# KOS_OP_JUMP_AND_FAST

- **Style**: EXTRA (FASTVAR, LABEL, NOARG, NOARG). Row = fast slot,
  target pc.
- **Stack**: `[...] -> [..., x]` on the jump, `[...] -> [...]` on the
  fall-through. Read the fast slot; if false, push the value and jump
  to the target; if true, advance with nothing pushed and no reference
  taken. Raises `NameError` when the slot is empty.
- **Invariants**: Jump locality for target.
- **Compiler form:** Post-compile only: the `convert_fast` lowering of
  [JUMP_AND_VAR](kos-op-jump-and-var.md) when the name has a fast slot.

**Example:** `(list (and x (f x)))`, `x` in fast slot 0 and `f` in 1.

| index | opcode         | arg | arg view                           | comment |
|------:|----------------|-----|------------------------------------|---------|
| 0     | JUMP_AND_FAST  | 0   | (fast 0 ("x"), -> 3, 0, 0)         | x false: push x, pc=3 |
| 1     | FAST           | 0   | 0 (local "x")                      | stack: [..., x] |
| 2     | CALL_FAST      | 1   | (fast 1 ("f"), 1, 0, 0)            | stack: [..., (f x)] |
| 3     | TAIL_CALL_FAST | 2   | (fast 2 (closure "list"), 1, 0, 0) | |
| 4     | RETURN         | 1   | 1                                  | |