CCInt.hash: replace OCaml Int64 loop with C stub (FNV-1)

Implement FNV-1 hashing for CCInt.hash as a C stub instead of a
pure-OCaml Int64-based loop. The core hash function operates on int64
with separate native/bytecode entry points for both int and int64.

- cc_stubs.c: cc_fnv_hash_int64 core, with int and int64 wrappers
- Uses [@untagged] for int args, [@unboxed] ready for int64
- OCAML_INT_MASK ensures correct byte extraction for 63-bit ints
- Hash values are unchanged from the previous OCaml implementation

Also adds a throughput benchmark in benchs/run_benchs.ml comparing
the old pure-OCaml FNV hash vs the new C stub (~2x faster).

benchs/dune

@@ -10,7 +10,6 @@
   qcheck
   oseq
   batteries
-  base
   sek)
  (flags :standard -warn-error -3-5 -w -60 -safe-string -color always)
  (optional)

benchs/run_benchs.ml

@@ -1824,4 +1824,35 @@ module Str = struct
            ])
 end
 
+module Hash = struct
+  let hash_ocaml (n : int) : int =
+    let offset_basis = 0xcbf29ce484222325L in
+    let prime = 0x100000001b3L in
+    let h = ref offset_basis in
+    for k = 0 to 7 do
+      h := Int64.(mul !h prime);
+      h := Int64.(logxor !h (of_int ((n lsr (k * 8)) land 0xff)))
+    done;
+    Int64.to_int !h land max_int
+
+  let bench_hash n =
+    let run_ocaml () =
+      for i = 0 to n - 1 do
+        opaque_ignore (hash_ocaml i)
+      done
+    and run_c_stub () =
+      for i = 0 to n - 1 do
+        opaque_ignore (CCInt.hash i)
+      done
+    in
+    B.throughputN 3 ~repeat
+      [
+        "ocaml_fnv", run_ocaml, ();
+        "c_stub", run_c_stub, ();
+      ]
+
+  let () =
+    B.Tree.register ("hash" @>>> [ "int" @>> app_ints bench_hash [ 1_000 ] ])
+end
+
 let () = try B.Tree.run_global () with Arg.Help msg -> print_endline msg

src/core/CCInt.ml

@@ -6,18 +6,8 @@ type 'a iter = ('a -> unit) -> unit
 
 (* use FNV:
    https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function *)
-let hash (n : int) : int =
-  let offset_basis = 0xcbf29ce484222325L in
-  let prime = 0x100000001b3L in
-
-  let h = ref offset_basis in
-  for k = 0 to 7 do
-    (h := Int64.(mul !h prime));
-    (* h := h xor (k-th byte of n) *)
-    h := Int64.(logxor !h (of_int ((n lsr (k * 8)) land 0xff)))
-  done;
-  (* truncate back to int and remove sign *)
-  Int64.to_int !h land max_int
+external hash : (int[@untagged]) -> (int[@untagged])
+  = "caml_cc_hash_int_byte" "caml_cc_hash_int" [@@noalloc]
 
 let range i j yield =
   let rec up i j yield =

src/core/cc_stubs.c

@@ -0,0 +1,48 @@
+#include <caml/mlvalues.h>
+#include <stdint.h>
+
+/* FNV-1a hash for a 64-bit integer.
+   https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function
+
+   Core routine: operates on all 8 bytes of an int64_t. */
+
+static inline int64_t cc_fnv_hash_int64(int64_t n) {
+  uint64_t un = (uint64_t)n;
+  uint64_t h = UINT64_C(0xcbf29ce484222325);
+  const uint64_t prime = UINT64_C(0x100000001b3);
+  for (int k = 0; k < 8; k++) {
+    h *= prime;
+    h ^= (un >> (k * 8)) & 0xff;
+  }
+  return (int64_t)h;
+}
+
+/* --- CCInt.hash entry points (int -> int) --- */
+
+/* Mask to the OCaml int range (63 bits on 64-bit, 31 on 32-bit)
+   before hashing, so negative OCaml ints hash the same as
+   the unsigned representation seen by OCaml's [lsr]. */
+#define OCAML_INT_MASK ((UINT64_C(1) << (8 * sizeof(value) - 1)) - 1)
+
+/* native: untagged int in, untagged int out */
+CAMLprim intnat caml_cc_hash_int(intnat n) {
+  int64_t projected = (int64_t)((uint64_t)n & OCAML_INT_MASK);
+  return (intnat)((uint64_t)cc_fnv_hash_int64(projected) & Max_long);
+}
+
+/* bytecode: boxed value in, boxed value out */
+CAMLprim value caml_cc_hash_int_byte(value v_n) {
+  return Val_long(caml_cc_hash_int(Long_val(v_n)));
+}
+
+/* --- int64 hash entry points (int64 -> int) --- */
+
+/* native: unboxed int64 in, untagged int out */
+CAMLprim intnat caml_cc_hash_int64(int64_t n) {
+  return (intnat)((uint64_t)cc_fnv_hash_int64(n) & Max_long);
+}
+
+/* bytecode: boxed int64 value in, boxed value out */
+CAMLprim value caml_cc_hash_int64_byte(value v_n) {
+  return Val_long(caml_cc_hash_int64(Int64_val(v_n)));
+}

src/core/dune

@@ -6,6 +6,10 @@
   (action
    (run %{project_root}/src/core/cpp/cpp.exe %{input-file})))
  (flags :standard -nolabels -open CCMonomorphic)
+ (foreign_stubs
+  (language c)
+  (flags :standard -std=c99 -O2)
+  (names cc_stubs))
  (libraries either containers.monomorphic containers.domain))
 
 (ocamllex