KOS_OP_JUMP_OR_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 true, push the value and jump to the target; if false, 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_OR_VAR when the name has a fast slot.

Example: (or x default) in value position, both in fast slots.

index	opcode	arg	arg view	comment
0	JUMP_OR_FAST	0	(fast 0 (“x”), -> 2, 0, 0)	x true: push x, pc=2
1	FAST	1	1 (local “default”)	stack: […, default]
2	RETURN	1	1