Commit de86652f authored by Ralf Jung's avatar Ralf Jung
Browse files

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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