Remove the Op API

README.md

@@ -120,21 +120,17 @@ val x : int Loc.t = <abstr>
 One can then manipulate the locations individually:
 
 ```ocaml
-# Loc.set a 6
+# Loc.set a 10
 - : unit = ()
 
 # Loc.get a
-- : int = 6
-```
-
-Attempt primitive operations over multiple locations:
+- : int = 10
 
-```ocaml
-# Op.atomically [
-    Op.make_cas a 6 10;
-    Op.make_cas b 0 52
-  ]
+# Loc.compare_and_set b 0 52
 - : bool = true
+
+# Loc.get b
+- : int = 52
 ```
 
 Block waiting for changes to locations:
@@ -170,16 +166,12 @@ And now we have it:
 The API of **Kcas** is divided into submodules. The main modules are
 
 - [`Loc`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Loc/index.html),
-  providing an abstraction of _shared memory locations_,
+  providing an abstraction of _shared memory locations_, and
 
 - [`Xt`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Xt/index.html),
-  providing _explicit transaction log passing_ over shared memory locations, and
-
-- [`Op`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Op/index.html),
-  providing an interface for _primitive operations_ over multiple shared memory
-  locations.
+  providing _explicit transaction log passing_ over shared memory locations.
 
-The following sections discuss each of the above in turn.
+The following sections discuss both of the above in turn.
 
 ### Creating and manipulating individual shared memory locations
 
@@ -192,9 +184,7 @@ In other words, an application that uses
 [`Atomic`](https://v2.ocaml.org/api/Atomic.html), but then needs to perform
 atomic operations over multiple atomic locations, could theoretically just
 rebind `module Atomic = Loc` and then use the
-[`Op`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Op/index.html),
-and/or
-[`Xt`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Xt/index.html) APIs
+[`Xt`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Xt/index.html) API
 to perform operations over multiple locations. This should not be done
 just-in-case, however, as, even though **Kcas** is efficient, it does naturally
 have higher overhead than the Stdlib
@@ -449,10 +439,8 @@ val a_queue : int queue = {front = <abstr>; back = <abstr>}
 
 #### Composing transactions
 
-The main benefit of the
+The main feature of the
 [`Xt`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Xt/index.html) API
-over the
-[`Op`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Op/index.html) API
 is that transactions are composable. In fact, we already wrote transactions that
 recorded multiple primitive shared memory accesses to the explicitly passed
 transaction log. Nothing prevents us from writing transactions calling other
@@ -1049,72 +1037,6 @@ val a_cache : (int, string) cache =
 As an exercise, implement an operation to `remove` associations from a cache and
 an operation to change the capacity of the cache.
 
-### Programming with primitive operations
-
-In addition to the transactional interface, **Kcas** also provides the
-[`Op`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Op/index.html)
-interface for performing a list of primitive operations. To program with
-primitive operations one simply makes a list of CAS operations using
-[`make_cas`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Op/index.html#val-make_cas)
-and then attempts them using
-[`atomically`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Op/index.html#val-atomically).
-Typically that needs to be done inside a loop of some kind as such an attempt
-can naturally fail.
-
-Let's first
-[`make`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Loc/index.html#val-make)
-two locations representing stacks:
-
-```ocaml
-# let stack_a = Loc.make [19]
-  and stack_b = Loc.make [76]
-val stack_a : int list Loc.t = <abstr>
-val stack_b : int list Loc.t = <abstr>
-```
-
-Here is a function that can atomically move an element from given `source` stack
-to the given `target` stack:
-
-```ocaml
-# let rec move ?(backoff = Backoff.default)
-               source
-               target =
-    match Loc.get source with
-    | [] -> raise Exit
-    | (elem::rest) as old_source ->
-      let old_target = Loc.get target in
-      let ops = [
-        Op.make_cas source old_source rest;
-        Op.make_cas target old_target (elem::old_target)
-      ] in
-      if not (Op.atomically ops) then
-        let backoff = Backoff.once backoff in
-        move ~backoff source target
-val move : ?backoff:Backoff.t -> 'a list Loc.t -> 'a list Loc.t -> unit =
-  <fun>
-```
-
-Note that we also used the
-[`Backoff`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Backoff/index.html)
-module provided by **Kcas** above.
-
-Now we can simply call `move`:
-
-```ocaml
-# move stack_a stack_b
-- : unit = ()
-
-# Loc.get stack_a
-- : int list = []
-
-# Loc.get stack_b
-- : int list = [19; 76]
-```
-
-As one can see, the API provided by
-[`Op`](https://ocaml-multicore.github.io/kcas/doc/kcas/Kcas/Op/index.html) is
-quite low-level and is not intended for application level programming.
-
 ## Designing lock-free algorithms with k-CAS
 
 The key benefit of k-CAS, or k-CAS-n-CMP, and transactions in particular, is

bench/main.ml

@@ -1,6 +1,5 @@
 let benchmarks =
   [
-    ("Kcas Op", Bench_op.run_suite);
     ("Kcas Xt", Bench_xt.run_suite);
     ("Kcas parallel CMP", Bench_parallel_cmp.run_suite);
     ("Kcas_data Hashtbl", Bench_hashtbl.run_suite);

src/kcas/kcas.ml

@@ -348,7 +348,6 @@ let[@inline] determine_for_owner casn cass =
       fenceless_get casn == After
 
 let[@inline never] impossible () = failwith "impossible"
-let[@inline never] overlap () = failwith "kcas: location overlap"
 let[@inline never] invalid_retry () = failwith "kcas: invalid use of retry"
 
 type splay = Miss : splay | Hit : 'a loc * 'a state -> splay
@@ -615,63 +614,6 @@ module Loc = struct
   let fenceless_get loc = eval (fenceless_get (as_atomic loc))
 end
 
-let[@inline] insert cass loc state =
-  let x = loc.id in
-  match cass with
-  | CASN { loc = a; lt = NIL; _ } when x < a.id ->
-      CASN { loc; state; lt = NIL; gt = cass; awaiters = [] }
-  | CASN { loc = a; gt = NIL; _ } when a.id < x ->
-      CASN { loc; state; lt = cass; gt = NIL; awaiters = [] }
-  | _ -> begin
-      match splay ~hit_parent:false x cass with
-      | _, Hit _, _ -> overlap ()
-      | lt, Miss, gt -> CASN { loc; state; lt; gt; awaiters = [] }
-    end
-
-module Op = struct
-  type t = CAS : 'a Loc.t * 'a * 'a -> t
-
-  let[@inline] make_cas loc before after = CAS (loc, before, after)
-  let[@inline] make_cmp loc expected = CAS (loc, expected, expected)
-
-  let[@inline] is_on_loc op loc =
-    match op with CAS (loc', _, _) -> Obj.magic loc' == loc
-
-  let[@inline] get_id = function CAS (loc, _, _) -> loc.id
-
-  let atomic = function
-    | CAS (loc, before, after) ->
-        if before == after then Loc.get loc == before
-        else Loc.compare_and_set loc before after
-
-  let atomically ?(mode = Mode.lock_free) = function
-    | [] -> true
-    | [ op ] -> atomic op
-    | first :: rest ->
-        let casn = Atomic.make (mode :> status) in
-        let rec run cass = function
-          | [] -> determine_for_owner casn cass
-          | CAS (loc, before, after) :: rest ->
-              if before == after && is_obstruction_free casn loc then
-                (* Fenceless is safe as there are fences in [determine]. *)
-                let state = fenceless_get (as_atomic loc) in
-                before == eval state && run (insert cass loc state) rest
-              else
-                run
-                  (insert cass loc { before; after; casn; awaiters = [] })
-                  rest
-        in
-        let (CAS (loc, before, after)) = first in
-        if before == after && is_obstruction_free casn loc then
-          (* Fenceless is safe as there are fences in [determine]. *)
-          let state = fenceless_get (as_atomic loc) in
-          before == eval state
-          && run (CASN { loc; state; lt = NIL; gt = NIL; awaiters = [] }) rest
-        else
-          let state = { before; after; casn; awaiters = [] } in
-          run (CASN { loc; state; lt = NIL; gt = NIL; awaiters = [] }) rest
-end
-
 module Xt = struct
   (* NOTE: You can adjust comment blocks below to select whether or not to use
      an unsafe cast to avoid a level of indirection due to [Atomic.t]. *)
@@ -791,6 +733,9 @@ module Xt = struct
     unsafe_update ~xt loc (fun actual ->
         if actual == before then after else actual)
 
+  let compare_and_set ~xt loc before after =
+    compare_and_swap ~xt loc before after == before
+
   let exchange ~xt loc after = unsafe_update ~xt loc (fun _ -> after)
   let fetch_and_add ~xt loc n = unsafe_update ~xt loc (( + ) n)
   let incr ~xt loc = unsafe_update ~xt loc inc |> ignore