perf pool: restore non-hashtable lookup for self-queuing

src/pool.ml

@@ -1,13 +1,5 @@
 module WSQ = Ws_deque_
 module A = Atomic_
-
-module Int_tbl = Hashtbl.Make (struct
-  type t = int
-
-  let equal : t -> t -> bool = ( = )
-  let hash : t -> int = Hashtbl.hash
-end)
-
 include Runner
 
 let ( let@ ) = ( @@ )
@@ -31,7 +23,6 @@ type mut_cond = {
 type state = {
   active: bool Atomic.t;  (** Becomes [false] when the pool is shutdown. *)
   workers: worker_state array;  (** Fixed set of workers. *)
-  worker_by_id: worker_state Int_tbl.t;
   main_q: task Queue.t;  (** Main queue for tasks coming from the outside *)
   mc: mut_cond;  (** Used to block on [main_q] *)
 }
@@ -47,9 +38,18 @@ let num_tasks_ (self : state) : int =
   Array.iter (fun w -> n := !n + WSQ.size w.q) self.workers;
   !n
 
+exception Got_worker of worker_state
+
 let[@inline] find_current_worker_ (self : state) : worker_state option =
   let self_id = Thread.id @@ Thread.self () in
-  Int_tbl.find_opt self.worker_by_id self_id
+  try
+    (* see if we're in one of the worker threads *)
+    for i = 0 to Array.length self.workers - 1 do
+      let w = self.workers.(i) in
+      if Thread.id w.thread = self_id then raise_notrace (Got_worker w)
+    done;
+    None
+  with Got_worker w -> Some w
 
 (** Run [task] as is, on the pool. *)
 let run_direct_ (self : state) (w : worker_state option) (task : task) : unit =
@@ -285,7 +285,6 @@ let create ?(on_init_thread = default_thread_init_exit_)
     {
       active = A.make true;
       workers;
-      worker_by_id = Int_tbl.create 8;
       main_q = Queue.create ();
       mc = { mutex = Mutex.create (); cond = Condition.create () };
     }
@@ -352,11 +351,7 @@ let create ?(on_init_thread = default_thread_init_exit_)
   for _j = 1 to num_threads do
     let i, th = Bb_queue.pop receive_threads in
     let worker_state = pool.workers.(i) in
-    worker_state.thread <- th;
+    worker_state.thread <- th
-
-    Mutex.lock pool.mc.mutex;
-    Int_tbl.add pool.worker_by_id (Thread.id th) worker_state;
-    Mutex.unlock pool.mc.mutex
   done;
 
   runner