boot2

072-cg-snapshot-rewind.scm (1838B)

---

 ;; tests/cc-cg/72-cg-snapshot-rewind.scm — cg-snapshot / cg-rewind
 ;; primitives. The pair underpins parse-side `sizeof EXPR` (no-eval):
 ;; the parser learns the expression's type without retaining the code
 ;; emission or vstack push the operand parse produced.
 ;;
 ;; Models, in C terms:
 ;;     int x = 5;
 ;;     /* "speculative" load+inc+store sequence, then rewound */
 ;;     /* speculative pushes onto vstack, then rewound */
 ;;     return x;          /* must still be 5 */
 ;;
 ;; Concretely:
 ;;   1. x = 5 (committed).
 ;;   2. cg-snapshot — capture vstack/fn-buf/frame-hi state.
 ;;   3. Emit a postinc on x (loads, stores x+1) AND push extra opnds
 ;;      onto the vstack.
 ;;   4. cg-rewind — discard the emitted bytes and the vstack pushes.
 ;;   5. return x — must observe pre-snapshot value, exit 5.
 ;;
 ;; If snapshot/rewind fail to undo the fn-buf bytes, x is incremented
 ;; to 6 and we'd exit 6 instead of 5. If the vstack rewind fails, the
 ;; final cg-return sees the wrong opnd on top.
 
 (let ((cg (cg-init)))
   (cg-fn-begin cg "main" '() %t-i32)
   (let* ((off-x (cg-alloc-slot cg 4 4))
          (sym-x (%sym "x" 'var 'auto %t-i32 off-x)))
     ;; x = 5 (committed)
     (cg-push-sym cg sym-x)
     (cg-push-imm cg %t-i32 5)
     (cg-assign cg) (cg-pop cg)
     ;; snapshot: capture state before the speculative work
     (let ((tag (cg-snapshot cg)))
       ;; speculative emission: postinc on x — would make x = 6
       (cg-push-sym cg sym-x)
       (cg-postinc cg)
       (cg-pop cg)
       ;; speculative vstack push, never assigned: an extra imm
       (cg-push-imm cg %t-i32 999)
       ;; rewind: discard the postinc bytes AND the speculative pushes
       (cg-rewind cg tag))
     ;; return x — should still be 5
     (cg-push-sym cg sym-x) (cg-load cg)
     (cg-return cg))
   (cg-fn-end cg)
   (write-bv-fd 1 (cg-finish cg)))