Replace global jump with catch handler implementation

When a global jump occurs, look at the catch handler table to determine where to jump.
In that case, `pc` already shows the following instruction, but since the table shows `begin_offset ... end_offset`, the comparison is done with `begin_offset < pc && pc <= end_offset`.
If there is a corresponding handler, move `pc` to `handler.target_offset` and continue running the VM.

When a global jump across `ensure` is made by `return`, `break`, `next`, `redo` and `retry`, the extended `RBreak` object saves and restores the C-level execution position.
This extended `RBreak` can have tag information, which makes it a pseudo coroutine (the "tag" mimics CRuby).

The implementation of pseudo coroutines by `RBreak` is summarized by `CHECKPOINT_RESTORE ... CHECKPOINT_MAIN ... CHECKPOINT_END` and `throw_tagged_break` / `unwind_ensure` macros.
The restart of processing is branched by `RBREAK_TAG_FOREACH(DISPATCH_CHECKPOINTS)`.

- Not only `rescue` blocks but also `ensure` blocks are now sandwiched between `OP_EXCEPT` and `OP_RAISEIF`.

- Remove the function `ecall()`.
  It is no longer necessary to re-enter the VM to perform an "ensure block".

  This will resolves #1888.

- Added instruction `OP_JUW` (Jump while UnWind).

  It jumps unconditionally like `OP_JMP`, but searches the catch handler table and executes the ensure block.
  Since it searches the catch handler table, it is much heavier than `OP_JMP`.

3 files changed, 34 insertions, 8 deletions

diff --git a/include/mruby.h b/include/mruby.h
index c9425e8a7..9f1d1aa30 100644
--- a/include/mruby.h
+++ b/include/mruby.h
@@ -149,8 +149,6 @@ typedef struct {
   mrb_sym mid;
   const struct RProc *proc;
   mrb_value *stackent;
-  uint16_t ridx;
-  uint16_t epos;
   struct REnv *env;
   const mrb_code *pc;           /* return address */
   const mrb_code *err;          /* error position */
@@ -177,11 +175,6 @@ struct mrb_context {
   mrb_callinfo *ci;
   mrb_callinfo *cibase, *ciend;
 
-  uint16_t *rescue;                       /* exception handler stack */
-  uint16_t rsize;
-  struct RProc **ensure;                  /* ensure handler stack */
-  uint16_t esize, eidx;
-
   enum mrb_fiber_state status : 4;
   mrb_bool vmexec : 1;
   struct RFiber *fib;
@@ -301,7 +294,6 @@ typedef struct mrb_state {
   mrb_atexit_func *atexit_stack;
 #endif
   uint16_t atexit_stack_len;
-  uint16_t ecall_nest;                    /* prevent infinite recursive ecall() */
 } mrb_state;
 
 /**
diff --git a/include/mruby/error.h b/include/mruby/error.h
index 72f6c5f3d..67a0a539e 100644
--- a/include/mruby/error.h
+++ b/include/mruby/error.h
@@ -66,6 +66,39 @@ mrb_break_value_set(struct RBreak *brk, mrb_value val)
 #define mrb_break_proc_get(brk) ((brk)->proc)
 #define mrb_break_proc_set(brk, p) ((brk)->proc = p)
 
+#define RBREAK_TAG_FOREACH(f) \
+  f(RBREAK_TAG_BREAK, 0) \
+  f(RBREAK_TAG_BREAK_UPPER, 1) \
+  f(RBREAK_TAG_BREAK_INTARGET, 2) \
+  f(RBREAK_TAG_RETURN_BLOCK, 3) \
+  f(RBREAK_TAG_RETURN, 4) \
+  f(RBREAK_TAG_RETURN_TOPLEVEL, 5) \
+  f(RBREAK_TAG_JUMP, 6) \
+  f(RBREAK_TAG_STOP, 7)
+
+#define RBREAK_TAG_DEFINE(tag, i) tag = i,
+enum {
+  RBREAK_TAG_FOREACH(RBREAK_TAG_DEFINE)
+};
+#undef RBREAK_TAG_DEFINE
+
+#define RBREAK_TAG_BIT          3
+#define RBREAK_TAG_BIT_OFF      8
+#define RBREAK_TAG_MASK         (~(~UINT32_C(0) << RBREAK_TAG_BIT))
+
+static inline uint32_t
+mrb_break_tag_get(struct RBreak *brk)
+{
+  return (brk->flags >> RBREAK_TAG_BIT_OFF) & RBREAK_TAG_MASK;
+}
+
+static inline void
+mrb_break_tag_set(struct RBreak *brk, uint32_t tag)
+{
+  brk->flags &= ~(RBREAK_TAG_MASK << RBREAK_TAG_BIT_OFF);
+  brk->flags |= (tag & RBREAK_TAG_MASK) << RBREAK_TAG_BIT_OFF;
+}
+
 /**
  * Protect
  *
diff --git a/include/mruby/ops.h b/include/mruby/ops.h
index 8ff44088e..770e54f3d 100644
--- a/include/mruby/ops.h
+++ b/include/mruby/ops.h
@@ -49,6 +49,7 @@ OPCODE(JMP,        S)        /* pc=a */
 OPCODE(JMPIF,      BS)       /* if R(a) pc=b */
 OPCODE(JMPNOT,     BS)       /* if !R(a) pc=b */
 OPCODE(JMPNIL,     BS)       /* if R(a)==nil pc=b */
+OPCODE(JUW,        S)        /* unwind_and_jump_to(a) */
 OPCODE(EXCEPT,     B)        /* R(a) = exc */
 OPCODE(RESCUE,     BB)       /* R(b) = R(a).isa?(R(b)) */
 OPCODE(RAISEIF,    B)        /* raise(R(a)) if R(a) */