diff --git a/middle_end/flambda2/from_lambda/lambda_to_flambda.ml b/middle_end/flambda2/from_lambda/lambda_to_flambda.ml
index 664d3f972b0..31b38eda6bc 100644
--- a/middle_end/flambda2/from_lambda/lambda_to_flambda.ml
+++ b/middle_end/flambda2/from_lambda/lambda_to_flambda.ml
@@ -580,13 +580,13 @@ let primitive_can_raise (prim : Lambda.primitive) =
   | Pbytes_set_32 false
   | Pbytes_set_64 false
   | Pbytes_set_128 { unsafe = false; _ }
-  | Pbigstring_load_16 false
-  | Pbigstring_load_32 (false, _)
-  | Pbigstring_load_64 (false, _)
+  | Pbigstring_load_16 { unsafe = false }
+  | Pbigstring_load_32 { unsafe = false; mode = _; boxed = _ }
+  | Pbigstring_load_64 { unsafe = false; mode = _; boxed = _ }
   | Pbigstring_load_128 { unsafe = false; _ }
-  | Pbigstring_set_16 false
-  | Pbigstring_set_32 false
-  | Pbigstring_set_64 false
+  | Pbigstring_set_16 { unsafe = false }
+  | Pbigstring_set_32 { unsafe = false; boxed = _ }
+  | Pbigstring_set_64 { unsafe = false; boxed = _ }
   | Pbigstring_set_128 { unsafe = false; _ }
   | Pdivbint { is_safe = Safe; _ }
   | Pmodbint { is_safe = Safe; _ }
@@ -645,13 +645,13 @@ let primitive_can_raise (prim : Lambda.primitive) =
   | Pbytes_set_32 true
   | Pbytes_set_64 true
   | Pbytes_set_128 { unsafe = true; _ }
-  | Pbigstring_load_16 true
-  | Pbigstring_load_32 (true, _)
-  | Pbigstring_load_64 (true, _)
+  | Pbigstring_load_16 { unsafe = true }
+  | Pbigstring_load_32 { unsafe = true; mode = _; boxed = _ }
+  | Pbigstring_load_64 { unsafe = true; mode = _; boxed = _ }
   | Pbigstring_load_128 { unsafe = true; _ }
-  | Pbigstring_set_16 true
-  | Pbigstring_set_32 true
-  | Pbigstring_set_64 true
+  | Pbigstring_set_16 { unsafe = true }
+  | Pbigstring_set_32 { unsafe = true; boxed = _ }
+  | Pbigstring_set_64 { unsafe = true; boxed = _ }
   | Pbigstring_set_128 { unsafe = true; _ }
   | Pctconst _ | Pbswap16 | Pbbswap _ | Pint_as_pointer _ | Popaque _
   | Pprobe_is_enabled _ | Pobj_dup | Pobj_magic _ | Pbox_float _ | Punbox_float
diff --git a/middle_end/flambda2/from_lambda/lambda_to_flambda_primitives.ml b/middle_end/flambda2/from_lambda/lambda_to_flambda_primitives.ml
index 430253a1b35..e657de0ecc3 100644
--- a/middle_end/flambda2/from_lambda/lambda_to_flambda_primitives.ml
+++ b/middle_end/flambda2/from_lambda/lambda_to_flambda_primitives.ml
@@ -479,14 +479,19 @@ let checked_bigstring_access ~dbg ~size_int ~access_size ~primitive arg1 arg2 =
       [bigstring_access_validity_condition ~size_int arg1 access_size arg2]
 
 (* String-like loads *)
-let string_like_load_unsafe ~access_size kind mode string index ~current_region
-    =
+let string_like_load_unsafe ~access_size kind mode ~boxed string index
+    ~current_region =
   let wrap =
     match (access_size : Flambda_primitive.string_accessor_width), mode with
-    | (Eight | Sixteen), None -> tag_int
-    | Thirty_two, Some mode -> box_bint Pint32 mode ~current_region
-    | Sixty_four, Some mode -> box_bint Pint64 mode ~current_region
-    | One_twenty_eight _, Some mode -> box_vec128 mode ~current_region
+    | (Eight | Sixteen), None ->
+      assert (not boxed);
+      tag_int
+    | Thirty_two, Some mode ->
+      if boxed then box_bint Pint32 mode ~current_region else Fun.id
+    | Sixty_four, Some mode ->
+      if boxed then box_bint Pint64 mode ~current_region else Fun.id
+    | One_twenty_eight _, Some mode ->
+      if boxed then box_vec128 mode ~current_region else Fun.id
     | (Eight | Sixteen), Some _
     | (Thirty_two | Sixty_four | One_twenty_eight _), None ->
       Misc.fatal_error "Inconsistent alloc_mode for string or bytes load"
@@ -497,51 +502,55 @@ let get_header obj mode ~current_region =
   let wrap hd = box_bint Pnativeint mode hd ~current_region in
   wrap (Unary (Get_header, obj))
 
-let string_like_load_safe ~dbg ~size_int ~access_size kind mode str index
+let string_like_load_safe ~dbg ~size_int ~access_size kind mode ~boxed str index
     ~current_region =
   match (kind : P.string_like_value) with
   | String ->
     checked_string_or_bytes_access ~dbg ~size_int ~access_size String
       ~primitive:
-        (string_like_load_unsafe ~access_size String mode str index
+        (string_like_load_unsafe ~access_size String mode ~boxed str index
            ~current_region)
       str index
   | Bytes ->
     checked_string_or_bytes_access ~dbg ~size_int ~access_size Bytes
       ~primitive:
-        (string_like_load_unsafe ~access_size Bytes mode str index
+        (string_like_load_unsafe ~access_size Bytes mode ~boxed str index
            ~current_region)
       str index
   | Bigstring ->
     checked_bigstring_access ~dbg ~size_int ~access_size
       ~primitive:
-        (string_like_load_unsafe ~access_size Bigstring mode str index
+        (string_like_load_unsafe ~access_size Bigstring mode ~boxed str index
            ~current_region)
       str index
 
 (* Bytes-like set *)
-let bytes_like_set_unsafe ~access_size kind bytes index new_value =
+let bytes_like_set_unsafe ~access_size kind ~boxed bytes index new_value =
   let wrap =
     match (access_size : Flambda_primitive.string_accessor_width) with
-    | Eight | Sixteen -> untag_int
-    | Thirty_two -> unbox_bint Pint32
-    | Sixty_four -> unbox_bint Pint64
-    | One_twenty_eight _ -> unbox_vec128
+    | Eight | Sixteen ->
+      assert (not boxed);
+      untag_int
+    | Thirty_two -> if boxed then unbox_bint Pint32 else Fun.id
+    | Sixty_four -> if boxed then unbox_bint Pint64 else Fun.id
+    | One_twenty_eight _ -> if boxed then unbox_vec128 else Fun.id
   in
   H.Ternary
     (Bytes_or_bigstring_set (kind, access_size), bytes, index, wrap new_value)
 
-let bytes_like_set_safe ~dbg ~size_int ~access_size kind bytes index new_value =
+let bytes_like_set_safe ~dbg ~size_int ~access_size kind ~boxed bytes index
+    new_value =
   match (kind : P.bytes_like_value) with
   | Bytes ->
     checked_string_or_bytes_access ~dbg ~size_int ~access_size Bytes
       ~primitive:
-        (bytes_like_set_unsafe ~access_size Bytes bytes index new_value)
+        (bytes_like_set_unsafe ~access_size Bytes ~boxed bytes index new_value)
       bytes index
   | Bigstring ->
     checked_bigstring_access ~dbg ~size_int ~access_size
       ~primitive:
-        (bytes_like_set_unsafe ~access_size Bigstring bytes index new_value)
+        (bytes_like_set_unsafe ~access_size Bigstring ~boxed bytes index
+           new_value)
       bytes index
 
 (* Bigarray accesses *)
@@ -1041,92 +1050,95 @@ let convert_lprim ~big_endian (prim : L.primitive) (args : Simple.t list list)
   | Pstringlength, [[arg]] -> [tag_int (Unary (String_length String, arg))]
   | Pbyteslength, [[arg]] -> [tag_int (Unary (String_length Bytes, arg))]
   | Pstringrefu, [[str]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Eight String None str index
-        ~current_region ]
+    [ string_like_load_unsafe ~access_size:Eight String None ~boxed:false str
+        index ~current_region ]
   | Pbytesrefu, [[bytes]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Eight Bytes None bytes index
-        ~current_region ]
-  | Pstringrefs, [[str]; [index]] ->
-    [ string_like_load_safe ~dbg ~size_int ~access_size:Eight String None str
+    [ string_like_load_unsafe ~access_size:Eight Bytes None ~boxed:false bytes
         index ~current_region ]
+  | Pstringrefs, [[str]; [index]] ->
+    [ string_like_load_safe ~dbg ~size_int ~access_size:Eight String
+        ~boxed:false None str index ~current_region ]
   | Pbytesrefs, [[bytes]; [index]] ->
-    [ string_like_load_safe ~dbg ~size_int ~access_size:Eight Bytes None bytes
-        index ~current_region ]
+    [ string_like_load_safe ~dbg ~size_int ~access_size:Eight Bytes ~boxed:false
+        None bytes index ~current_region ]
   | Pstring_load_16 true (* unsafe *), [[str]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Sixteen String None str index
-        ~current_region ]
+    [ string_like_load_unsafe ~access_size:Sixteen String ~boxed:false None str
+        index ~current_region ]
   | Pbytes_load_16 true (* unsafe *), [[bytes]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Sixteen Bytes None bytes index
-        ~current_region ]
-  | Pstring_load_32 (true (* unsafe *), mode), [[str]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Thirty_two String (Some mode) str
+    [ string_like_load_unsafe ~access_size:Sixteen Bytes ~boxed:false None bytes
         index ~current_region ]
+  | Pstring_load_32 (true (* unsafe *), mode), [[str]; [index]] ->
+    [ string_like_load_unsafe ~access_size:Thirty_two String ~boxed:true
+        (Some mode) str index ~current_region ]
   | Pbytes_load_32 (true (* unsafe *), mode), [[bytes]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Thirty_two Bytes (Some mode) bytes
-        index ~current_region ]
+    [ string_like_load_unsafe ~access_size:Thirty_two Bytes ~boxed:true
+        (Some mode) bytes index ~current_region ]
   | Pstring_load_64 (true (* unsafe *), mode), [[str]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Sixty_four String (Some mode) str
-        index ~current_region ]
+    [ string_like_load_unsafe ~access_size:Sixty_four String ~boxed:true
+        (Some mode) str index ~current_region ]
   | Pbytes_load_64 (true (* unsafe *), mode), [[bytes]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Sixty_four Bytes (Some mode) bytes
-        index ~current_region ]
+    [ string_like_load_unsafe ~access_size:Sixty_four Bytes ~boxed:true
+        (Some mode) bytes index ~current_region ]
   | Pstring_load_128 { unsafe = true; mode }, [[str]; [index]] ->
     [ string_like_load_unsafe
         ~access_size:(One_twenty_eight { aligned = false })
-        String (Some mode) str index ~current_region ]
+        String ~boxed:true (Some mode) str index ~current_region ]
   | Pbytes_load_128 { unsafe = true; mode }, [[str]; [index]] ->
     [ string_like_load_unsafe
         ~access_size:(One_twenty_eight { aligned = false })
-        Bytes (Some mode) str index ~current_region ]
+        Bytes ~boxed:true (Some mode) str index ~current_region ]
   | Pstring_load_16 false (* safe *), [[str]; [index]] ->
-    [ string_like_load_safe ~dbg ~size_int ~access_size:Sixteen String None str
-        index ~current_region ]
+    [ string_like_load_safe ~dbg ~size_int ~access_size:Sixteen String
+        ~boxed:false None str index ~current_region ]
   | Pstring_load_32 (false (* safe *), mode), [[str]; [index]] ->
     [ string_like_load_safe ~dbg ~size_int ~access_size:Thirty_two String
-        (Some mode) str index ~current_region ]
+        (Some mode) ~boxed:true str index ~current_region ]
   | Pstring_load_64 (false (* safe *), mode), [[str]; [index]] ->
     [ string_like_load_safe ~dbg ~size_int ~access_size:Sixty_four String
-        (Some mode) str index ~current_region ]
+        (Some mode) ~boxed:true str index ~current_region ]
   | Pstring_load_128 { unsafe = false; mode }, [[str]; [index]] ->
     [ string_like_load_safe ~dbg ~size_int
         ~access_size:(One_twenty_eight { aligned = false })
-        String (Some mode) str index ~current_region ]
+        String (Some mode) ~boxed:true str index ~current_region ]
   | Pbytes_load_16 false (* safe *), [[bytes]; [index]] ->
-    [ string_like_load_safe ~dbg ~size_int ~access_size:Sixteen Bytes None bytes
-        index ~current_region ]
+    [ string_like_load_safe ~dbg ~size_int ~access_size:Sixteen Bytes
+        ~boxed:false None bytes index ~current_region ]
   | Pbytes_load_32 (false (* safe *), mode), [[bytes]; [index]] ->
     [ string_like_load_safe ~dbg ~size_int ~access_size:Thirty_two Bytes
-        (Some mode) bytes index ~current_region ]
+        ~boxed:true (Some mode) bytes index ~current_region ]
   | Pbytes_load_64 (false (* safe *), mode), [[bytes]; [index]] ->
     [ string_like_load_safe ~dbg ~size_int ~access_size:Sixty_four Bytes
-        (Some mode) bytes index ~current_region ]
+        ~boxed:true (Some mode) bytes index ~current_region ]
   | Pbytes_load_128 { unsafe = false; mode }, [[bytes]; [index]] ->
     [ string_like_load_safe ~dbg ~size_int
         ~access_size:(One_twenty_eight { aligned = false })
-        Bytes (Some mode) bytes index ~current_region ]
+        Bytes ~boxed:true (Some mode) bytes index ~current_region ]
   | Pbytes_set_16 true (* unsafe *), [[bytes]; [index]; [new_value]] ->
-    [bytes_like_set_unsafe ~access_size:Sixteen Bytes bytes index new_value]
+    [ bytes_like_set_unsafe ~access_size:Sixteen Bytes ~boxed:false bytes index
+        new_value ]
   | Pbytes_set_32 true (* unsafe *), [[bytes]; [index]; [new_value]] ->
-    [bytes_like_set_unsafe ~access_size:Thirty_two Bytes bytes index new_value]
+    [ bytes_like_set_unsafe ~access_size:Thirty_two Bytes ~boxed:true bytes
+        index new_value ]
   | Pbytes_set_64 true (* unsafe *), [[bytes]; [index]; [new_value]] ->
-    [bytes_like_set_unsafe ~access_size:Sixty_four Bytes bytes index new_value]
+    [ bytes_like_set_unsafe ~access_size:Sixty_four Bytes ~boxed:true bytes
+        index new_value ]
   | Pbytes_set_128 { unsafe = true }, [[bytes]; [index]; [new_value]] ->
     [ bytes_like_set_unsafe
         ~access_size:(One_twenty_eight { aligned = false })
-        Bytes bytes index new_value ]
+        Bytes ~boxed:true bytes index new_value ]
   | Pbytes_set_16 false (* safe *), [[bytes]; [index]; [new_value]] ->
-    [ bytes_like_set_safe ~dbg ~size_int ~access_size:Sixteen Bytes bytes index
-        new_value ]
+    [ bytes_like_set_safe ~dbg ~size_int ~access_size:Sixteen Bytes ~boxed:false
+        bytes index new_value ]
   | Pbytes_set_32 false (* safe *), [[bytes]; [index]; [new_value]] ->
-    [ bytes_like_set_safe ~dbg ~size_int ~access_size:Thirty_two Bytes bytes
-        index new_value ]
+    [ bytes_like_set_safe ~dbg ~size_int ~access_size:Thirty_two Bytes
+        ~boxed:true bytes index new_value ]
   | Pbytes_set_64 false (* safe *), [[bytes]; [index]; [new_value]] ->
-    [ bytes_like_set_safe ~dbg ~size_int ~access_size:Sixty_four Bytes bytes
-        index new_value ]
+    [ bytes_like_set_safe ~dbg ~size_int ~access_size:Sixty_four Bytes
+        ~boxed:true bytes index new_value ]
   | Pbytes_set_128 { unsafe = false }, [[bytes]; [index]; [new_value]] ->
     [ bytes_like_set_safe ~dbg ~size_int
         ~access_size:(One_twenty_eight { aligned = false })
-        Bytes bytes index new_value ]
+        Bytes ~boxed:true bytes index new_value ]
   | Pisint { variant_only }, [[arg]] ->
     [tag_int (Unary (Is_int { variant_only }, arg))]
   | Pisout, [[arg1]; [arg2]] ->
@@ -1288,10 +1300,11 @@ let convert_lprim ~big_endian (prim : L.primitive) (args : Simple.t list list)
         (match_on_array_set_kind ~array array_set_kind
            (array_set_unsafe ~array ~index ~new_value)) ]
   | Pbytessetu (* unsafe *), [[bytes]; [index]; [new_value]] ->
-    [bytes_like_set_unsafe ~access_size:Eight Bytes bytes index new_value]
-  | Pbytessets, [[bytes]; [index]; [new_value]] ->
-    [ bytes_like_set_safe ~dbg ~size_int ~access_size:Eight Bytes bytes index
+    [ bytes_like_set_unsafe ~access_size:Eight Bytes ~boxed:false bytes index
         new_value ]
+  | Pbytessets, [[bytes]; [index]; [new_value]] ->
+    [ bytes_like_set_safe ~dbg ~size_int ~access_size:Eight Bytes ~boxed:false
+        bytes index new_value ]
   | Poffsetref n, [[block]] ->
     let block_access : P.Block_access_kind.t =
       Values
@@ -1422,62 +1435,66 @@ let convert_lprim ~big_endian (prim : L.primitive) (args : Simple.t list list)
          with an unknown layout should have been removed by Lambda_to_flambda.")
   | Pbigarraydim dimension, [[arg]] ->
     [tag_int (Unary (Bigarray_length { dimension }, arg))]
-  | Pbigstring_load_16 true (* unsafe *), [[big_str]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Sixteen Bigstring None big_str index
-        ~current_region ]
-  | Pbigstring_load_32 (true (* unsafe *), mode), [[big_str]; [index]] ->
-    [ string_like_load_unsafe ~access_size:Thirty_two Bigstring (Some mode)
+  | Pbigstring_load_16 { unsafe = true }, [[big_str]; [index]] ->
+    [ string_like_load_unsafe ~access_size:Sixteen Bigstring ~boxed:false None
         big_str index ~current_region ]
-  | Pbigstring_load_64 (true (* unsafe *), mode), [[big_str]; [index]] ->
+  | Pbigstring_load_32 { unsafe = true; mode; boxed }, [[big_str]; [index]] ->
+    [ string_like_load_unsafe ~access_size:Thirty_two Bigstring (Some mode)
+        ~boxed big_str index ~current_region ]
+  | Pbigstring_load_64 { unsafe = true; mode; boxed }, [[big_str]; [index]] ->
     [ string_like_load_unsafe ~access_size:Sixty_four Bigstring (Some mode)
-        big_str index ~current_region ]
-  | Pbigstring_load_128 { unsafe = true; aligned; mode }, [[big_str]; [index]]
-    ->
+        ~boxed big_str index ~current_region ]
+  | ( Pbigstring_load_128 { unsafe = true; aligned; mode; boxed },
+      [[big_str]; [index]] ) ->
     [ string_like_load_unsafe
         ~access_size:(One_twenty_eight { aligned })
-        Bigstring (Some mode) big_str index ~current_region ]
-  | Pbigstring_load_16 false (* safe *), [[big_str]; [index]] ->
-    [ string_like_load_safe ~dbg ~size_int ~access_size:Sixteen Bigstring None
-        big_str index ~current_region ]
-  | Pbigstring_load_32 (false (* safe *), mode), [[big_str]; [index]] ->
+        Bigstring (Some mode) ~boxed big_str index ~current_region ]
+  | Pbigstring_load_16 { unsafe = false }, [[big_str]; [index]] ->
+    [ string_like_load_safe ~dbg ~size_int ~access_size:Sixteen Bigstring
+        ~boxed:false None big_str index ~current_region ]
+  | Pbigstring_load_32 { unsafe = false; mode; boxed }, [[big_str]; [index]] ->
     [ string_like_load_safe ~dbg ~size_int ~access_size:Thirty_two Bigstring
-        (Some mode) big_str index ~current_region ]
-  | Pbigstring_load_64 (false (* safe *), mode), [[big_str]; [index]] ->
+        (Some mode) ~boxed big_str index ~current_region ]
+  | Pbigstring_load_64 { unsafe = false; mode; boxed }, [[big_str]; [index]] ->
     [ string_like_load_safe ~dbg ~size_int ~access_size:Sixty_four Bigstring
-        (Some mode) big_str index ~current_region ]
-  | Pbigstring_load_128 { unsafe = false; aligned; mode }, [[big_str]; [index]]
-    ->
+        (Some mode) ~boxed big_str index ~current_region ]
+  | ( Pbigstring_load_128 { unsafe = false; aligned; mode; boxed },
+      [[big_str]; [index]] ) ->
     [ string_like_load_safe ~dbg ~size_int
         ~access_size:(One_twenty_eight { aligned })
-        Bigstring (Some mode) big_str index ~current_region ]
-  | Pbigstring_set_16 true (* unsafe *), [[bigstring]; [index]; [new_value]] ->
-    [ bytes_like_set_unsafe ~access_size:Sixteen Bigstring bigstring index
-        new_value ]
-  | Pbigstring_set_32 true (* unsafe *), [[bigstring]; [index]; [new_value]] ->
-    [ bytes_like_set_unsafe ~access_size:Thirty_two Bigstring bigstring index
-        new_value ]
-  | Pbigstring_set_64 true (* unsafe *), [[bigstring]; [index]; [new_value]] ->
-    [ bytes_like_set_unsafe ~access_size:Sixty_four Bigstring bigstring index
-        new_value ]
-  | ( Pbigstring_set_128 { unsafe = true; aligned },
+        Bigstring (Some mode) ~boxed big_str index ~current_region ]
+  | Pbigstring_set_16 { unsafe = true }, [[bigstring]; [index]; [new_value]] ->
+    [ bytes_like_set_unsafe ~access_size:Sixteen Bigstring ~boxed:false
+        bigstring index new_value ]
+  | ( Pbigstring_set_32 { unsafe = true; boxed },
+      [[bigstring]; [index]; [new_value]] ) ->
+    [ bytes_like_set_unsafe ~access_size:Thirty_two Bigstring ~boxed bigstring
+        index new_value ]
+  | ( Pbigstring_set_64 { unsafe = true; boxed },
+      [[bigstring]; [index]; [new_value]] ) ->
+    [ bytes_like_set_unsafe ~access_size:Sixty_four Bigstring ~boxed bigstring
+        index new_value ]
+  | ( Pbigstring_set_128 { unsafe = true; aligned; boxed },
       [[bigstring]; [index]; [new_value]] ) ->
     [ bytes_like_set_unsafe
         ~access_size:(One_twenty_eight { aligned })
-        Bigstring bigstring index new_value ]
-  | Pbigstring_set_16 false (* safe *), [[bigstring]; [index]; [new_value]] ->
+        Bigstring ~boxed bigstring index new_value ]
+  | Pbigstring_set_16 { unsafe = false }, [[bigstring]; [index]; [new_value]] ->
     [ bytes_like_set_safe ~dbg ~size_int ~access_size:Sixteen Bigstring
-        bigstring index new_value ]
-  | Pbigstring_set_32 false (* safe *), [[bigstring]; [index]; [new_value]] ->
+        ~boxed:false bigstring index new_value ]
+  | ( Pbigstring_set_32 { unsafe = false; boxed },
+      [[bigstring]; [index]; [new_value]] ) ->
     [ bytes_like_set_safe ~dbg ~size_int ~access_size:Thirty_two Bigstring
-        bigstring index new_value ]
-  | Pbigstring_set_64 false (* safe *), [[bigstring]; [index]; [new_value]] ->
+        ~boxed bigstring index new_value ]
+  | ( Pbigstring_set_64 { unsafe = false; boxed },
+      [[bigstring]; [index]; [new_value]] ) ->
     [ bytes_like_set_safe ~dbg ~size_int ~access_size:Sixty_four Bigstring
-        bigstring index new_value ]
-  | ( Pbigstring_set_128 { unsafe = false; aligned },
+        ~boxed bigstring index new_value ]
+  | ( Pbigstring_set_128 { unsafe = false; aligned; boxed },
       [[bigstring]; [index]; [new_value]] ) ->
     [ bytes_like_set_safe ~dbg ~size_int
         ~access_size:(One_twenty_eight { aligned })
-        Bigstring bigstring index new_value ]
+        Bigstring ~boxed bigstring index new_value ]
   | Pcompare_ints, [[i1]; [i2]] ->
     [ tag_int
         (Binary
diff --git a/ocaml/lambda/lambda.ml b/ocaml/lambda/lambda.ml
index 13db7d21382..d517c4e73c4 100644
--- a/ocaml/lambda/lambda.ml
+++ b/ocaml/lambda/lambda.ml
@@ -238,14 +238,15 @@ type primitive =
   | Pbytes_set_128 of { unsafe : bool }
   (* load/set 16,32,64,128 bits from a
      (char, int8_unsigned_elt, c_layout) Bigarray.Array1.t : (unsafe) *)
-  | Pbigstring_load_16 of bool
-  | Pbigstring_load_32 of bool * alloc_mode
-  | Pbigstring_load_64 of bool * alloc_mode
-  | Pbigstring_load_128 of { aligned : bool; unsafe : bool; mode: alloc_mode }
-  | Pbigstring_set_16 of bool
-  | Pbigstring_set_32 of bool
-  | Pbigstring_set_64 of bool
-  | Pbigstring_set_128 of { aligned : bool; unsafe : bool }
+  | Pbigstring_load_16 of { unsafe : bool }
+  | Pbigstring_load_32 of { unsafe : bool; mode: alloc_mode; boxed : bool }
+  | Pbigstring_load_64 of { unsafe : bool; mode: alloc_mode; boxed : bool }
+  | Pbigstring_load_128 of { aligned : bool; unsafe : bool; mode: alloc_mode;
+      boxed : bool }
+  | Pbigstring_set_16 of { unsafe : bool }
+  | Pbigstring_set_32 of { unsafe : bool; boxed : bool }
+  | Pbigstring_set_64 of { unsafe : bool; boxed : bool }
+  | Pbigstring_set_128 of { aligned : bool; unsafe : bool; boxed : bool }
   (* Compile time constants *)
   | Pctconst of compile_time_constant
   (* byte swap *)
@@ -745,8 +746,18 @@ let layout_unboxed_nativeint = Punboxed_int Pnativeint
 let layout_unboxed_int32 = Punboxed_int Pint32
 let layout_unboxed_int64 = Punboxed_int Pint64
 let layout_string = Pvalue Pgenval
+let layout_unboxed_int ubi = Punboxed_int ubi
 let layout_boxedint bi = Pvalue (Pboxedintval bi)
 
+let layout_unboxed_vector (v : Primitive.boxed_vector) =
+  match v with
+  | Pvec128 Int8x16 -> Punboxed_vector (Pvec128 Int8x16)
+  | Pvec128 Int16x8 -> Punboxed_vector (Pvec128 Int16x8)
+  | Pvec128 Int32x4 -> Punboxed_vector (Pvec128 Int32x4)
+  | Pvec128 Int64x2 -> Punboxed_vector (Pvec128 Int64x2)
+  | Pvec128 Float32x4 -> Punboxed_vector (Pvec128 Float32x4)
+  | Pvec128 Float64x2 -> Punboxed_vector (Pvec128 Float64x2)
+
 let layout_boxed_vector : Primitive.boxed_vector -> layout = function
   | Pvec128 Int8x16 -> Pvalue (Pboxedvectorval (Pvec128 Int8x16))
   | Pvec128 Int16x8 -> Pvalue (Pboxedvectorval (Pvec128 Int16x8))
@@ -1557,8 +1568,12 @@ let primitive_may_allocate : primitive -> alloc_mode option = function
   | Pget_header m -> Some m
   | Pbytes_set_16 _ | Pbytes_set_32 _ | Pbytes_set_64 _ | Pbytes_set_128 _ -> None
   | Pbigstring_load_16 _ -> None
-  | Pbigstring_load_32 (_,m) | Pbigstring_load_64 (_,m)
-  | Pbigstring_load_128 { mode = m; _ } -> Some m
+  | Pbigstring_load_32 { mode = m; boxed = true; _ }
+  | Pbigstring_load_64 { mode = m; boxed = true; _ }
+  | Pbigstring_load_128 { mode = m; boxed = true; _ } -> Some m
+  | Pbigstring_load_32 { boxed = false; _ }
+  | Pbigstring_load_64 { boxed = false; _ }
+  | Pbigstring_load_128 { boxed = false; _ } -> None
   | Pbigstring_set_16 _ | Pbigstring_set_32 _
   | Pbigstring_set_64 _ | Pbigstring_set_128 _ -> None
   | Pctconst _ -> None
@@ -1669,12 +1684,19 @@ let primitive_result_layout (p : primitive) =
   | Pbbswap (bi, _) | Pbox_int (bi, _) ->
       layout_boxedint bi
   | Punbox_int bi -> Punboxed_int bi
-  | Pstring_load_32 _ | Pbytes_load_32 _ | Pbigstring_load_32 _ ->
+  | Pstring_load_32 _ | Pbytes_load_32 _
+  | Pbigstring_load_32 { boxed = true; _ } ->
       layout_boxedint Pint32
-  | Pstring_load_64 _ | Pbytes_load_64 _ | Pbigstring_load_64 _ ->
+  | Pstring_load_64 _ | Pbytes_load_64 _
+  | Pbigstring_load_64 { boxed = true; _ } ->
       layout_boxedint Pint64
-  | Pstring_load_128 _ | Pbytes_load_128 _ | Pbigstring_load_128 _ ->
+  | Pstring_load_128 _ | Pbytes_load_128 _
+  | Pbigstring_load_128 { boxed = true; _ } ->
       layout_boxed_vector (Pvec128 Int8x16)
+  | Pbigstring_load_32 { boxed = false; _ } -> layout_unboxed_int Pint32
+  | Pbigstring_load_64 { boxed = false; _ } -> layout_unboxed_int Pint64
+  | Pbigstring_load_128 { boxed = false; _ } ->
+      layout_unboxed_vector (Pvec128 Int8x16)
   | Pbigarrayref (_, _, kind, _) ->
       begin match kind with
       | Pbigarray_unknown -> layout_any_value
diff --git a/ocaml/lambda/lambda.mli b/ocaml/lambda/lambda.mli
index 16e63492c8f..04abe07df9e 100644
--- a/ocaml/lambda/lambda.mli
+++ b/ocaml/lambda/lambda.mli
@@ -200,14 +200,15 @@ type primitive =
   | Pbytes_set_128 of { unsafe : bool }
   (* load/set 16,32,64,128 bits from a
      (char, int8_unsigned_elt, c_layout) Bigarray.Array1.t : (unsafe) *)
-  | Pbigstring_load_16 of bool
-  | Pbigstring_load_32 of bool * alloc_mode
-  | Pbigstring_load_64 of bool * alloc_mode
-  | Pbigstring_load_128 of { aligned : bool; unsafe : bool; mode: alloc_mode }
-  | Pbigstring_set_16 of bool
-  | Pbigstring_set_32 of bool
-  | Pbigstring_set_64 of bool
-  | Pbigstring_set_128 of { aligned : bool; unsafe : bool }
+  | Pbigstring_load_16 of { unsafe : bool }
+  | Pbigstring_load_32 of { unsafe : bool; mode: alloc_mode; boxed : bool }
+  | Pbigstring_load_64 of { unsafe : bool; mode: alloc_mode; boxed : bool }
+  | Pbigstring_load_128 of { aligned : bool; unsafe : bool; mode: alloc_mode;
+      boxed : bool }
+  | Pbigstring_set_16 of { unsafe : bool }
+  | Pbigstring_set_32 of { unsafe : bool; boxed : bool }
+  | Pbigstring_set_64 of { unsafe : bool; boxed : bool }
+  | Pbigstring_set_128 of { aligned : bool; unsafe : bool; boxed : bool }
   (* Compile time constants *)
   | Pctconst of compile_time_constant
   (* byte swap *)
diff --git a/ocaml/lambda/printlambda.ml b/ocaml/lambda/printlambda.ml
index a820284a176..3d860a53a22 100644
--- a/ocaml/lambda/printlambda.ml
+++ b/ocaml/lambda/printlambda.ml
@@ -550,13 +550,13 @@ let primitive ppf = function
      fprintf ppf "bytes.unsafe_unaligned_set128"
   | Pbytes_set_128 {unsafe = false} ->
      fprintf ppf "bytes.unaligned_set128"
-  | Pbigstring_load_16(unsafe) ->
+  | Pbigstring_load_16 { unsafe } ->
      if unsafe then fprintf ppf "bigarray.array1.unsafe_get16"
      else fprintf ppf "bigarray.array1.get16"
-  | Pbigstring_load_32(unsafe,m) ->
+  | Pbigstring_load_32 { unsafe; mode = m } ->
      if unsafe then fprintf ppf "bigarray.array1.unsafe_get32%s" (alloc_kind m)
      else fprintf ppf "bigarray.array1.get32%s" (alloc_kind m)
-  | Pbigstring_load_64(unsafe,m) ->
+  | Pbigstring_load_64 { unsafe; mode = m } ->
      if unsafe then fprintf ppf "bigarray.array1.unsafe_get64%s" (alloc_kind m)
      else fprintf ppf "bigarray.array1.get64%s" (alloc_kind m)
   | Pbigstring_load_128 {unsafe = true; aligned = false; mode} ->
@@ -567,13 +567,13 @@ let primitive ppf = function
      fprintf ppf "bigarray.array1.unsafe_aligned_get128%s" (alloc_kind mode)
   | Pbigstring_load_128 {unsafe = false; aligned = true; mode} ->
      fprintf ppf "bigarray.array1.aligned_get128%s" (alloc_kind mode)
-  | Pbigstring_set_16(unsafe) ->
+  | Pbigstring_set_16 { unsafe } ->
      if unsafe then fprintf ppf "bigarray.array1.unsafe_set16"
      else fprintf ppf "bigarray.array1.set16"
-  | Pbigstring_set_32(unsafe) ->
+  | Pbigstring_set_32 { unsafe } ->
      if unsafe then fprintf ppf "bigarray.array1.unsafe_set32"
      else fprintf ppf "bigarray.array1.set32"
-  | Pbigstring_set_64(unsafe) ->
+  | Pbigstring_set_64 { unsafe } ->
      if unsafe then fprintf ppf "bigarray.array1.unsafe_set64"
      else fprintf ppf "bigarray.array1.set64"
   | Pbigstring_set_128 {unsafe = true; aligned = false} ->
diff --git a/ocaml/lambda/translprim.ml b/ocaml/lambda/translprim.ml
index 81863a604db..4561cb34c60 100644
--- a/ocaml/lambda/translprim.ml
+++ b/ocaml/lambda/translprim.ml
@@ -399,34 +399,97 @@ let lookup_primitive loc poly pos p =
       Primitive ((Pbytes_set_128 {unsafe = false}), 3)
     | "%caml_bytes_setu128u" ->
       Primitive ((Pbytes_set_128 {unsafe = true}), 3)
-    | "%caml_bigstring_get16" -> Primitive ((Pbigstring_load_16(false)), 2)
-    | "%caml_bigstring_get16u" -> Primitive ((Pbigstring_load_16(true)), 2)
-    | "%caml_bigstring_get32" -> Primitive ((Pbigstring_load_32(false, mode)), 2)
-    | "%caml_bigstring_get32u" -> Primitive ((Pbigstring_load_32(true, mode)), 2)
-    | "%caml_bigstring_get64" -> Primitive ((Pbigstring_load_64(false, mode)), 2)
-    | "%caml_bigstring_get64u" -> Primitive ((Pbigstring_load_64(true, mode)), 2)
+    | "%caml_bigstring_get16" ->
+      Primitive ((Pbigstring_load_16 { unsafe = false }), 2)
+    | "%caml_bigstring_get16u" ->
+      Primitive ((Pbigstring_load_16 { unsafe = true }), 2)
+    | "%caml_bigstring_get32" ->
+      Primitive ((Pbigstring_load_32 { unsafe = false; mode; boxed = true }), 2)
+    | "%caml_bigstring_get32u" ->
+      Primitive ((Pbigstring_load_32 { unsafe = true; mode; boxed = true }), 2)
+    | "%caml_bigstring_get64" ->
+      Primitive ((Pbigstring_load_64 { unsafe = false; mode; boxed = true }), 2)
+    | "%caml_bigstring_get64u" ->
+      Primitive ((Pbigstring_load_64 { unsafe = true; mode; boxed = true }), 2)
     | "%caml_bigstring_getu128" ->
-      Primitive ((Pbigstring_load_128 {aligned = false; unsafe = false; mode}), 2)
+      Primitive ((Pbigstring_load_128 {aligned = false; unsafe = false; mode;
+        boxed = true }), 2)
     | "%caml_bigstring_getu128u" ->
-      Primitive ((Pbigstring_load_128 {aligned = false; unsafe = true; mode}), 2)
+      Primitive ((Pbigstring_load_128 {aligned = false; unsafe = true; mode;
+        boxed = true }), 2)
     | "%caml_bigstring_geta128" ->
-      Primitive ((Pbigstring_load_128 {aligned = true; unsafe = false; mode}), 2)
+      Primitive ((Pbigstring_load_128 {aligned = true; unsafe = false; mode;
+        boxed = true }), 2)
     | "%caml_bigstring_geta128u" ->
-      Primitive ((Pbigstring_load_128 {aligned = true; unsafe = true; mode}), 2)
-    | "%caml_bigstring_set16" -> Primitive ((Pbigstring_set_16(false)), 3)
-    | "%caml_bigstring_set16u" -> Primitive ((Pbigstring_set_16(true)), 3)
-    | "%caml_bigstring_set32" -> Primitive ((Pbigstring_set_32(false)), 3)
-    | "%caml_bigstring_set32u" -> Primitive ((Pbigstring_set_32(true)), 3)
-    | "%caml_bigstring_set64" -> Primitive ((Pbigstring_set_64(false)), 3)
-    | "%caml_bigstring_set64u" -> Primitive ((Pbigstring_set_64(true)), 3)
+      Primitive ((Pbigstring_load_128 {aligned = true; unsafe = true; mode;
+        boxed = true }), 2)
+    | "%caml_bigstring_set16" ->
+      Primitive ((Pbigstring_set_16 { unsafe = false }), 3)
+    | "%caml_bigstring_set16u" ->
+      Primitive ((Pbigstring_set_16 { unsafe = true }), 3)
+    | "%caml_bigstring_set32" ->
+      Primitive ((Pbigstring_set_32 { unsafe = false; boxed = true }), 3)
+    | "%caml_bigstring_set32u" ->
+      Primitive ((Pbigstring_set_32 { unsafe = true; boxed = true }), 3)
+    | "%caml_bigstring_set64" ->
+      Primitive ((Pbigstring_set_64 { unsafe = false; boxed = true }), 3)
+    | "%caml_bigstring_set64u" ->
+      Primitive ((Pbigstring_set_64 { unsafe = true; boxed = true }), 3)
     | "%caml_bigstring_setu128" ->
-      Primitive ((Pbigstring_set_128 {aligned = false; unsafe = false}), 3)
+      Primitive ((Pbigstring_set_128 {aligned = false; unsafe = false;
+        boxed = true}), 3)
     | "%caml_bigstring_setu128u" ->
-      Primitive ((Pbigstring_set_128 {aligned = false; unsafe = true}), 3)
+      Primitive ((Pbigstring_set_128 {aligned = false; unsafe = true;
+        boxed = true}), 3)
     | "%caml_bigstring_seta128" ->
-      Primitive ((Pbigstring_set_128 {aligned = true; unsafe = false}), 3)
+      Primitive ((Pbigstring_set_128 {aligned = true; unsafe = false;
+        boxed = true}), 3)
     | "%caml_bigstring_seta128u" ->
-      Primitive ((Pbigstring_set_128 {aligned = true; unsafe = true}), 3)
+      Primitive ((Pbigstring_set_128 {aligned = true; unsafe = true;
+        boxed = true}), 3)
+    | "%caml_bigstring_get32#" ->
+      Primitive ((Pbigstring_load_32 { unsafe = false; mode; boxed = false }),
+        2)
+    | "%caml_bigstring_get32u#" ->
+      Primitive ((Pbigstring_load_32 { unsafe = true; mode; boxed = false }),
+        2)
+    | "%caml_bigstring_get64#" ->
+      Primitive ((Pbigstring_load_64 { unsafe = false; mode; boxed = false }),
+        2)
+    | "%caml_bigstring_get64u#" ->
+      Primitive ((Pbigstring_load_64 { unsafe = true; mode; boxed = false }), 2)
+    | "%caml_bigstring_getu128#" ->
+      Primitive ((Pbigstring_load_128 {aligned = false; unsafe = false; mode;
+        boxed = false }), 2)
+    | "%caml_bigstring_getu128u#" ->
+      Primitive ((Pbigstring_load_128 {aligned = false; unsafe = true; mode;
+        boxed = false }), 2)
+    | "%caml_bigstring_geta128#" ->
+      Primitive ((Pbigstring_load_128 {aligned = true; unsafe = false; mode;
+        boxed = false }), 2)
+    | "%caml_bigstring_geta128u#" ->
+      Primitive ((Pbigstring_load_128 {aligned = true; unsafe = true; mode;
+        boxed = false }), 2)
+    | "%caml_bigstring_set32#" ->
+      Primitive ((Pbigstring_set_32 { unsafe = false; boxed = false }), 3)
+    | "%caml_bigstring_set32u#" ->
+      Primitive ((Pbigstring_set_32 { unsafe = true; boxed = false }), 3)
+    | "%caml_bigstring_set64#" ->
+      Primitive ((Pbigstring_set_64 { unsafe = false; boxed = false }), 3)
+    | "%caml_bigstring_set64u#" ->
+      Primitive ((Pbigstring_set_64 { unsafe = true; boxed = false }), 3)
+    | "%caml_bigstring_setu128#" ->
+      Primitive ((Pbigstring_set_128 {aligned = false; unsafe = false;
+        boxed = false}), 3)
+    | "%caml_bigstring_setu128u#" ->
+      Primitive ((Pbigstring_set_128 {aligned = false; unsafe = true;
+        boxed = false}), 3)
+    | "%caml_bigstring_seta128#" ->
+      Primitive ((Pbigstring_set_128 {aligned = true; unsafe = false;
+        boxed = false}), 3)
+    | "%caml_bigstring_seta128u#" ->
+      Primitive ((Pbigstring_set_128 {aligned = true; unsafe = true;
+        boxed = false}), 3)
     | "%bswap16" -> Primitive (Pbswap16, 1)
     | "%bswap_int32" -> Primitive ((Pbbswap(Pint32, mode)), 1)
     | "%bswap_int64" -> Primitive ((Pbbswap(Pint64, mode)), 1)
diff --git a/ocaml/middle_end/convert_primitives.ml b/ocaml/middle_end/convert_primitives.ml
index d186ebb8a0f..531669181de 100644
--- a/ocaml/middle_end/convert_primitives.ml
+++ b/ocaml/middle_end/convert_primitives.ml
@@ -145,17 +145,17 @@ let convert (prim : Lambda.primitive) : Clambda_primitives.primitive =
       Pbytes_set (Thirty_two, convert_unsafety is_unsafe)
   | Pbytes_set_64 is_unsafe ->
       Pbytes_set (Sixty_four, convert_unsafety is_unsafe)
-  | Pbigstring_load_16 is_unsafe ->
+  | Pbigstring_load_16 { unsafe = is_unsafe } ->
       Pbigstring_load (Sixteen, convert_unsafety is_unsafe, Lambda.alloc_heap)
-  | Pbigstring_load_32 (is_unsafe, m) ->
+  | Pbigstring_load_32 { unsafe = is_unsafe; mode = m; boxed = true } ->
       Pbigstring_load (Thirty_two, convert_unsafety is_unsafe, m)
-  | Pbigstring_load_64 (is_unsafe, m) ->
+  | Pbigstring_load_64 { unsafe = is_unsafe; mode = m; boxed = true } ->
       Pbigstring_load (Sixty_four, convert_unsafety is_unsafe, m)
-  | Pbigstring_set_16 is_unsafe ->
+  | Pbigstring_set_16 { unsafe = is_unsafe } ->
       Pbigstring_set (Sixteen, convert_unsafety is_unsafe)
-  | Pbigstring_set_32 is_unsafe ->
+  | Pbigstring_set_32 { unsafe = is_unsafe; boxed = true } ->
       Pbigstring_set (Thirty_two, convert_unsafety is_unsafe)
-  | Pbigstring_set_64 is_unsafe ->
+  | Pbigstring_set_64 { unsafe = is_unsafe; boxed = true } ->
       Pbigstring_set (Sixty_four, convert_unsafety is_unsafe)
   | Pbigarraydim dim -> Pbigarraydim dim
   | Pbswap16 -> Pbswap16
@@ -194,6 +194,10 @@ let convert (prim : Lambda.primitive) : Clambda_primitives.primitive =
   | Pgetpredef _
   | Parray_to_iarray
   | Parray_of_iarray
+  | Pbigstring_load_32 _
+  | Pbigstring_set_32 _
+  | Pbigstring_load_64 _
+  | Pbigstring_set_64 _
   | Pbigstring_load_128 _
   | Pbigstring_set_128 _
   | Pstring_load_128 _