Commit 29c965ba authored by Ralf Jung's avatar Ralf Jung
Browse files

we can allow comparing more things -- and explain why

parent 670c101b
......@@ -367,18 +367,46 @@ Definition bin_op_eval (op : bin_op) (v1 v2 : val) : option val :=
| _, _ => None
(** Return whether it is possible to use CAS to compare vl (current value) with v1 (netest value). *)
(** Return whether it is possible to use CAS to compare vl (current value) with
v1 (netest value).
We assume the following encoding of values to 64-bit words: The least 3
significant bits of every word are a "tag", and we have 61 bits of payload,
which is enough if all pointers are 8-byte-aligned (commong on 64bit
architectures). The tags have the following meaning:
0: Payload is one of the following finitely many values, which 61 bits are more
than enough to encode: LitV LitUnit, LitV (LitBool _), NONEV, SOMEV (LitV
LitUnit), SOMEV (LitV (LitBool _)).
1: Payload is the data for a LitV (LitInt _).
2: Payload is the data for a SOMEV (LitV (LitInt _)).
3: Payload is the data for a LitV (LitLoc _).
4: Payload is the data for a SOMEV (LitV (LitLoc _)).
5: Value is boxed, i.e., payload is a pointer to some read-only memory area on
the heap which stores whether this is a RecV, PairV, InjLV or InjRV and the
relevant data for those cases. However, the boxed representation is never
used if any of the above representations could be used.
6: Unused.
7: Unused.
Ignoring (as usual) the fact that we have to fit the infinite Z/loc into 61
bits, this means every value is machine-word-sized and can hence be atomically
read and written. It also justifies the comparisons allowed for CAS: Whenever
[vals_cas_compare_safe vl v1] holds, equality of the one-word representation of
[vl] and [v1] is equivalent to equality of the abstract value represented. This
is clear for [LitV _ == LitV _] and [SOMEV (LitV _) == SOMEV (LitV _)] because
none of these are boxed. For [NONEV == v], we can't actually atomically load and
compare the data for boxed values, but that's okay because we only have to know
if they are equal to [NONEV] which is distinct from all boxed values.
Definition vals_cas_compare_safe (vl v1 : val) : Prop :=
match vl, v1 with
(* We allow comparing literals with each other. *)
(* We allow comparing literals with each other, also when wrapped in a SOMEV. *)
| LitV _, LitV _ => True
(* 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 _ => True
| SOMEV _, NONEV => True
| SOMEV (LitV _), SOMEV (LitV _) => True
(* We allow comparing NONEV to anything. *)
| NONEV, _ => True
| _, NONEV => True
(* We don't allow comparing anything else. *)
| _, _ => False
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