Add heap stats to -dgc-timings (ocaml-flambda#1174)

This adds a bunch of heap stats from https://v2.ocaml.org/api/Gc.html to
`-dgc-timings`. These will be consumed and stored by our build stats
pipeline.

Co-authored-by: Roman Leshchinskiy <rleshchinskiy@janestreet.com>

driver/optmaindriver.ml

@@ -162,11 +162,30 @@ let main unix argv ppf ~flambda2 =
     if !Flambda_backend_flags.gc_timings then begin
       let minor = Gc_timings.gc_minor_ns () in
       let major = Gc_timings.gc_major_ns () in
+      let stats = Gc.quick_stat () in
       let secs x = x *. 1e-9 in
       let precision = !Clflags.timings_precision in
+      let w2b n = n * (Sys.word_size / 8) in
+      let fw2b x = w2b (Float.to_int x) in
       Format.fprintf Format.std_formatter "%0.*fs gc\n" precision (secs (minor +. major));
       Format.fprintf Format.std_formatter "  %0.*fs minor\n" precision (secs minor);
-      Format.fprintf Format.std_formatter "  %0.*fs major\n" precision (secs major)
+      Format.fprintf Format.std_formatter "  %0.*fs major\n" precision (secs major);
+      Format.fprintf Format.std_formatter "- heap\n";
+      (* Having minor + major + promoted = total alloc make more sense for
+         hierarchical stats. *)
+      Format.fprintf Format.std_formatter "  %ib alloc\n"
+        (fw2b stats.minor_words + (fw2b stats.major_words - fw2b stats.promoted_words));
+      Format.fprintf Format.std_formatter "    %ib minor\n"
+        (fw2b stats.minor_words - fw2b stats.promoted_words);
+      Format.fprintf Format.std_formatter "    %ib major\n"
+        (fw2b stats.major_words - fw2b stats.promoted_words);
+      Format.fprintf Format.std_formatter "    %ib promoted\n"
+        (fw2b stats.promoted_words);
+      Format.fprintf Format.std_formatter "  %ib top\n" (w2b stats.top_heap_words);
+      Format.fprintf Format.std_formatter "  %i collections\n"
+        (stats.minor_collections + stats.major_collections);
+      Format.fprintf Format.std_formatter "    %i minor\n" stats.minor_collections;
+      Format.fprintf Format.std_formatter "    %i major\n" stats.major_collections;
     end;
     Profile.print Format.std_formatter !Clflags.profile_columns ~timings_precision:!Clflags.timings_precision;
     0