diff --git a/sledge/cli/frontend.ml b/sledge/cli/frontend.ml
index e730d9405..e427dac58 100644
--- a/sledge/cli/frontend.ml
+++ b/sledge/cli/frontend.ml
@@ -256,15 +256,27 @@ let rec xlate_type : x -> Llvm.lltype -> Typ.t =
       | Struct ->
           let llelts = Llvm.struct_element_types llt in
           let len = Array.length llelts in
+          let fld_off i =
+            match
+              Int64.unsigned_to_int
+                (Llvm_target.DataLayout.offset_of_element llt i
+                   x.lldatalayout)
+            with
+            | Some i -> i
+            | None -> todo "offset too large: %a" pp_lltype llt ()
+          in
           if Llvm.is_literal llt then
             let elts =
-              IArray.map ~f:(xlate_type x) (IArray.of_array llelts)
+              IArray.mapi
+                ~f:(fun i elt -> (fld_off i, xlate_type x elt))
+                (IArray.of_array llelts)
             in
             Typ.tuple elts ~bits ~byts
           else
             let name = struct_name llt in
             let elts =
-              IArray.init len ~f:(fun i -> lazy (xlate_type x llelts.(i)))
+              IArray.init len ~f:(fun i ->
+                  lazy (fld_off i, xlate_type x llelts.(i)) )
             in
             Typ.struct_ ~name elts ~bits ~byts
       | Function -> fail "expected to be unsized: %a" pp_lltype llt ()
@@ -633,7 +645,7 @@ and xlate_opcode stk :
           match (typ : Typ.t) with
           | Tuple {elts} | Struct {elts} ->
               ( Exp.select typ rcd indices.(i)
-              , IArray.get elts indices.(i)
+              , snd (IArray.get elts indices.(i))
               , Exp.update typ ~rcd indices.(i) )
           | Array {elt} ->
               ( Exp.select typ rcd indices.(i)
@@ -822,7 +834,9 @@ let landingpad_typs : x -> Llvm.llvalue -> Typ.t * Typ.t * Llvm.lltype =
 let exception_typs =
   let pi8 = Typ.pointer ~elt:Typ.byt in
   let i32 = Typ.integer ~bits:32 ~byts:4 in
-  let exc = Typ.tuple (IArray.of_array [|pi8; i32|]) ~bits:96 ~byts:12 in
+  let exc =
+    Typ.tuple (IArray.of_array [|(0, pi8); (8, i32)|]) ~bits:96 ~byts:12
+  in
   (pi8, i32, exc)
 
 (** Translate a control transfer from instruction [instr] to block [dst] to
diff --git a/sledge/nonstdlib/array.ml b/sledge/nonstdlib/array.ml
index 4966a71a7..57be2e838 100644
--- a/sledge/nonstdlib/array.ml
+++ b/sledge/nonstdlib/array.ml
@@ -28,6 +28,7 @@ let of_list_rev = function
 
 let is_empty = function [||] -> true | _ -> false
 let map xs ~f = map ~f xs
+let mapi xs ~f = mapi ~f xs
 let map_endo xs ~f = map_endo map xs ~f
 
 let reduce_adjacent xs ~f =
diff --git a/sledge/nonstdlib/array.mli b/sledge/nonstdlib/array.mli
index f449ddf1d..dfeb69667 100644
--- a/sledge/nonstdlib/array.mli
+++ b/sledge/nonstdlib/array.mli
@@ -13,6 +13,7 @@ type 'a t = 'a array [@@deriving compare, equal, sexp]
 val of_ : 'a -> 'a t
 val of_list_rev : 'a list -> 'a t
 val map : 'a t -> f:('a -> 'b) -> 'b t
+val mapi : 'a t -> f:(int -> 'a -> 'b) -> 'b t
 
 val map_endo : 'a t -> f:('a -> 'a) -> 'a t
 (** Like [map], but specialized to require [f] to be an endofunction, which
diff --git a/sledge/nonstdlib/iArray.mli b/sledge/nonstdlib/iArray.mli
index 94ad7576c..f08db3781 100644
--- a/sledge/nonstdlib/iArray.mli
+++ b/sledge/nonstdlib/iArray.mli
@@ -41,6 +41,7 @@ val init : int -> f:(int -> 'a) -> 'a t
 val sub : 'a t -> pos:int -> len:int -> 'a t
 val concat : 'a t list -> 'a t
 val map : 'a t -> f:('a -> 'b) -> 'b t
+val mapi : 'a t -> f:(int -> 'a -> 'b) -> 'b t
 
 val map_endo : 'a t -> f:('a -> 'a) -> 'a t
 (** Like map, but specialized to require [f] to be an endofunction, which
diff --git a/sledge/src/llair/exp.ml b/sledge/src/llair/exp.ml
index 6f2adc27b..3af26601c 100644
--- a/sledge/src/llair/exp.ml
+++ b/sledge/src/llair/exp.ml
@@ -208,7 +208,7 @@ let rec invariant exp =
       match typ with
       | Tuple {elts} | Struct {elts} ->
           assert (valid_idx idx elts) ;
-          assert (Typ.castable (IArray.get elts idx) (typ_of elt))
+          assert (Typ.castable (snd (IArray.get elts idx)) (typ_of elt))
       | Array {elt= typ_elt} -> assert (Typ.castable typ_elt (typ_of elt))
       | _ -> assert false )
   | Ap2 (op, typ, x, y) -> (
@@ -237,7 +237,7 @@ let rec invariant exp =
     | Tuple {elts} | Struct {elts} ->
         assert (IArray.length elts = IArray.length args) ;
         assert (
-          IArray.for_all2_exn elts args ~f:(fun typ arg ->
+          IArray.for_all2_exn elts args ~f:(fun (_, typ) arg ->
               Typ.castable typ (typ_of arg) ) )
     | _ -> assert false )
   | RecRecord _ -> ()
@@ -253,7 +253,7 @@ and typ_of exp =
   | Ap1 (Select idx, typ, _) -> (
     match typ with
     | Array {elt} -> elt
-    | Tuple {elts} | Struct {elts} -> IArray.get elts idx
+    | Tuple {elts} | Struct {elts} -> snd (IArray.get elts idx)
     | _ -> violates invariant exp )
   | Ap2
       ( (Eq | Dq | Gt | Ge | Lt | Le | Ugt | Uge | Ult | Ule | Ord | Uno)
diff --git a/sledge/src/llair/typ.ml b/sledge/src/llair/typ.ml
index 9e2f800c4..3a6adffc0 100644
--- a/sledge/src/llair/typ.ml
+++ b/sledge/src/llair/typ.ml
@@ -13,10 +13,10 @@ type t =
   | Float of {bits: int; byts: int; enc: [`IEEE | `Extended | `Pair]}
   | Pointer of {elt: t}
   | Array of {elt: t; len: int; bits: int; byts: int}
-  | Tuple of {elts: t iarray; bits: int; byts: int}
+  | Tuple of {elts: (int * t) iarray; bits: int; byts: int}
   | Struct of
       { name: string
-      ; elts: t iarray (* possibly cyclic, name unique *)
+      ; elts: (int * t) iarray (* possibly cyclic, name unique *)
             [@compare.ignore] [@equal.ignore] [@sexp_drop_if fun _ -> true]
       ; bits: int
       ; byts: int }
@@ -42,14 +42,15 @@ let rec pp fs typ =
       pf "f%i%s" bits enc_str
   | Pointer {elt} -> pf "%a*" pp elt
   | Array {elt; len} -> pf "[%i x %a]" len pp elt
-  | Tuple {elts} -> pf "{ @[%a@] }" pps elts
+  | Tuple {elts} -> pf "{ @[%a@] }" pp_flds elts
   | Struct {name} | Opaque {name} -> pf "%%%s" name
 
 and pps fs typs = IArray.pp ",@ " pp fs typs
+and pp_flds fs flds = IArray.pp ",@ " (fun fs (_, fld) -> pp fs fld) fs flds
 
 let pp_defn fs = function
   | Struct {name; elts} ->
-      Format.fprintf fs "@[<2>%%%s =@ @[{ %a@] }@]" name pps elts
+      Format.fprintf fs "@[<2>%%%s =@ @[{ %a@] }@]" name pp_flds elts
   | Opaque {name} -> Format.fprintf fs "@[<2>%%%s =@ opaque@]" name
   | typ -> pp fs typ
 
@@ -67,7 +68,8 @@ let invariant t =
       assert (Option.for_all ~f:is_sized return) ;
       assert (IArray.for_all ~f:is_sized args)
   | Array {elt} -> assert (is_sized elt)
-  | Tuple {elts} | Struct {elts} -> assert (IArray.for_all ~f:is_sized elts)
+  | Tuple {elts} | Struct {elts} ->
+      assert (IArray.for_all ~f:(fun (_, t) -> is_sized t) elts)
   | Integer {bits} | Float {bits} -> assert (bits > 0)
   | Pointer _ | Opaque _ -> assert true
 
@@ -93,7 +95,7 @@ let struct_ =
     | None ->
         (* Add placeholder defn to prevent computing [elts] in calls to
            [struct] from [elts] for recursive occurrences of [name]. *)
-        let elts = Array.make (IArray.length elt_thks) dummy_typ in
+        let elts = Array.make (IArray.length elt_thks) (0, dummy_typ) in
         let typ = Struct {name; elts= IArray.of_array elts; bits; byts} in
         String.Tbl.set defns ~key:name ~data:typ ;
         IArray.iteri elt_thks ~f:(fun i (lazy elt) -> elts.(i) <- elt) ;
diff --git a/sledge/src/llair/typ.mli b/sledge/src/llair/typ.mli
index c25f0601a..1944b25e2 100644
--- a/sledge/src/llair/typ.mli
+++ b/sledge/src/llair/typ.mli
@@ -16,10 +16,11 @@ type t = private
   | Pointer of {elt: t}  (** Pointer to element type. *)
   | Array of {elt: t; len: int; bits: int; byts: int}
       (** Statically-sized array of [len] elements of type [elt]. *)
-  | Tuple of {elts: t iarray; bits: int; byts: int}
+  | Tuple of {elts: (int * t) iarray; bits: int; byts: int}
       (** Anonymous aggregate of heterogeneous types. *)
-  | Struct of {name: string; elts: t iarray; bits: int; byts: int}
-      (** Uniquely named aggregate of heterogeneous types. Every cycle of
+  | Struct of {name: string; elts: (int * t) iarray; bits: int; byts: int}
+      (** Uniquely named aggregate of heterogeneous types. Elements are
+          specified by their byte offset and their type. Every cycle of
           recursive types contains a [Struct]. NOTE: recursive [Struct]
           types are represented by cyclic values. *)
   | Opaque of {name: string}
@@ -43,8 +44,11 @@ val integer : bits:int -> byts:int -> t
 val float : bits:int -> byts:int -> enc:[`Extended | `IEEE | `Pair] -> t
 val pointer : elt:t -> t
 val array : elt:t -> len:int -> bits:int -> byts:int -> t
-val tuple : t iarray -> bits:int -> byts:int -> t
-val struct_ : name:string -> bits:int -> byts:int -> t lazy_t iarray -> t
+val tuple : (int * t) iarray -> bits:int -> byts:int -> t
+
+val struct_ :
+  name:string -> bits:int -> byts:int -> (int * t) lazy_t iarray -> t
+
 val opaque : name:string -> t
 
 (** Queries *)