diff --git a/interpreter/binary/decode.ml b/interpreter/binary/decode.ml
index 0686703613..5a7aa6f5c5 100644
--- a/interpreter/binary/decode.ml
+++ b/interpreter/binary/decode.ml
@@ -5,12 +5,11 @@ type stream =
   name : string;
   bytes : string;
   pos : int ref;
-  need_data_count : bool ref;
 }
 
 exception EOS
 
-let stream name bs = {name; bytes = bs; pos = ref 0; need_data_count = ref false}
+let stream name bs = {name; bytes = bs; pos = ref 0}
 
 let len s = String.length s.bytes
 let pos s = !(s.pos)
@@ -208,9 +207,8 @@ let misc_instr s =
   | 0x08 ->
     let x = at var s in
     zero_flag s;
-    s.need_data_count := true;
     memory_init x
-  | 0x09 -> s.need_data_count := true; data_drop (at var s)
+  | 0x09 -> data_drop (at var s)
   | 0x0a -> zero_flag s; zero_flag s; memory_copy
   | 0x0b -> zero_flag s; memory_fill
   | 0x0c ->
@@ -727,7 +725,8 @@ let module_ s =
     s (len s) "function and code section have inconsistent lengths";
   require (data_count = None || data_count = Some (Lib.List32.length datas))
     s (len s) "data count and data section have inconsistent lengths";
-  require (not !(s.need_data_count) || data_count <> None)
+  require (data_count <> None ||
+    List.for_all Free.(fun f -> (func f).datas = Set.empty) func_bodies)
     s (len s) "data count section required";
   let funcs =
     List.map2 Source.(fun t f -> {f.it with ftype = t} @@ f.at)
diff --git a/interpreter/binary/encode.ml b/interpreter/binary/encode.ml
index 87abace93d..6d60a74b21 100644
--- a/interpreter/binary/encode.ml
+++ b/interpreter/binary/encode.ml
@@ -515,8 +515,8 @@ let encode m =
       section 11 (vec memory_segment) datas (datas <> [])
 
     (* Data count section *)
-    let data_count_section datas =
-      section 12 len (List.length datas) true
+    let data_count_section datas m =
+      section 12 len (List.length datas) Free.((module_ m).datas <> Set.empty)
 
     (* Module *)
 
@@ -532,7 +532,7 @@ let encode m =
       export_section m.it.exports;
       start_section m.it.start;
       elem_section m.it.elems;
-      data_count_section m.it.datas;
+      data_count_section m.it.datas m;
       code_section m.it.funcs;
       data_section m.it.datas
   end
diff --git a/interpreter/syntax/free.ml b/interpreter/syntax/free.ml
new file mode 100644
index 0000000000..46727fc634
--- /dev/null
+++ b/interpreter/syntax/free.ml
@@ -0,0 +1,141 @@
+open Source
+open Ast
+
+module Set = Set.Make(Int32)
+
+type t =
+{
+  types : Set.t;
+  globals : Set.t;
+  tables : Set.t;
+  memories : Set.t;
+  funcs : Set.t;
+  elems : Set.t;
+  datas : Set.t;
+  locals : Set.t;
+  labels : Set.t;
+}
+
+let empty : t =
+{
+  types = Set.empty;
+  globals = Set.empty;
+  tables = Set.empty;
+  memories = Set.empty;
+  funcs = Set.empty;
+  elems = Set.empty;
+  datas = Set.empty;
+  locals = Set.empty;
+  labels = Set.empty;
+}
+
+let union (s1 : t) (s2 : t) : t =
+{
+  types = Set.union s1.types s2.types;
+  globals = Set.union s1.globals s2.globals;
+  tables = Set.union s1.tables s2.tables;
+  memories = Set.union s1.memories s2.memories;
+  funcs = Set.union s1.funcs s2.funcs;
+  elems = Set.union s1.elems s2.elems;
+  datas = Set.union s1.datas s2.datas;
+  locals = Set.union s1.locals s2.locals;
+  labels = Set.union s1.labels s2.labels;
+}
+
+let types s = {empty with types = s}
+let globals s = {empty with globals = s}
+let tables s = {empty with tables = s}
+let memories s = {empty with memories = s}
+let funcs s = {empty with funcs = s}
+let elems s = {empty with elems = s}
+let datas s = {empty with datas = s}
+let locals s = {empty with locals = s}
+let labels s = {empty with labels = s}
+
+let var x = Set.singleton x.it
+let zero = Set.singleton 0l
+let shift s = Set.map (Int32.add (-1l)) (Set.remove 0l s)
+
+let (++) = union
+let list free xs = List.fold_left union empty (List.map free xs)
+
+let rec instr (e : instr) =
+  match e.it with
+  | Unreachable | Nop | Drop | Select -> empty
+  | Const _ | Test _ | Compare _ | Unary _ | Binary _ | Convert _ -> empty
+  | Block (_, es) | Loop (_, es) -> block es
+  | If (_, es1, es2) -> block es1 ++ block es2
+  | Br x | BrIf x -> labels (var x)
+  | BrTable (xs, x) -> list (fun x -> labels (var x)) (x::xs)
+  | Return -> empty
+  | Call x -> funcs (var x)
+  | CallIndirect x -> types (var x) ++ tables zero
+  | LocalGet x | LocalSet x | LocalTee x -> locals (var x)
+  | GlobalGet x | GlobalSet x -> globals (var x)
+  | Load _ | Store _ | MemorySize | MemoryGrow | MemoryCopy | MemoryFill ->
+    memories zero
+  | MemoryInit x -> memories zero ++ datas (var x)
+  | TableCopy -> tables zero
+  | TableInit x -> tables zero ++ elems (var x)
+  | DataDrop x -> datas (var x)
+  | ElemDrop x -> elems (var x)
+
+and block (es : instr list) =
+  let free = list instr es in {free with labels = shift free.labels}
+
+let const (c : const) = block c.it
+
+let global (g : global) = const g.it.value
+let func (f : func) = {(block f.it.body) with locals = Set.empty}
+let table (t : table) = empty
+let memory (m : memory) = empty
+
+let elem (e : elem) =
+  match e.it with
+  | Null -> empty
+  | Func x -> funcs (var x)
+
+let table_segment (s : table_segment) =
+  match s.it with
+  | Active {index; offset; init} ->
+    tables (var index) ++ const offset ++ list elem init
+  | Passive {etype; data} -> list elem data
+
+let memory_segment (s : memory_segment) =
+  match s.it with
+  | Active {index; offset; init} -> memories (var index) ++ const offset
+  | Passive {etype; data} -> empty
+
+let type_ (t : type_) = empty
+
+let export_desc (d : export_desc) =
+  match d.it with
+  | FuncExport x -> funcs (var x)
+  | TableExport x -> tables (var x)
+  | MemoryExport x -> memories (var x)
+  | GlobalExport x -> globals (var x)
+
+let import_desc (d : import_desc) =
+  match d.it with
+  | FuncImport x -> types (var x)
+  | TableImport tt -> empty
+  | MemoryImport mt -> empty
+  | GlobalImport gt -> empty
+
+let export (e : export) = export_desc e.it.edesc
+let import (i : import) = import_desc i.it.idesc
+
+let start (s : var option) =
+  funcs (Lib.Option.get (Lib.Option.map var s) Set.empty)
+
+let module_ (m : module_) =
+  list type_ m.it.types ++
+  list global m.it.globals ++
+  list table m.it.tables ++
+  list memory m.it.memories ++
+  list func m.it.funcs ++
+  start m.it.start ++
+  list table_segment m.it.elems ++
+  list memory_segment m.it.datas ++
+  list import m.it.imports ++
+  list export m.it.exports
diff --git a/interpreter/syntax/free.mli b/interpreter/syntax/free.mli
new file mode 100644
index 0000000000..b7ea2d7480
--- /dev/null
+++ b/interpreter/syntax/free.mli
@@ -0,0 +1,34 @@
+module Set : Set.S with type elt = int32
+
+type t =
+{
+  types : Set.t;
+  globals : Set.t;
+  tables : Set.t;
+  memories : Set.t;
+  funcs : Set.t;
+  elems : Set.t;
+  datas : Set.t;
+  locals : Set.t;
+  labels : Set.t;
+}
+
+val empty : t
+val union : t -> t -> t
+
+val instr : Ast.instr -> t
+val block : Ast.instr list -> t
+val const : Ast.const -> t
+
+val type_ : Ast.type_ -> t
+val global : Ast.global -> t
+val func : Ast.func -> t
+val table : Ast.table -> t
+val memory : Ast.memory -> t
+val table_segment : Ast.table_segment -> t
+val memory_segment : Ast.memory_segment -> t
+val export : Ast.export -> t
+val import : Ast.import -> t
+val start : Ast.var option -> t
+
+val module_ : Ast.module_ -> t