Update linker in prep for updating LLVM

- Implement extended name section
  (https://github.com/WebAssembly/extended-name-section/blob/master/proposals/extended-name-section/Overview.md)

- Handle `start` functions in the RTS module in linker

These changes are needed to update to LLVM 12: #2542

src/codegen/compile.ml

@@ -8205,13 +8205,10 @@ and conclude_module env start_fi_o =
     let open Wasm_exts.CustomModule in
     { module_;
       dylink = None;
-      name = {
-        module_ = None;
-        function_names =
-            List.mapi (fun i (f,n,_) -> Int32.(add ni' (of_int i), n)) funcs;
-        locals_names =
-            List.mapi (fun i (f,_,ln) -> Int32.(add ni' (of_int i), ln)) funcs;
-        };
+      name = { empty_name_section with function_names =
+                 List.mapi (fun i (f,n,_) -> Int32.(add ni' (of_int i), n)) funcs;
+               locals_names =
+                 List.mapi (fun i (f,_,ln) -> Int32.(add ni' (of_int i), ln)) funcs; };
       motoko = {
         labels = E.get_labs env;
       };

src/linking/linkModule.ml

@@ -598,29 +598,38 @@ let fill_table_base_import new_base : module_' -> module_' = fun m ->
 
 (* Concatenation of modules *)
 
-let join_modules (em1 : extended_module) (m2 : module_') (ns2 : name_section) : extended_module =
-  assert (m2.start = None);
+let join_modules
+      (em1 : extended_module) (m2 : module_') (ns2 : name_section)
+      (type_indices : (Wasm.Types.func_type, int32) Hashtbl.t) : extended_module =
   let m1 = em1.module_ in
-  { em1 with
-    module_ = {
-      types = m1.types @ m2.types;
-      globals = m1.globals @ m2.globals;
-      tables = m1.tables @ m2.tables;
-      memories = m1.memories @ m2.memories;
-      funcs = m1.funcs @ m2.funcs;
-      start = m1.start;
-      elems = m1.elems @ m2.elems;
-      data = m1.data @ m2.data;
-      imports = m1.imports @ m2.imports;
-      exports = m1.exports @ m2.exports;
-    };
-    name = {
-      em1.name with
-      function_names = em1.name.function_names @ ns2.function_names;
-      locals_names = em1.name.locals_names @ ns2.locals_names;
+  let joined = 
+    { em1 with
+      module_ = {
+        types = m1.types @ m2.types;
+        globals = m1.globals @ m2.globals;
+        tables = m1.tables @ m2.tables;
+        memories = m1.memories @ m2.memories;
+        funcs = m1.funcs @ m2.funcs;
+        start = m1.start;
+        elems = m1.elems @ m2.elems;
+        data = m1.data @ m2.data;
+        imports = m1.imports @ m2.imports;
+        exports = m1.exports @ m2.exports;
       };
-    motoko = em1.motoko;
-  }
+      name = {
+        em1.name with
+        function_names = em1.name.function_names @ ns2.function_names;
+        locals_names = em1.name.locals_names @ ns2.locals_names;
+      };
+      motoko = em1.motoko;
+    }
+  in
+  (* If second module has a start, prepend it to the first module's start.
+     OK to use `Hashtbl.find` below as the first module will have a start, so
+     we'll have the unit function in the type section already. *)
+  match m2.start with
+  | None -> joined
+  | Some fi -> prepend_to_start fi.it (Hashtbl.find type_indices (Wasm.Types.FuncType ([], []))) joined
 
 (* The main linking function *)
 
@@ -873,8 +882,6 @@ let link (em1 : extended_module) libname (em2 : extended_module) =
     | Some fi -> prepend_to_start (funs2 fi) (add_or_get_ty (Wasm.Types.FuncType ([], [])))
   in
 
-  assert (dm2.globals = []);
-
   let new_table_size =
     Int32.add (Int32.add lib_table_start dylink.table_size) (Int32.of_int (List.length got_func_imports))
   in
@@ -904,6 +911,7 @@ let link (em1 : extended_module) libname (em2 : extended_module) =
     |> remove_fun_imports_name_section fun_resolved21
     |> rename_funcs_name_section funs2
     )
+    type_indices
   |> add_call_ctors
   |> remove_non_ic_exports (* only sane if no additional files get linked in *)
   in

src/wasm-exts/customModule.ml

@@ -8,12 +8,26 @@ type name_section = {
   module_ : string option;
   function_names : (int32 * string) list;
   locals_names : (int32 * (int32 * string) list) list;
+  label_names : (int32 * (int32 * string) list) list;
+  type_names : (int32 * string) list;
+  table_names : (int32 * string) list;
+  memory_names : (int32 * string) list;
+  global_names : (int32 * string) list;
+  elem_segment_names : (int32 * string) list;
+  data_segment_names : (int32 * string) list;
 }
 
 let empty_name_section : name_section = {
   module_ = None;
   function_names = [];
   locals_names = [];
+  label_names = [];
+  type_names = [];
+  table_names = [];
+  memory_names = [];
+  global_names = [];
+  elem_segment_names = [];
+  data_segment_names = [];
 }
 
 type dylink_section = {

src/wasm-exts/customModuleDecode.ml

@@ -142,7 +142,7 @@ let name s =
   try Utf8.decode (string s) with Utf8.Utf8 ->
     error s pos "malformed UTF-8 encoding"
 
-let sized f s =
+let sized (f : int -> stream -> 'a) (s : stream) =
   let size = len32 s in
   let start = pos s in
   let x = f size s in
@@ -663,7 +663,7 @@ let data_section s =
 
 (* Custom sections *)
 
-let custom_section name_pred f default s =
+let custom_section (name_pred : int list -> bool) (f : int -> stream -> 'a) (default : 'a) (s : stream) =
   let p = pos s in
   match id s with
   | Some `CustomSection ->
@@ -715,7 +715,7 @@ let assoc_list f = vec (fun s ->
 let name_map = assoc_list string
 let indirect_name_map = assoc_list name_map
 
-let name_section_subsection (ns : name_section) s =
+let name_section_subsection (ns : name_section) (s : stream) : name_section =
   match u8 s with
   | 0 -> (* module name *)
     let mod_name = sized (fun _ -> string) s in
@@ -726,17 +726,30 @@ let name_section_subsection (ns : name_section) s =
   | 2 -> (* local names *)
     let loc_names = sized (fun _ -> indirect_name_map) s in
     { ns with locals_names = ns.locals_names @ loc_names }
-
-  (* We ignore additional name subsections for now, despite newer
-     LLVM seemingly producing them.
-
-     We should check if these sections are indeed as spec'ed in
+  (* The following subsections are not in the standard yet, but from the extended-name-section proposal
      https://github.com/WebAssembly/extended-name-section/blob/master/proposals/extended-name-section/Overview.md
-     and implement them, for better debugging.
   *)
-  | 7 ->
-    let _global_names = sized (fun _ -> name_map) s in
-    ns
+  | 3 -> (* label names *)
+    let label_names = sized (fun _ -> indirect_name_map) s in
+    { ns with label_names = ns.label_names @ label_names }
+  | 4 -> (* type names *)
+    let type_names = sized (fun _ -> name_map) s in
+    { ns with type_names = ns.type_names @ type_names }
+  | 5 -> (* table names *)
+    let table_names = sized (fun _ -> name_map) s in
+    { ns with table_names = ns.table_names @ table_names }
+  | 6 -> (* memory names *)
+    let memory_names = sized (fun _ -> name_map) s in
+    { ns with memory_names = ns.memory_names @ memory_names }
+  | 7 -> (* global names *)
+    let global_names = sized (fun _ -> name_map) s in
+    { ns with global_names = ns.global_names @ global_names }
+  | 8 -> (* elem segment names *)
+    let elem_segment_names = sized (fun _ -> name_map) s in
+    { ns with elem_segment_names = ns.elem_segment_names @ elem_segment_names }
+  | 9 -> (* data segment names *)
+    let data_segment_names = sized (fun _ -> name_map) s in
+    { ns with data_segment_names = ns.data_segment_names @ data_segment_names }
   | i -> error s (pos s) (Printf.sprintf "unknown name section subsection id %d" i)
 
 let name_section_content p_end s =