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`.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/codedump.c b/src/codedump.c
index 81212921d..095028a39 100644
--- a/src/codedump.c
+++ b/src/codedump.c
@@ -261,6 +261,9 @@ codedump(mrb_state *mrb, const mrb_irep *irep)
     CASE(OP_JMP, S);
       printf("OP_JMP\t\t%03d\n", a);
       break;
+    CASE(OP_JUW, S);
+      printf("OP_JUW\t\t%03d\n", a);
+      break;
     CASE(OP_JMPIF, BS);
       printf("OP_JMPIF\tR%d\t%03d\t", a, b);
       print_lv_a(mrb, irep, a);