, mrb_value self, mrb_sym mid, int argc, mr
 
   if (!mrb->jmp) {
     jmp_buf c_jmp;
-    mrb_callinfo *old_ci = mrb->ci;
+    mrb_callinfo *old_ci = mrb->c->ci;
 
     if (setjmp(c_jmp) != 0) { /* error */
-      while (old_ci != mrb->ci) {
-        mrb->stack = mrb->stbase + mrb->ci->stackidx;
+      while (old_ci != mrb->c->ci) {
+        mrb->c->stack = mrb->c->stbase + mrb->c->ci->stackidx;
         cipop(mrb);
       }
       mrb->jmp = 0;
@@ -332,10 +339,10 @@ mrb_funcall_with_block(mrb_state *mrb, mrb_value self, mrb_sym mid, int argc, mr
     mrb_callinfo *ci;
     int n;
 
-    if (!mrb->stack) {
+    if (!mrb->c->stack) {
       stack_init(mrb);
     }
-    n = mrb->ci->nregs;
+    n = mrb->c->ci->nregs;
     if (argc < 0) {
       mrb_raisef(mrb, E_ARGUMENT_ERROR, "negative argc for funcall (%S)", mrb_fixnum_value(argc));
     }
@@ -350,7 +357,7 @@ mrb_funcall_with_block(mrb_state *mrb, mrb_value self, mrb_sym mid, int argc, mr
     ci = cipush(mrb);
     ci->mid = mid;
     ci->proc = p;
-    ci->stackidx = mrb->stack - mrb->stbase;
+    ci->stackidx = mrb->c->stack - mrb->c->stbase;
     ci->argc = argc;
     ci->target_class = p->target_class;
     if (MRB_PROC_CFUNC_P(p)) {
@@ -360,25 +367,25 @@ mrb_funcall_with_block(mrb_state *mrb, mrb_value self, mrb_sym mid, int argc, mr
       ci->nregs = p->body.irep->nregs + n;
     }
     ci->acc = -1;
-    mrb->stack = mrb->stack + n;
+    mrb->c->stack = mrb->c->stack + n;
 
     stack_extend(mrb, ci->nregs, 0);
-    mrb->stack[0] = self;
+    mrb->c->stack[0] = self;
     if (undef) {
-      mrb->stack[1] = mrb_symbol_value(undef);
-      stack_copy(mrb->stack+2, argv, argc-1);
+      mrb->c->stack[1] = mrb_symbol_value(undef);
+      stack_copy(mrb->c->stack+2, argv, argc-1);
     }
     else if (argc > 0) {
-      stack_copy(mrb->stack+1, argv, argc);
+      stack_copy(mrb->c->stack+1, argv, argc);
     }
-    mrb->stack[argc+1] = blk;
+    mrb->c->stack[argc+1] = blk;
 
     if (MRB_PROC_CFUNC_P(p)) {
       int ai = mrb_gc_arena_save(mrb);
       val = p->body.func(mrb, self);
       mrb_gc_arena_restore(mrb, ai);
       mrb_gc_protect(mrb, val);
-      mrb->stack = mrb->stbase + mrb->ci->stackidx;
+      mrb->c->stack = mrb->c->stbase + mrb->c->ci->stackidx;
       cipop(mrb);
     }
     else {
@@ -398,9 +405,9 @@ mrb_value
 mrb_yield_internal(mrb_state *mrb, mrb_value b, int argc, mrb_value *argv, mrb_value self, struct RClass *c)
 {
   struct RProc *p;
-  mrb_sym mid = mrb->ci->mid;
+  mrb_sym mid = mrb->c->ci->mid;
   mrb_callinfo *ci;
-  int n = mrb->ci->nregs;
+  int n = mrb->c->ci->nregs;
   mrb_value val;
 
   if (mrb_nil_p(b)) {
@@ -410,7 +417,7 @@ mrb_yield_internal(mrb_state *mrb, mrb_value b, int argc, mrb_value *argv, mrb_v
   ci = cipush(mrb);
   ci->mid = mid;
   ci->proc = p;
-  ci->stackidx = mrb->stack - mrb->stbase;
+  ci->stackidx = mrb->c->stack - mrb->c->stbase;
   ci->argc = argc;
   ci->target_class = c;
   if (MRB_PROC_CFUNC_P(p)) {
@@ -420,18 +427,18 @@ mrb_yield_internal(mrb_state *mrb, mrb_value b, int argc, mrb_value *argv, mrb_v
     ci->nregs = p->body.irep->nregs + 1;
   }
   ci->acc = -1;
-  mrb->stack = mrb->stack + n;
+  mrb->c->stack = mrb->c->stack + n;
 
   stack_extend(mrb, ci->nregs, 0);
-  mrb->stack[0] = self;
+  mrb->c->stack[0] = self;
   if (argc > 0) {
-    stack_copy(mrb->stack+1, argv, argc);
+    stack_copy(mrb->c->stack+1, argv, argc);
   }
-  mrb->stack[argc+1] = mrb_nil_value();
+  mrb->c->stack[argc+1] = mrb_nil_value();
 
   if (MRB_PROC_CFUNC_P(p)) {
     val = p->body.func(mrb, self);
-    mrb->stack = mrb->stbase + mrb->ci->stackidx;
+    mrb->c->stack = mrb->c->stbase + mrb->c->ci->stackidx;
     cipop(mrb);
   }
   else {
@@ -445,7 +452,7 @@ mrb_yield_argv(mrb_state *mrb, mrb_value b, int argc, mrb_value *argv)
 {
   struct RProc *p = mrb_proc_ptr(b);
 
-  return mrb_yield_internal(mrb, b, argc, argv, mrb->stack[0], p->target_class);
+  return mrb_yield_internal(mrb, b, argc, argv, mrb->c->stack[0], p->target_class);
 }
 
 mrb_value
@@ -453,7 +460,7 @@ mrb_yield(mrb_state *mrb, mrb_value b, mrb_value arg)
 {
   struct RProc *p = mrb_proc_ptr(b);
 
-  return mrb_yield_internal(mrb, b, 1, &arg, mrb->stack[0], p->target_class);
+  return mrb_yield_internal(mrb, b, 1, &arg, mrb->c->stack[0], p->target_class);
 }
 
 typedef enum {
@@ -484,14 +491,14 @@ argnum_error(mrb_state *mrb, int num)
   mrb_value exc;
   mrb_value str;
 
-  if (mrb->ci->mid) {
+  if (mrb->c->ci->mid) {
     str = mrb_format(mrb, "'%S': wrong number of arguments (%S for %S)",
-                  mrb_sym2str(mrb, mrb->ci->mid),
-                  mrb_fixnum_value(mrb->ci->argc), mrb_fixnum_value(num));
+                  mrb_sym2str(mrb, mrb->c->ci->mid),
+                  mrb_fixnum_value(mrb->c->ci->argc), mrb_fixnum_value(num));
   }
   else {
     str = mrb_format(mrb, "wrong number of arguments (%S for %S)",
-                  mrb_fixnum_value(mrb->ci->argc), mrb_fixnum_value(num));
+                  mrb_fixnum_value(mrb->c->ci->argc), mrb_fixnum_value(num));
   }
   exc = mrb_exc_new3(mrb, E_ARGUMENT_ERROR, str);
   mrb->exc = mrb_obj_ptr(exc);
@@ -577,13 +584,13 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
   else {
     goto L_RAISE;
   }
-  if (!mrb->stack) {
+  if (!mrb->c->stack) {
     stack_init(mrb);
   }
   stack_extend(mrb, irep->nregs, irep->nregs);
-  mrb->ci->proc = proc;
-  mrb->ci->nregs = irep->nregs + 1;
-  regs = mrb->stack;
+  mrb->c->ci->proc = proc;
+  mrb->c->ci->nregs = irep->nregs + 1;
+  regs = mrb->c->stack;
   regs[0] = self;
 
   INIT_DISPATCH {
@@ -769,12 +776,12 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
 
     CASE(OP_ONERR) {
       /* sBx    pc+=sBx on exception */
-      if (mrb->rsize <= mrb->ci->ridx) {
-        if (mrb->rsize == 0) mrb->rsize = 16;
-        else mrb->rsize *= 2;
-        mrb->rescue = (mrb_code **)mrb_realloc(mrb, mrb->rescue, sizeof(mrb_code*) * mrb->rsize);
+      if (mrb->c->rsize <= mrb->c->ci->ridx) {
+        if (mrb->c->rsize == 0) mrb->c->rsize = 16;
+        else mrb->c->rsize *= 2;
+        mrb->c->rescue = (mrb_code **)mrb_realloc(mrb, mrb->c->rescue, sizeof(mrb_code*) * mrb->c->rsize);
       }
-      mrb->rescue[mrb->ci->ridx++] = pc + GETARG_sBx(i);
+      mrb->c->rescue[mrb->c->ci->ridx++] = pc + GETARG_sBx(i);
       NEXT;
     }
 
@@ -789,7 +796,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       int a = GETARG_A(i);
 
       while (a--) {
-        mrb->ci->ridx--;
+        mrb->c->ci->ridx--;
       }
       NEXT;
     }
@@ -806,12 +813,12 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
 
       p = mrb_closure_new(mrb, mrb->irep[irep->idx+GETARG_Bx(i)]);
       /* push ensure_stack */
-      if (mrb->esize <= mrb->ci->eidx) {
-        if (mrb->esize == 0) mrb->esize = 16;
-        else mrb->esize *= 2;
-        mrb->ensure = (struct RProc **)mrb_realloc(mrb, mrb->ensure, sizeof(struct RProc*) * mrb->esize);
+      if (mrb->c->esize <= mrb->c->ci->eidx) {
+        if (mrb->c->esize == 0) mrb->c->esize = 16;
+        else mrb->c->esize *= 2;
+        mrb->c->ensure = (struct RProc **)mrb_realloc(mrb, mrb->c->ensure, sizeof(struct RProc*) * mrb->c->esize);
       }
-      mrb->ensure[mrb->ci->eidx++] = p;
+      mrb->c->ensure[mrb->c->ci->eidx++] = p;
       mrb_gc_arena_restore(mrb, ai);
       NEXT;
     }
@@ -822,7 +829,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       int a = GETARG_A(i);
 
       for (n=0; n<a; n++) {
-        ecall(mrb, --mrb->ci->eidx);
+        ecall(mrb, --mrb->c->ci->eidx);
       }
       mrb_gc_arena_restore(mrb, ai);
       NEXT;
@@ -880,7 +887,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       ci = cipush(mrb);
       ci->mid = mid;
       ci->proc = m;
-      ci->stackidx = mrb->stack - mrb->stbase;
+      ci->stackidx = mrb->c->stack - mrb->c->stbase;
       if (n == CALL_MAXARGS) {
         ci->argc = -1;
       }
@@ -898,7 +905,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       ci->acc = a;
 
       /* prepare stack */
-      mrb->stack += a;
+      mrb->c->stack += a;
 
       if (MRB_PROC_CFUNC_P(m)) {
         if (n == CALL_MAXARGS) {
@@ -908,17 +915,24 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
           ci->nregs = n + 2;
         }
         result = m->body.func(mrb, recv);
-        mrb->stack[0] = result;
+        mrb->c->stack[0] = result;
         mrb_gc_arena_restore(mrb, ai);
         if (mrb->exc) goto L_RAISE;
         /* pop stackpos */
-        regs = mrb->stack = mrb->stbase + mrb->ci->stackidx;
+        ci = mrb->c->ci;
+        if (!MRB_PROC_CFUNC_P(ci[-1].proc)) {
+          irep = ci[-1].proc->body.irep;
+          pool = irep->pool;
+          syms = irep->syms;
+        }
+        regs = mrb->c->stack = mrb->c->stbase + ci->stackidx;
+        pc = ci->pc;
         cipop(mrb);
-        NEXT;
+        JUMP;
       }
       else {
         /* setup environment for calling method */
-        proc = mrb->ci->proc = m;
+        proc = mrb->c->ci->proc = m;
         irep = m->body.irep;
         pool = irep->pool;
         syms = irep->syms;
@@ -929,7 +943,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         else {
           stack_extend(mrb, irep->nregs,  ci->argc+2);
         }
-        regs = mrb->stack;
+        regs = mrb->c->stack;
         pc = irep->iseq;
         JUMP;
       }
@@ -943,11 +957,11 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
     CASE(OP_CALL) {
       /* A      R(A) := self.call(frame.argc, frame.argv) */
       mrb_callinfo *ci;
-      mrb_value recv = mrb->stack[0];
+      mrb_value recv = mrb->c->stack[0];
       struct RProc *m = mrb_proc_ptr(recv);
 
       /* replace callinfo */
-      ci = mrb->ci;
+      ci = mrb->c->ci;
       ci->target_class = m->target_class;
       ci->proc = m;
       if (m->env) {
@@ -955,7 +969,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
           ci->mid = m->env->mid;
         }
         if (!m->env->stack) {
-          m->env->stack = mrb->stack;
+          m->env->stack = mrb->c->stack;
         }
       }
 
@@ -965,12 +979,12 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         mrb_gc_arena_restore(mrb, ai);
         if (mrb->exc) goto L_RAISE;
         /* pop stackpos */
-        ci = mrb->ci;
-        regs = mrb->stack = mrb->stbase + ci->stackidx;
+        ci = mrb->c->ci;
+        regs = mrb->c->stack = mrb->c->stbase + ci->stackidx;
         regs[ci->acc] = recv;
         pc = ci->pc;
         cipop(mrb);
-        irep = mrb->ci->proc->body.irep;
+        irep = mrb->c->ci->proc->body.irep;
         pool = irep->pool;
         syms = irep->syms;
         JUMP;
@@ -980,7 +994,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         proc = m;
         irep = m->body.irep;
         if (!irep) {
-          mrb->stack[0] = mrb_nil_value();
+          mrb->c->stack[0] = mrb_nil_value();
           goto L_RETURN;
         }
         pool = irep->pool;
@@ -992,7 +1006,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         else {
           stack_extend(mrb, irep->nregs,  ci->argc+2);
         }
-        regs = mrb->stack;
+        regs = mrb->c->stack;
         regs[0] = m->env->stack[0];
         pc = m->body.irep->iseq;
         JUMP;
@@ -1002,7 +1016,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
     CASE(OP_SUPER) {
       /* A B C  R(A) := super(R(A+1),... ,R(A+C-1)) */
       mrb_value recv;
-      mrb_callinfo *ci = mrb->ci;
+      mrb_callinfo *ci = mrb->c->ci;
       struct RProc *m;
       struct RClass *c;
       mrb_sym mid = ci->mid;
@@ -1010,7 +1024,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       int n = GETARG_C(i);
 
       recv = regs[0];
-      c = mrb->ci->target_class->super;
+      c = mrb->c->ci->target_class->super;
       m = mrb_method_search_vm(mrb, &c, mid);
       if (!m) {
         mid = mrb_intern2(mrb, "method_missing", 14);
@@ -1028,7 +1042,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       ci = cipush(mrb);
       ci->mid = mid;
       ci->proc = m;
-      ci->stackidx = mrb->stack - mrb->stbase;
+      ci->stackidx = mrb->c->stack - mrb->c->stbase;
       if (n == CALL_MAXARGS) {
         ci->argc = -1;
       }
@@ -1039,15 +1053,15 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       ci->pc = pc + 1;
 
       /* prepare stack */
-      mrb->stack += a;
-      mrb->stack[0] = recv;
+      mrb->c->stack += a;
+      mrb->c->stack[0] = recv;
 
       if (MRB_PROC_CFUNC_P(m)) {
-        mrb->stack[0] = m->body.func(mrb, recv);
+        mrb->c->stack[0] = m->body.func(mrb, recv);
         mrb_gc_arena_restore(mrb, ai);
         if (mrb->exc) goto L_RAISE;
         /* pop stackpos */
-        regs = mrb->stack = mrb->stbase + mrb->ci->stackidx;
+        regs = mrb->c->stack = mrb->c->stbase + mrb->c->ci->stackidx;
         cipop(mrb);
         NEXT;
       }
@@ -1067,7 +1081,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         else {
           stack_extend(mrb, irep->nregs,  ci->argc+2);
         }
-        regs = mrb->stack;
+        regs = mrb->c->stack;
         pc = irep->iseq;
         JUMP;
       }
@@ -1138,7 +1152,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       int kd = (ax>>1)&0x1;
       int b  = (ax>>0)& 0x1;
       */
-      int argc = mrb->ci->argc;
+      int argc = mrb->c->ci->argc;
       mrb_value *argv = regs+1;
       mrb_value *argv0 = argv;
       int len = m1 + o + r + m2;
@@ -1150,7 +1164,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         argc = ary->len;
         mrb_gc_protect(mrb, regs[1]);
       }
-      if (mrb->ci->proc && MRB_PROC_STRICT_P(mrb->ci->proc)) {
+      if (mrb->c->ci->proc && MRB_PROC_STRICT_P(mrb->c->ci->proc)) {
         if (argc >= 0) {
           if (argc < m1 + m2 || (r == 0 && argc > len)) {
             argnum_error(mrb, m1+m2);
@@ -1162,7 +1176,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         argc = mrb_ary_ptr(argv[0])->len;
         argv = mrb_ary_ptr(argv[0])->ptr;
       }
-      mrb->ci->argc = len;
+      mrb->c->ci->argc = len;
       if (argc < len) {
         regs[len+1] = *blk; /* move block */
         if (argv0 != argv) {
@@ -1219,11 +1233,11 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         int eidx;
 
       L_RAISE:
-        ci = mrb->ci;
+        ci = mrb->c->ci;
         mrb_obj_iv_ifnone(mrb, mrb->exc, mrb_intern2(mrb, "lastpc", 6), mrb_voidp_value(pc));
-        mrb_obj_iv_ifnone(mrb, mrb->exc, mrb_intern2(mrb, "ciidx", 5), mrb_fixnum_value(ci - mrb->cibase));
+        mrb_obj_iv_ifnone(mrb, mrb->exc, mrb_intern2(mrb, "ciidx", 5), mrb_fixnum_value(ci - mrb->c->cibase));
         eidx = ci->eidx;
-        if (ci == mrb->cibase) {
+        if (ci == mrb->c->cibase) {
           if (ci->ridx == 0) goto L_STOP;
           goto L_RESCUE;
         }
@@ -1232,8 +1246,8 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         }
         while (ci[0].ridx == ci[-1].ridx) {
           cipop(mrb);
-          ci = mrb->ci;
-          mrb->stack = mrb->stbase + ci[1].stackidx;
+          ci = mrb->c->ci;
+          mrb->c->stack = mrb->c->stbase + ci[1].stackidx;
           if (ci[1].acc < 0 && prev_jmp) {
             mrb->jmp = prev_jmp;
             longjmp(*(jmp_buf*)mrb->jmp, 1);
@@ -1241,9 +1255,9 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
           while (eidx > ci->eidx) {
             ecall(mrb, --eidx);
           }
-          if (ci == mrb->cibase) {
+          if (ci == mrb->c->cibase) {
             if (ci->ridx == 0) {
-              regs = mrb->stack = mrb->stbase;
+              regs = mrb->c->stack = mrb->c->stbase;
               goto L_STOP;
             }
             break;
@@ -1253,12 +1267,12 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         irep = ci->proc->body.irep;
         pool = irep->pool;
         syms = irep->syms;
-        regs = mrb->stack = mrb->stbase + ci[1].stackidx;
-        pc = mrb->rescue[--ci->ridx];
+        regs = mrb->c->stack = mrb->c->stbase + ci[1].stackidx;
+        pc = mrb->c->rescue[--ci->ridx];
       }
       else {
-        mrb_callinfo *ci = mrb->ci;
-        int acc, eidx = mrb->ci->eidx;
+        mrb_callinfo *ci = mrb->c->ci;
+        int acc, eidx = mrb->c->ci->eidx;
         mrb_value v = regs[GETARG_A(i)];
 
         switch (GETARG_B(i)) {
@@ -1271,45 +1285,48 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
               localjump_error(mrb, LOCALJUMP_ERROR_RETURN);
               goto L_RAISE;
             }
-            ci = mrb->cibase + e->cioff;
-            if (ci == mrb->cibase) {
+            ci = mrb->c->cibase + e->cioff;
+            if (ci == mrb->c->cibase) {
               localjump_error(mrb, LOCALJUMP_ERROR_RETURN);
               goto L_RAISE;
             }
-            mrb->ci = ci;
+            mrb->c->ci = ci;
             break;
           }
         case OP_R_NORMAL:
-          if (ci == mrb->cibase) {
-            localjump_error(mrb, LOCALJUMP_ERROR_RETURN);
-            goto L_RAISE;
+          if (ci == mrb->c->cibase) {
+            if (!mrb->c->prev) { /* toplevel return */
+              localjump_error(mrb, LOCALJUMP_ERROR_RETURN);
+              goto L_RAISE;
+            }
+            mrb->c = mrb->c->prev; /* automatic yield at the end */
           }
-          ci = mrb->ci;
+          ci = mrb->c->ci;
           break;
         case OP_R_BREAK:
           if (proc->env->cioff < 0) {
             localjump_error(mrb, LOCALJUMP_ERROR_BREAK);
             goto L_RAISE;
           }
-          ci = mrb->ci = mrb->cibase + proc->env->cioff + 1;
+          ci = mrb->c->ci = mrb->c->cibase + proc->env->cioff + 1;
           break;
         default:
           /* cannot happen */
           break;
         }
-        while (eidx > mrb->ci[-1].eidx) {
+        while (eidx > mrb->c->ci[-1].eidx) {
           ecall(mrb, --eidx);
         }
         cipop(mrb);
         acc = ci->acc;
         pc = ci->pc;
-        regs = mrb->stack = mrb->stbase + ci->stackidx;
+        regs = mrb->c->stack = mrb->c->stbase + ci->stackidx;
         if (acc < 0) {
           mrb->jmp = prev_jmp;
           return v;
         }
         DEBUG(printf("from :%s\n", mrb_sym2name(mrb, ci->mid)));
-        proc = mrb->ci->proc;
+        proc = mrb->c->ci->proc;
         irep = proc->body.irep;
         pool = irep->pool;
         syms = irep->syms;
@@ -1347,7 +1364,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       }
 
       /* replace callinfo */
-      ci = mrb->ci;
+      ci = mrb->c->ci;
       ci->mid = mid;
       ci->target_class = m->target_class;
       if (n == CALL_MAXARGS) {
@@ -1358,10 +1375,10 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       }
 
       /* move stack */
-      value_move(mrb->stack, &regs[a], ci->argc+1);
+      value_move(mrb->c->stack, &regs[a], ci->argc+1);
 
       if (MRB_PROC_CFUNC_P(m)) {
-        mrb->stack[0] = m->body.func(mrb, recv);
+        mrb->c->stack[0] = m->body.func(mrb, recv);
         mrb_gc_arena_restore(mrb, ai);
         goto L_RETURN;
       }
@@ -1376,7 +1393,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         else {
           stack_extend(mrb, irep->nregs,  ci->argc+2);
         }
-        regs = mrb->stack;
+        regs = mrb->c->stack;
         pc = irep->iseq;
       }
       JUMP;
@@ -1854,7 +1871,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       base = regs[a];
       super = regs[a+1];
       if (mrb_nil_p(base)) {
-        base = mrb_obj_value(mrb->ci->target_class);
+        base = mrb_obj_value(mrb->c->ci->target_class);
       }
       c = mrb_vm_define_class(mrb, base, super, id);
       regs[a] = mrb_obj_value(c);
@@ -1871,7 +1888,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
 
       base = regs[a];
       if (mrb_nil_p(base)) {
-        base = mrb_obj_value(mrb->ci->target_class);
+        base = mrb_obj_value(mrb->c->ci->target_class);
       }
       c = mrb_vm_define_module(mrb, base, id);
       regs[a] = mrb_obj_value(c);
@@ -1891,23 +1908,23 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       ci->pc = pc + 1;
       ci->acc = a;
       ci->mid = 0;
-      ci->stackidx = mrb->stack - mrb->stbase;
+      ci->stackidx = mrb->c->stack - mrb->c->stbase;
       ci->argc = 0;
       ci->target_class = mrb_class_ptr(recv);
 
       /* prepare stack */
-      mrb->stack += a;
+      mrb->c->stack += a;
 
       p = mrb_proc_new(mrb, mrb->irep[irep->idx+GETARG_Bx(i)]);
       p->target_class = ci->target_class;
       ci->proc = p;
 
       if (MRB_PROC_CFUNC_P(p)) {
-        mrb->stack[0] = p->body.func(mrb, recv);
+        mrb->c->stack[0] = p->body.func(mrb, recv);
         mrb_gc_arena_restore(mrb, ai);
         if (mrb->exc) goto L_RAISE;
         /* pop stackpos */
-        regs = mrb->stack = mrb->stbase + mrb->ci->stackidx;
+        regs = mrb->c->stack = mrb->c->stbase + mrb->c->ci->stackidx;
         cipop(mrb);
         NEXT;
       }
@@ -1917,7 +1934,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
         syms = irep->syms;
         stack_extend(mrb, irep->nregs, 1);
         ci->nregs = irep->nregs;
-        regs = mrb->stack;
+        regs = mrb->c->stack;
         pc = irep->iseq;
         JUMP;
       }
@@ -1942,13 +1959,13 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
 
     CASE(OP_TCLASS) {
       /* A B    R(A) := target_class */
-      if (!mrb->ci->target_class) {
+      if (!mrb->c->ci->target_class) {
         static const char msg[] = "no target class or module";
         mrb_value exc = mrb_exc_new(mrb, E_TYPE_ERROR, msg, sizeof(msg) - 1);
         mrb->exc = mrb_obj_ptr(exc);
         goto L_RAISE;
       }
-      regs[GETARG_A(i)] = mrb_obj_value(mrb->ci->target_class);
+      regs[GETARG_A(i)] = mrb_obj_value(mrb->c->ci->target_class);
       NEXT;
     }
 
@@ -1974,7 +1991,7 @@ mrb_run(mrb_state *mrb, struct RProc *proc, mrb_value self)
       /*        stop VM */
     L_STOP:
       {
-        int n = mrb->ci->eidx;
+        int n = mrb->c->ci->eidx;
 
         while (n--) {
           ecall(mrb, n);