Fix small CR about immutability

Author: mshinwell

backend/cmm_helpers.ml

@@ -1114,6 +1114,9 @@ let int_array_ref arr ofs dbg =
 let unboxed_float_array_ref arr ofs dbg =
   Cop (mk_load_mut Double, [array_indexing log2_size_float arr ofs dbg], dbg)
 
+let unboxed_immutable_float_array_ref arr ofs dbg =
+  Cop (mk_load_immut Double, [array_indexing log2_size_float arr ofs dbg], dbg)
+
 let float_array_ref mode arr ofs dbg =
   box_float dbg mode (unboxed_float_array_ref arr ofs dbg)
 

backend/cmm_helpers.mli

@@ -220,6 +220,9 @@ val int_array_ref : expression -> expression -> Debuginfo.t -> expression
 val unboxed_float_array_ref :
   expression -> expression -> Debuginfo.t -> expression
 
+val unboxed_immutable_float_array_ref :
+  expression -> expression -> Debuginfo.t -> expression
+
 val float_array_ref :
   Lambda.alloc_mode -> expression -> expression -> Debuginfo.t -> expression
 

middle_end/flambda2/to_cmm/to_cmm_primitive.ml

@@ -123,10 +123,7 @@ let block_load ~dbg (kind : P.Block_access_kind.t) (mutability : Mutability.t)
     match field_kind with
     | Tagged_immediate ->
       C.get_field_computed Immediate mutability ~block ~index dbg
-    | Naked_float ->
-      (* CR layouts v5.1: We should use the mutability here to generate better
-         code if the load is immutable. *)
-      C.unboxed_float_array_ref block index dbg
+    | Naked_float -> C.unboxed_immutable_float_array_ref block index dbg
     | Naked_float32 -> C.get_field_unboxed_float32 mutability ~block ~index dbg
     | Naked_int32 -> C.get_field_unboxed_int32 mutability ~block ~index dbg
     | Naked_int64 | Naked_nativeint ->