flambda-backend: Don't add a module to the environment when saving it (#1667)

When saving a .cmi, we produce all the same data we would if we were importing
it and add this data to the environment. This is done only so that the CRC for
the module is added to our CRCs and we can look up the CRC later. This is wholly
unnecessary, however: we can just add the CRC to the `Consistbl`. Removing the
extra steps
  * gets rid of a decidedly awkward code path,
  * keeps the environment clean (we currently never observe the fact that a
    saved .cmi is treated as bound in the environment, but it's not good!), and
  * will also enable further simplification, since `sign_of_cmi` now freshens
    unconditionally (the only call with `~freshen:false` is gone).

This patch removes the saved .cmi from the persistent environment, and also
changes `crc_of_unit` to look up in the global `crc_units` table first so that
it bypasses `find_pers_struct` for a saved .cmi.

Author: lukemaurer

typing/env.ml

@@ -972,8 +972,6 @@ let sign_of_cmi ~freshen { Persistent_env.Persistent_signature.cmi; _ } =
 
 let read_sign_of_cmi = sign_of_cmi ~freshen:true
 
-let save_sign_of_cmi = sign_of_cmi ~freshen:false
-
 let persistent_env : module_data Persistent_env.t ref =
   s_table Persistent_env.empty ()
 
@@ -2670,10 +2668,8 @@ let save_signature_with_transform cmi_transform ~alerts sg modname filename =
   let cmi =
     Persistent_env.make_cmi !persistent_env modname sg alerts
     |> cmi_transform in
-  let pm = save_sign_of_cmi
-      { Persistent_env.Persistent_signature.cmi; filename } in
   Persistent_env.save_cmi !persistent_env
-    { Persistent_env.Persistent_signature.filename; cmi } pm;
+    { Persistent_env.Persistent_signature.filename; cmi };
   cmi
 
 let save_signature ~alerts sg modname filename =

typing/persistent_env.ml

@@ -168,10 +168,9 @@ let fold {persistent_structures; _} f x =
 
 (* Reading persistent structures from .cmi files *)
 
-let save_pers_struct penv crc ps pm =
-  let {persistent_structures; crc_units; _} = penv in
+let save_pers_struct penv crc ps =
+  let {crc_units; _} = penv in
   let modname = CU.name ps.ps_name in
-  Hashtbl.add persistent_structures modname (Found (ps, pm));
   List.iter
     (function
         | Rectypes -> ()
@@ -334,13 +333,16 @@ module Array = struct
 end
 
 let crc_of_unit penv f name =
-  let (ps, _pm) = find_pers_struct penv f true name in
-  match Array.find_opt (Import_info.has_name ~name) ps.ps_crcs with
-  | None -> assert false
-  | Some import_info ->
-    match Import_info.crc import_info with
+  match Consistbl.find penv.crc_units name with
+  | Some (_, crc) -> crc
+  | None ->
+    let (ps, _pm) = find_pers_struct penv f true name in
+    match Array.find_opt (Import_info.has_name ~name) ps.ps_crcs with
     | None -> assert false
-    | Some crc -> crc
+    | Some import_info ->
+      match Import_info.crc import_info with
+      | None -> assert false
+      | Some crc -> crc
 
 let imports {imported_units; crc_units; _} =
   let imports =
@@ -377,7 +379,7 @@ let make_cmi penv modname sign alerts =
     cmi_flags = flags
   }
 
-let save_cmi penv psig pm =
+let save_cmi penv psig =
   let { Persistent_signature.filename; cmi } = psig in
   Misc.try_finally (fun () ->
       let {
@@ -390,7 +392,7 @@ let save_cmi penv psig pm =
         output_to_file_via_temporary (* see MPR#7472, MPR#4991 *)
           ~mode: [Open_binary] filename
           (fun temp_filename oc -> output_cmi temp_filename oc cmi) in
-      (* Enter signature in persistent table so that imports()
+      (* Enter signature in consistbl so that imports()
          will also return its crc *)
       let ps =
         { ps_name = modname;
@@ -401,7 +403,7 @@ let save_cmi penv psig pm =
           ps_filename = filename;
           ps_flags = flags;
         } in
-      save_pers_struct penv crc ps pm
+      save_pers_struct penv crc ps
     )
     ~exceptionally:(fun () -> remove_file filename)
 

typing/persistent_env.mli

@@ -89,7 +89,7 @@ val register_import_as_opaque : 'a t -> Compilation_unit.Name.t -> unit
 val make_cmi : 'a t -> Compilation_unit.t -> Subst.Lazy.signature -> alerts
   -> Cmi_format.cmi_infos_lazy
 
-val save_cmi : 'a t -> Persistent_signature.t -> 'a -> unit
+val save_cmi : 'a t -> Persistent_signature.t -> unit
 
 val can_load_cmis : 'a t -> can_load_cmis
 val set_can_load_cmis : 'a t -> can_load_cmis -> unit