spectre.isle

;; Rewrites for `select_spectre_guard` -- check these rules carefully!
;;
;; This instruction prohibits all speculation on the controlling value when
;; determining which input to use as the result. Rewrites must respect that
;; requirement.

;; If we statically know which value will be the result, it's safe to just use
;; that value. No speculation on the controlling value is possible if we simply
;; don't depend on that value at all.
(rule (simplify (select_spectre_guard _ _ x x))
      (subsume x))
(rule (simplify (select_spectre_guard _ (iconst_u _ (u64_when_non_zero)) x _))
      (subsume x))
(rule (simplify (select_spectre_guard _ (iconst_u _ 0) _ y))
      (subsume y))