Compact heap before calling C linker (ocaml-flambda#33)

Reduces peak memory usage, which is important when a build system
tries to run many large links in parallel.

backend/asmlink.ml

@@ -353,6 +353,16 @@ let call_linker file_list startup_file output_name =
   if not (exitcode = 0)
   then raise(Error(Linking_error exitcode))
 
+let reset () =
+  Cmi_consistbl.clear crc_interfaces;
+  Cmx_consistbl.clear crc_implementations;
+  String.Tbl.reset implementations_defined;
+  cmx_required := [];
+  String.Tbl.reset interfaces;
+  implementations := [];
+  lib_ccobjs := [];
+  lib_ccopts := []
+
 (* Main entry point *)
 
 let link ~ppf_dump objfiles output_name =
@@ -386,6 +396,20 @@ let link ~ppf_dump objfiles output_name =
       ~asm_filename:startup ~keep_asm:!Clflags.keep_startup_file
       ~obj_filename:startup_obj
       (fun () -> make_startup_file ~ppf_dump units_tolink);
+    (* Clear all state and compact before calling the linker, because the linker
+       can use a lot of memory, and this reduces the peak memory usage by freeing
+       most of the memory from this process before the linker starts using memory.
+
+       On a link where this frees up around 1.1GB of memory this takes around 0.6s. We
+       only take this time on large links where the number of parallel linking jobs is
+       likely to be constrained by total system memory. *)
+    let _minor, _promoted, major_words = Gc.counters () in
+    (* Uses [major_words] because it doesn't require a heap traversal to compute and 
+       for this workload a majority of major words are live at this point. *)
+    if major_words > 500_000_000.0 /. 8.0 then 
+      Profile.record_call "asmlink_compact" (fun () ->
+        reset ();
+        Gc.compact ());
     Misc.try_finally
       (fun () ->
          call_linker (List.map object_file_name objfiles)
@@ -458,13 +482,3 @@ let () =
       | Error err -> Some (Location.error_of_printer_file report_error err)
       | _ -> None
     )
-
-let reset () =
-  Cmi_consistbl.clear crc_interfaces;
-  Cmx_consistbl.clear crc_implementations;
-  String.Tbl.reset implementations_defined;
-  cmx_required := [];
-  String.Tbl.reset interfaces;
-  implementations := [];
-  lib_ccobjs := [];
-  lib_ccopts := []