Commit de86652f authored by Ralf Jung's avatar Ralf Jung

allow comparing more things in CAS

See the comments in the code for why that makes sense
parent 6b0f3bfb
......@@ -370,13 +370,16 @@ Definition bin_op_eval (op : bin_op) (v1 v2 : val) : option val :=
(** Return whether it is possible to use CAS to compare vl (current value) with v1 (netest value). *)
Definition vals_cas_compare_safe (vl v1 : val) : Prop :=
match vl, v1 with
(* We allow comparing literals with each other. *)
| LitV _, LitV _ => True
(* We want to support CAS'ing [NONEV] to [SOMEV #l]. An implementation of
this is possible if literals have an invalid bit pattern that can be used to
represent NONE. *)
(* We assume that [NONEV] is represented as a NULL-pointer and [SOMEV x] as a
pointer to a location (never written to after allocation) storing [x]. Then
comparing [NONEV] with [NONEV] or [SOMEV x] is possible atomically by testing
for NULL. Comparing [SOMEV x] with [SOMEV y] is not possible though. *)
| NONEV, NONEV => True
| NONEV, SOMEV (LitV _) => True
| SOMEV (LitV _), NONEV => True
| NONEV, SOMEV _ => True
| SOMEV _, NONEV => True
(* We don't allow comparing anything else. *)
| _, _ => False
end.
(** Just a sanity check. *)
......
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