Hello all,
I'm seeing some very strange behaviour from `atomic-box-swap!` (but not
`atomic-box-compare-and-swap!`) on Guile 3.0.9 from Homebrew on OSX
Sonoma using an M3 Pro cpu. The issue does not seem to manifest on
x86_64. Could it be some interaction between Guile and M3 CPUs?
Or am I just doing something very silly that shouldn't work at all and
just happens to look like it works on x86_64?
Here's the program that fails. It will run for a few hundred million
rounds and then yield "q null in get". Note that using CAS seems to
work, but plain old swap doesn't.
;;--
;; Eventually this fails with "q null in get" if `atomic-box-swap!` is
;; used where marked (*) below. It takes usually between hundreds of
;; millions and a few billion increments to fail.
;;
;; It does NOT fail if the line marked (*) is commented out and the line
;; below it mentioning `atomic-box-compare-and-swap!` is uncommented and
;; used instead.
;;
;; The failure happens on OSX Sonoma 14.4.1 on a MacBook Pro running an
;; M3 Pro CPU using Guile version 3.0.9 from Homebrew as of 2024-04-17.
;;
;; It does NOT happen on AMD x86_64 Debian linux with Guile 3.0.9 from
;; Debian packaging.
(use-modules (ice-9 atomic))
(define r (make-atomic-box '(0)))
(let loop ()
(let ((v (let ((q (atomic-box-ref r)))
(when (null? q) (error "q null in get"))
(unless (eq? (atomic-box-compare-and-swap! r q (cdr q)) q)
(error "CAS failed in get"))
(car q))))
(when (zero? (remainder v 10000000)) (write v) (newline))
(unless (null?
(atomic-box-swap! r (list (+ v 1))) ;; (*)
;; (atomic-box-compare-and-swap! r '() (list (+ v 1)))
)
(error "swap failed in put"))
(loop)))
