diff --git a/src/core/CCInt64.ml b/src/core/CCInt64.ml
index 8bb45ead..0c3f2405 100644
--- a/src/core/CCInt64.ml
+++ b/src/core/CCInt64.ml
@@ -11,21 +11,13 @@ let max : t -> t -> t = Stdlib.max
 
 let sign i = compare i zero
 
-(* use FNV:
+(* use FNV-1:
    https://en.wikipedia.org/wiki/Fowler%E2%80%93Noll%E2%80%93Vo_hash_function *)
-let hash_to_int64 (n : t) =
-  let offset_basis = 0xcbf29ce484222325L in
-  let prime = 0x100000001b3L in
+external hash_to_int64 : (int64[@unboxed]) -> (int64[@unboxed])
+  = "caml_cc_hash_int64_to_int64_byte" "caml_cc_hash_int64_to_int64"
 
-  let h = ref offset_basis in
-  for k = 0 to 7 do
-    h := mul !h prime;
-    (* h := h xor (k-th byte of n) *)
-    h := logxor !h (logand (shift_right n (k * 8)) 0xffL)
-  done;
-  logand !h max_int
-
-let[@inline] hash (n : t) : int = to_int (hash_to_int64 n) land Stdlib.max_int
+external hash : (int64[@unboxed]) -> (int[@untagged])
+  = "caml_cc_hash_int64_byte" "caml_cc_hash_int64" [@@noalloc]
 
 (* see {!CCInt.popcount} for more details *)
 let[@inline] popcount (b : t) : int =
diff --git a/src/core/cc_stubs.c b/src/core/cc_stubs.c
index 9c4426ea..af8ee084 100644
--- a/src/core/cc_stubs.c
+++ b/src/core/cc_stubs.c
@@ -1,3 +1,4 @@
+#include <caml/alloc.h>
 #include <caml/mlvalues.h>
 #include <stdint.h>
 
@@ -51,3 +52,16 @@ CAMLprim intnat caml_cc_hash_int64(int64_t n) {
 CAMLprim value caml_cc_hash_int64_byte(value v_n) {
   return Val_long(caml_cc_hash_int64(Int64_val(v_n)));
 }
+
+/* --- int64 -> int64 entry points (for hash_to_int64) --- */
+
+/* native: unboxed int64 in, unboxed int64 out.
+   Masks to non-negative int64 (matches OCaml's Int64.max_int). */
+CAMLprim int64_t caml_cc_hash_int64_to_int64(int64_t n) {
+  return cc_fnv_hash_int64(n) & INT64_MAX;
+}
+
+/* bytecode: boxed int64 in, boxed int64 out */
+CAMLprim value caml_cc_hash_int64_to_int64_byte(value v_n) {
+  return caml_copy_int64(cc_fnv_hash_int64(Int64_val(v_n)) & INT64_MAX);
+}
diff --git a/tests/core/t_int64.ml b/tests/core/t_int64.ml
index 72078a15..64d27bd9 100644
--- a/tests/core/t_int64.ml
+++ b/tests/core/t_int64.ml
@@ -101,3 +101,40 @@ eq' 63 (popcount max_int);;
 eq' 1 (popcount min_int);;
 eq' 10 (popcount 0b1110010110110001010L);;
 eq' 5 (popcount 0b1101110000000000L)
+
+(* hash tests *)
+let ( >= ) = Stdlib.( >= );;
+let ( = ) = Stdlib.( = );;
+let ( <> ) = Stdlib.( <> );;
+
+(* hash is non-negative *)
+t @@ fun () -> hash 0L >= 0;;
+t @@ fun () -> hash 1L >= 0;;
+t @@ fun () -> hash (-1L) >= 0;;
+t @@ fun () -> hash max_int >= 0;;
+t @@ fun () -> hash min_int >= 0;;
+
+(* hash_to_int64 is non-negative *)
+t @@ fun () -> CCInt64.compare (hash_to_int64 0L) 0L >= 0;;
+t @@ fun () -> CCInt64.compare (hash_to_int64 (-1L)) 0L >= 0;;
+t @@ fun () -> CCInt64.compare (hash_to_int64 min_int) 0L >= 0;;
+
+(* hash is consistent with hash_to_int64 *)
+t @@ fun () ->
+  hash 42L = Stdlib.(Int64.to_int (hash_to_int64 42L) land max_int);;
+t @@ fun () ->
+  hash (-1L) = Stdlib.(Int64.to_int (hash_to_int64 (-1L)) land max_int);;
+
+(* different inputs produce different hashes *)
+t @@ fun () -> hash 0L <> hash 1L;;
+t @@ fun () -> hash 1L <> hash 2L;;
+t @@ fun () -> hash 1L <> hash (-1L);;
+t @@ fun () -> hash_to_int64 0L <> hash_to_int64 1L;;
+
+(* deterministic *)
+t @@ fun () -> hash 123L = hash 123L;;
+t @@ fun () -> hash_to_int64 123L = hash_to_int64 123L;;
+
+(* quickcheck: hash is always non-negative *)
+q Q.(map Int64.of_int int) (fun n -> hash n >= 0);;
+q Q.(map Int64.of_int int) (fun n -> CCInt64.compare (hash_to_int64 n) 0L >= 0)