diff --git a/src/alias.ml b/src/alias.ml
index 6caa2055..e2b4685c 100644
--- a/src/alias.ml
+++ b/src/alias.ml
@@ -45,9 +45,6 @@ let default = make "DEFAULT"
 let runtest = make "runtest"
 let install = make "install"
 
-let lib_cm_all ~dir lib_name cm_kind =
-  make (sprintf "%s%s-all" lib_name (Cm_kind.ext cm_kind)) ~dir
-
 let recursive_aliases =
   [ default
   ; runtest
diff --git a/src/alias.mli b/src/alias.mli
index 515955cf..441a3333 100644
--- a/src/alias.mli
+++ b/src/alias.mli
@@ -6,8 +6,6 @@ val default : dir:Path.t -> t
 val runtest : dir:Path.t -> t
 val install : dir:Path.t -> t
 
-val lib_cm_all : dir:Path.t -> string -> Cm_kind.t -> t
-
 val dep : t -> ('a, 'a) Build.t
 val file : t -> Path.t
 
diff --git a/src/cm_kind.ml b/src/cm_kind.ml
index e754bbf3..fb3efe92 100644
--- a/src/cm_kind.ml
+++ b/src/cm_kind.ml
@@ -10,3 +10,22 @@ let choose cmi cmo cmx = function
 let ext = choose ".cmi" ".cmo" ".cmx"
 
 let source = choose Ml_kind.Intf Impl Impl
+
+module Dict = struct
+  type 'a t =
+    { cmi : 'a
+    ; cmo : 'a
+    ; cmx : 'a
+    }
+
+  let get t = function
+    | Cmi -> t.cmi
+    | Cmo -> t.cmo
+    | Cmx -> t.cmx
+
+  let of_func f =
+    { cmi = f ~cm_kind:Cmi
+    ; cmo = f ~cm_kind:Cmo
+    ; cmx = f ~cm_kind:Cmx
+    }
+end
diff --git a/src/cm_kind.mli b/src/cm_kind.mli
index 0de11f47..7d248c1e 100644
--- a/src/cm_kind.mli
+++ b/src/cm_kind.mli
@@ -4,3 +4,17 @@ val all : t list
 
 val ext : t -> string
 val source : t -> Ml_kind.t
+
+module Dict : sig
+  type cm_kind = t
+
+  type 'a t =
+    { cmi : 'a
+    ; cmo : 'a
+    ; cmx : 'a
+    }
+
+  val get : 'a t -> cm_kind -> 'a
+
+  val of_func : (cm_kind:cm_kind -> 'a) -> 'a t
+end with type cm_kind := t
diff --git a/src/findlib.ml b/src/findlib.ml
index 9829fe01..f527f14a 100644
--- a/src/findlib.ml
+++ b/src/findlib.ml
@@ -128,7 +128,6 @@ type package =
   ; jsoo_runtime     : string list
   ; requires         : package list
   ; ppx_runtime_deps : package list
-  ; has_headers      : bool
   }
 
 module Package_not_available = struct
@@ -201,10 +200,9 @@ type present_or_not_available =
   | Not_available of Package_not_available.t
 
 type t =
-  { stdlib_dir  : Path.t
-  ; path        : Path.t list
-  ; packages    : (string, present_or_not_available) Hashtbl.t
-  ; has_headers : (Path.t, bool                    ) Hashtbl.t
+  { stdlib_dir    : Path.t
+  ; path          : Path.t list
+  ; packages      : (string, present_or_not_available) Hashtbl.t
   }
 
 let path t = t.path
@@ -212,23 +210,9 @@ let path t = t.path
 let create ~stdlib_dir ~path =
   { stdlib_dir
   ; path
-  ; packages    = Hashtbl.create 1024
-  ; has_headers = Hashtbl.create 1024
+  ; packages      = Hashtbl.create 1024
   }
 
-let has_headers t ~dir =
-  match Hashtbl.find t.has_headers dir with
-  | Some x -> x
-  | None ->
-    let x =
-      match Path.readdir dir with
-      | exception _ -> false
-      | files ->
-        List.exists files ~f:(fun fn -> Filename.check_suffix fn ".h")
-    in
-    Hashtbl.add t.has_headers ~key:dir ~data:x;
-    x
-
 module Pkg_step1 = struct
   type t =
     { package          : package
@@ -261,14 +245,13 @@ let parse_package t ~name ~parent_dir ~vars ~required_by =
   let pkg =
     { name
     ; dir
-    ; has_headers = has_headers t ~dir
     ; version     = Vars.get vars "version" []
     ; description = Vars.get vars "description" []
     ; archives    = archives "archive" preds
     ; jsoo_runtime
     ; plugins     = Mode.Dict.map2 ~f:(@)
-                      (archives "archive" ("plugin" :: preds))
-                      (archives "plugin" preds)
+                       (archives "archive" ("plugin" :: preds))
+                       (archives "plugin" preds)
     ; requires    = []
     ; ppx_runtime_deps = []
     }
diff --git a/src/findlib.mli b/src/findlib.mli
index 996015f3..a5433e13 100644
--- a/src/findlib.mli
+++ b/src/findlib.mli
@@ -55,7 +55,6 @@ type package =
   ; jsoo_runtime     : string list
   ; requires         : package list
   ; ppx_runtime_deps : package list
-  ; has_headers      : bool
   }
 
 val find     : t -> required_by:string list -> string -> package option
diff --git a/src/gen_rules.ml b/src/gen_rules.ml
index 152fcee6..5a9c53a5 100644
--- a/src/gen_rules.ml
+++ b/src/gen_rules.ml
@@ -92,15 +92,6 @@ module Gen(P : Params) = struct
            ; Dyn (fun (cm_files, _) -> Deps cm_files)
            ]))
 
-  let mk_lib_cm_all (lib : Library.t) ~dir ~modules cm_kind =
-    let deps =
-      String_map.fold modules ~init:[] ~f:(fun ~key:_ ~data:m acc ->
-        Module.cm_file m ~dir cm_kind :: acc)
-    in
-    Alias.add_deps (SC.aliases sctx)
-      (Alias.lib_cm_all ~dir lib.name cm_kind)
-      deps
-
   let expand_includes ~dir includes =
     Arg_spec.As (List.concat_map includes ~f:(fun s ->
       ["-I"; SC.expand_vars sctx ~dir s]))
@@ -113,9 +104,7 @@ module Gen(P : Params) = struct
        >>>
        Build.fanout
          (SC.expand_and_eval_set ~dir lib.c_flags ~standard:(Utils.g ()))
-         (requires
-          >>>
-          Build.dyn_paths (Build.arr Lib.header_files))
+         requires
        >>>
        Build.run ~context:ctx
          (* We have to execute the rule in the library directory as the .o is produced in
@@ -269,11 +258,14 @@ module Gen(P : Params) = struct
         ~modules:(String_map.singleton m.name m)
         ~dep_graph:(Ml_kind.Dict.make_both (Build.return (String_map.singleton m.name [])))
         ~requires:(
-          if String_map.is_empty modules then
-            (* Just so that we setup lib dependencies for empty libraries *)
-            requires
-          else
-            Build.return [])
+          let requires =
+            if String_map.is_empty modules then
+              (* Just so that we setup lib dependencies for empty libraries *)
+              requires
+            else
+              Build.return []
+          in
+          Cm_kind.Dict.of_func (fun ~cm_kind:_ -> requires))
         ~alias_module:None);
 
     if Library.has_stubs lib then begin
@@ -286,6 +278,10 @@ module Gen(P : Params) = struct
             None)
       in
       let o_files =
+        let requires =
+          Build.memoize "header files"
+            (requires >>> SC.Libs.file_deps sctx ~ext:".h")
+        in
         List.map lib.c_names   ~f:(build_c_file   lib ~dir ~requires ~h_files) @
         List.map lib.cxx_names ~f:(build_cxx_file lib ~dir ~requires ~h_files)
       in
@@ -324,7 +320,17 @@ module Gen(P : Params) = struct
         end
     end;
 
-    List.iter Cm_kind.all ~f:(mk_lib_cm_all lib ~dir ~modules);
+    List.iter Cm_kind.all ~f:(fun cm_kind ->
+      let files =
+        String_map.fold modules ~init:[] ~f:(fun ~key:_ ~data:m acc ->
+          Module.cm_file m ~dir cm_kind :: acc)
+      in
+      SC.Libs.setup_file_deps_alias sctx (dir, lib) ~ext:(Cm_kind.ext cm_kind)
+        files);
+    SC.Libs.setup_file_deps_group_alias sctx (dir, lib) ~exts:[".cmi"; ".cmx"];
+    SC.Libs.setup_file_deps_alias sctx (dir, lib) ~ext:".h"
+      (List.map lib.install_c_headers ~f:(fun header ->
+         Path.relative dir (header ^ ".h")));
 
     List.iter Mode.all ~f:(fun mode ->
       build_lib lib ~flags ~dir ~mode ~modules ~dep_graph);
diff --git a/src/lib.ml b/src/lib.ml
index f67e53ca..5994a13a 100644
--- a/src/lib.ml
+++ b/src/lib.ml
@@ -21,26 +21,11 @@ end
 
 include T
 module Set = Set.Make(T)
-(*
-let deps = function
-  | Internal (_, lib) -> lib.libraries
-  | External pkg -> pkg.requires
-*)
+
 let dir = function
   | Internal (dir, _) -> dir
   | External pkg -> pkg.dir
 
-let header_files ts =
-  List.fold_left ts ~init:[] ~f:(fun acc t ->
-      match t with
-      | External _ -> []
-      | Internal (dir, lib) ->
-        match lib.install_c_headers with
-        | [] -> acc
-        | l ->
-          List.fold_left l ~init:acc ~f:(fun acc fn ->
-              Path.relative dir (fn ^ ".h") :: acc))
-
 let include_paths ts =
   List.fold_left ts ~init:Path.Set.empty ~f:(fun acc t ->
     Path.Set.add (dir t) acc)
@@ -50,17 +35,10 @@ let include_flags ts =
   Arg_spec.S (List.concat_map (Path.Set.elements dirs) ~f:(fun dir ->
     [Arg_spec.A "-I"; Path dir]))
 
-let has_headers = function
-  | Internal (_, lib) -> lib.install_c_headers <> []
-  | External pkg -> pkg.has_headers
-
 let c_include_flags ts =
   let dirs =
     List.fold_left ts ~init:Path.Set.empty ~f:(fun acc t ->
-      if has_headers t then
-        Path.Set.add (dir t) acc
-      else
-        acc)
+      Path.Set.add (dir t) acc)
   in
   Arg_spec.S (List.concat_map (Path.Set.elements dirs) ~f:(fun dir ->
     [Arg_spec.A "-I"; Path dir]))
diff --git a/src/lib.mli b/src/lib.mli
index 486412f3..7bdc919b 100644
--- a/src/lib.mli
+++ b/src/lib.mli
@@ -12,8 +12,6 @@ module Set : Set.S with type elt := t
 
 (*val deps : t -> string list*)
 
-val header_files : t list -> Path.t list
-
 val include_paths : t list -> Path.Set.t
 
 val include_flags : t list -> _ Arg_spec.t
diff --git a/src/module_compilation.ml b/src/module_compilation.ml
index 7f4b59ca..44597fad 100644
--- a/src/module_compilation.ml
+++ b/src/module_compilation.ml
@@ -1,22 +1,8 @@
 open Import
-open Jbuild_types
 open Build.O
 
 module SC = Super_context
 
-let lib_cm_all ~dir (lib : Library.t) cm_kind =
-  Alias.file (Alias.lib_cm_all ~dir lib.name cm_kind)
-
-let lib_dependencies (libs : Lib.t list) ~(cm_kind : Cm_kind.t) =
-  List.concat_map libs ~f:(function
-    | External _ -> []
-    | Internal (dir, lib) ->
-      match cm_kind with
-      | Cmi | Cmo ->
-        [lib_cm_all ~dir lib Cmi]
-      | Cmx ->
-        [lib_cm_all ~dir lib Cmx])
-
 let build_cm sctx ?sandbox ~dynlink ~flags ~cm_kind ~(dep_graph:Ocamldep.dep_graph)
       ~requires ~(modules : Module.t String_map.t) ~dir ~alias_module (m : Module.t) =
   let ctx = SC.context sctx in
@@ -75,7 +61,6 @@ let build_cm sctx ?sandbox ~dynlink ~flags ~cm_kind ~(dep_graph:Ocamldep.dep_gra
         (Build.paths extra_deps >>>
          other_cm_files >>>
          requires >>>
-         Build.dyn_paths (Build.arr (lib_dependencies ~cm_kind)) >>>
          Build.run ~context:ctx (Dep compiler)
            ~extra_targets
            [ Ocaml_flags.get_for_cm flags ~cm_kind
@@ -92,16 +77,35 @@ let build_cm sctx ?sandbox ~dynlink ~flags ~cm_kind ~(dep_graph:Ocamldep.dep_gra
            ; A "-c"; Ml_kind.flag ml_kind; Dep src
            ])))
 
-let build_module sctx ?sandbox ~dynlink ~js_of_ocaml ~flags m ~dir ~dep_graph ~modules ~requires
-      ~alias_module =
+let build_module sctx ?sandbox ~dynlink ~js_of_ocaml ~flags m ~dir ~dep_graph
+      ~modules ~requires ~alias_module =
   List.iter Cm_kind.all ~f:(fun cm_kind ->
-    build_cm sctx ?sandbox ~dynlink ~flags ~dir ~dep_graph ~modules m ~cm_kind ~requires
-      ~alias_module);
+    let requires = Cm_kind.Dict.get requires cm_kind in
+    build_cm sctx ?sandbox ~dynlink ~flags ~dir ~dep_graph ~modules m ~cm_kind
+      ~requires ~alias_module);
   (* Build *.cmo.js *)
   let src = Module.cm_file m ~dir Cm_kind.Cmo in
   SC.add_rules sctx (Js_of_ocaml_rules.build_cm sctx ~dir ~js_of_ocaml ~src)
 
 let build_modules sctx ~dynlink ~js_of_ocaml ~flags ~dir ~dep_graph ~modules ~requires ~alias_module =
+  let cmi_requires =
+    Build.memoize "cmi library dependencies"
+      (requires
+       >>>
+       SC.Libs.file_deps sctx ~ext:".cmi")
+  in
+  let cmi_and_cmx_requires =
+    Build.memoize "cmi and cmx library dependencies"
+      (requires
+       >>>
+       SC.Libs.file_deps sctx ~ext:".cmi-and-.cmx")
+  in
+  let requires : _ Cm_kind.Dict.t =
+    { cmi = cmi_requires
+    ; cmo = cmi_requires
+    ; cmx = cmi_and_cmx_requires
+    }
+  in
   String_map.iter
     (match alias_module with
      | None -> modules
diff --git a/src/module_compilation.mli b/src/module_compilation.mli
index 7424ac79..823926cf 100644
--- a/src/module_compilation.mli
+++ b/src/module_compilation.mli
@@ -2,7 +2,10 @@
 
 open Import
 
-(** Setup rules to build a single module *)
+(** Setup rules to build a single module.
+
+    [requires] must declare dependencies on files of libraries.
+*)
 val build_module
   :  Super_context.t
   -> ?sandbox:bool
@@ -13,7 +16,7 @@ val build_module
   -> dir:Path.t
   -> dep_graph:Ocamldep.dep_graph
   -> modules:Module.t String_map.t
-  -> requires:(unit, Lib.t list) Build.t
+  -> requires:(unit, Lib.t list) Build.t Cm_kind.Dict.t
   -> alias_module:Module.t option
   -> unit
 
diff --git a/src/super_context.ml b/src/super_context.ml
index 0b068c28..4242b0f3 100644
--- a/src/super_context.ml
+++ b/src/super_context.ml
@@ -10,6 +10,33 @@ module Dir_with_jbuild = struct
     }
 end
 
+module External_dir = struct
+  (* Files in the directory, grouped by extension *)
+  type t = Path.t list String_map.t
+
+  let create ~dir : t =
+    match Path.readdir dir with
+    | exception _ -> String_map.empty
+    | files ->
+      List.map files ~f:(fun fn -> Filename.extension fn, Path.relative dir fn)
+      |> String_map.of_alist_multi
+  (* CR-someday jdimino: when we can have dynamic targets:
+
+     {[
+       |> String_map.mapi ~f:(fun ext files ->
+         lazy (
+           let alias =
+             Alias.make ~dir:Path.root (sprintf "external-files-%s%s" hash ext)
+           in
+           Alias.add_deps aliases alias files;
+           alias
+         ))
+     ]}
+  *)
+
+  let files t ~ext = String_map.find_default ext t ~default:[]
+end
+
 type t =
   { context                                 : Context.t
   ; libs                                    : Lib_db.t
@@ -26,6 +53,7 @@ type t =
   ; vars                                    : string String_map.t
   ; ppx_dir                                 : Path.t
   ; ppx_drivers                             : (string, Path.t) Hashtbl.t
+  ; external_dirs                           : (Path.t, External_dir.t) Hashtbl.t
   }
 
 let context t = t.context
@@ -40,6 +68,10 @@ let cxx_flags t = t.cxx_flags
 
 let expand_var_no_root t var = String_map.find var t.vars
 
+let get_external_dir t ~dir =
+  Hashtbl.find_or_add t.external_dirs dir ~f:(fun dir ->
+    External_dir.create ~dir)
+
 let expand_vars t ~dir s =
   String_with_vars.expand s ~f:(function
   | "ROOT" -> Some (Path.reach ~from:dir t.context.build_dir)
@@ -166,6 +198,7 @@ let create
   ; vars
   ; ppx_drivers = Hashtbl.create 32
   ; ppx_dir = Path.of_string (sprintf "_build/.ppx/%s" context.name)
+  ; external_dirs = Hashtbl.create 1024
   }
 
 let add_rule t ?sandbox build =
@@ -324,6 +357,29 @@ module Libs = struct
          Lib.remove_dups_preserve_order (rt_deps @ rt_deps_of_deps))
        >>>
        Build.store_vfile vruntime_deps)
+
+  let lib_files_alias ((dir, lib) : Lib.Internal.t) ~ext =
+    Alias.make (sprintf "lib-%s%s-all" lib.name ext) ~dir
+
+  let setup_file_deps_alias t lib ~ext files =
+    Alias.add_deps t.aliases (lib_files_alias lib ~ext) files
+
+  let setup_file_deps_group_alias t lib ~exts =
+    setup_file_deps_alias t lib
+      ~ext:(String.concat exts ~sep:"-and-")
+      (List.map exts ~f:(fun ext -> Alias.file (lib_files_alias lib ~ext)))
+
+  let file_deps t ~ext =
+    Build.dyn_paths (Build.arr (fun libs ->
+      List.fold_left libs ~init:[] ~f:(fun acc (lib : Lib.t) ->
+        match lib with
+        | External pkg -> begin
+            List.rev_append
+              (External_dir.files (get_external_dir t ~dir:pkg.dir) ~ext)
+              acc
+          end
+        | Internal lib ->
+          Alias.file (lib_files_alias lib ~ext) :: acc)))
 end
 
 module Deps = struct
diff --git a/src/super_context.mli b/src/super_context.mli
index 3223c1f3..7b0f6b36 100644
--- a/src/super_context.mli
+++ b/src/super_context.mli
@@ -95,6 +95,19 @@ module Libs : sig
     -> libraries:Lib_deps.t
     -> ppx_runtime_libraries:string list
     -> unit
+
+  (** [file_deps ~ext] is an arrow that record dependencies on all the files with
+      extension [ext] of the libraries given as input. *)
+  val file_deps : t -> ext:string -> (Lib.t list, Lib.t list) Build.t
+
+  (** Setup the alias that depends on all files with a given extension for a library *)
+  val setup_file_deps_alias : t -> Lib.Internal.t -> ext:string -> Path.t list -> unit
+
+  (** Setup an alias that depend on all files with the given extensions.
+
+      To depend on this alias, use [~ext:"ext1-and-ext2-...-extn"]
+  *)
+  val setup_file_deps_group_alias : t -> Lib.Internal.t -> exts:string list -> unit
 end
 
 (** Interpret dependencies written in jbuild files *)
diff --git a/src/utils.ml b/src/utils.ml
index c38135ee..7f3af677 100644
--- a/src/utils.ml
+++ b/src/utils.ml
@@ -75,8 +75,16 @@ let jbuild_name_in ~dir =
 
 let describe_target fn =
   match Path.extract_build_context fn with
-  | Some (".aliases", dir) ->
-    sprintf "alias %s" (Path.to_string dir)
+  | Some (".aliases", fn) ->
+    let name =
+      let fn = Path.to_string fn in
+      match String.rsplit2 fn ~on:'-' with
+      | None -> assert false
+      | Some (name, digest) ->
+        assert (String.length digest = 32);
+        name
+    in
+    sprintf "alias %s" name
   | _ ->
     Path.to_string fn