Optimize to use a single counter to keep track of when to validate

Validation is triggered whenever counter reaches next power of two which can be
detected when previous and new counter values have no bits set in common.

Author: polytypic

src/kcas/kcas.ml

@@ -557,8 +557,7 @@ module Xt = struct
   type 'x t = {
     mutable casn : casn;
     mutable cass : cass;
-    mutable validate_countdown : int;
-    mutable validate_period : int;
+    mutable validate_counter : int;
     mutable post_commit : Action.t;
   }
 
@@ -575,16 +574,12 @@ module Xt = struct
         validate_one casn loc state;
         validate_all casn gt
 
-  let validate_log xt =
-    let p = xt.validate_period * 2 in
-    xt.validate_countdown <- p;
-    xt.validate_period <- p;
-    validate_all xt.casn xt.cass
-    [@@inline never]
-
   let maybe_validate_log xt =
-    let c = xt.validate_countdown - 1 in
-    if 0 < c then xt.validate_countdown <- c else validate_log xt
+    let c0 = xt.validate_counter in
+    let c1 = c0 + 1 in
+    xt.validate_counter <- c1;
+    (* Validate whenever counter reaches next power of 2. *)
+    if c0 land c1 = 0 then validate_all xt.casn xt.cass
     [@@inline]
 
   let update0 loc f xt lt gt =
@@ -777,8 +772,7 @@ module Xt = struct
 
   let reset_quick xt =
     xt.cass <- NIL;
-    xt.validate_countdown <- initial_validate_period;
-    xt.validate_period <- initial_validate_period;
+    xt.validate_counter <- initial_validate_period;
     xt.post_commit <- Action.noop;
     xt
     [@@inline]
@@ -831,10 +825,9 @@ module Xt = struct
   let commit ?(backoff = Backoff.default) ?(mode = Mode.obstruction_free) tx =
     let casn = Atomic.make (mode :> status)
     and cass = NIL
-    and validate_countdown = initial_validate_period
-    and validate_period = initial_validate_period
+    and validate_counter = initial_validate_period
     and post_commit = Action.noop in
-    let xt = { casn; cass; validate_countdown; post_commit; validate_period } in
+    let xt = { casn; cass; validate_counter; post_commit } in
     commit backoff mode xt tx.tx
     [@@inline]
 end