Commit 3076a654 authored by Robbert Krebbers's avatar Robbert Krebbers

Comments for gen_heap.

parent 2578238d
......@@ -6,6 +6,57 @@ From iris.proofmode Require Import tactics.
Set Default Proof Using "Type".
Import uPred.
(** This file provides a generic mechanism for a point-to connective [l ↦{q} v]
with fractional permissions (where [l : L] and [v : V] over some abstract type
[L] for locations and [V] for values). This mechanism can be plugged into a
language by using the heap invariant [gen_heap_ctx σ] where [σ : gmap L V]. See
heap-lang for an example.
Next to the point-to connective [l ↦{q} v], which keeps track of the value [v]
of a location [l], this mechanism allows one to attach "meta" or "ghost" data to
locations. This is done as follows:
- When one allocates a location, in addition to the point-to connective [l ↦ v],
one also obtains the token [meta_token ⊤ l]. This token is an exclusive
resource that denotes that no meta data has been associated with the
namespaces in the mask [⊤] for the location [l].
- Meta data tokens can be split w.r.t. namespace masks, i.e.
[meta_token l (E1 ∪ E2) ⊣⊢ meta_token l E1 ∗ meta_token l E2] if [E1 ## E2].
- Meta data can be set using the update [meta_token l E ==∗ meta l N x] provided
[↑N ⊆ E], and [x : A] for any countable [A]. The [meta l N x] connective is
persistent and denotes the knowledge that the meta data [x] has been
associated with namespace [N] to the location [l].
To make the mechanism as flexible as possible, the [x : A] in [meta l N x] can
be of any countable type [A]. This means that you can associate e.g. single
ghost names, but also tuples of ghost names, etc.
To further increase flexibility, the [meta l N x] and [meta_token l E]
connectives are annotated with a namespace [N] and mask [E]. That way, one can
assign a map of meta information to a location. This is particularly useful when
building abstractions, then one can gradually assign more ghost information to a
location instead of having to do all of this at once. We use namespaces so that
these can be matched up with the invariant namespaces. *)
(** To implement this mechanism, we use three resource algebras:
- An authoritative RA over [gmap L (fracR * agreeR V)], which keeps track of the
values of locations.
- An authoritative RA over [gmap L (agree gname)], which keeps track of the meta
information of locations. This RA introduces an indirection, it keeps track of
a ghost name for each location.
- The ghost names in the aforementioned authoritative RA refer to namespace maps
[namespace_map (agree positive)], which store the actual meta information.
This indirection is needed because we cannot perform frame preserving updates
in an authoritative fragment without owning the full authoritative element
(in other words, without the indirection [meta_set] would need [gen_heap_ctx]
as a premise).
Note that in principle we could have used one big authoritative RA to keep track
of both values and ghost names for meta information, for example:
[gmap L (option (fracR * agreeR V) ∗ option (agree gname)]. Due to the [option]s,
this RA would be quite inconvenient to deal with. *)
Definition gen_heapUR (L V : Type) `{Countable L} : ucmraT :=
gmapUR L (prodR fracR (agreeR (leibnizC V))).
Definition gen_metaUR (L : Type) `{Countable L} : ucmraT :=
......@@ -47,6 +98,8 @@ Section definitions.
Context `{Countable L, hG : !gen_heapG L V Σ}.
Definition gen_heap_ctx (σ : gmap L V) : iProp Σ := ( m,
(* The [⊆] is used to avoid assigning ghost information to the locations in
the initial heap (see [gen_heap_init]). *)
dom _ m dom (gset L) σ
own (gen_heap_name hG) ( (to_gen_heap σ))
own (gen_meta_name hG) ( (to_gen_meta m)))%I.
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