Preanal: reduce garbage

Summary:
- do not `List.rev` for `List.last`
- `List.rev_filter_map` rather than `filter |> map |> rev`

Reviewed By: da319

Differential Revision: D8185619

fbshipit-source-id: aeb41a4

infer/src/IR/Procdesc.ml

@@ -111,7 +111,7 @@ module Node = struct
 
   (** Get the source location of the last instruction in the node *)
   let get_last_loc n =
-    match List.rev (get_instrs n) with instr :: _ -> Sil.instr_get_loc instr | [] -> n.loc
+    n |> get_instrs |> List.last |> Option.value_map ~f:Sil.instr_get_loc ~default:n.loc
 
 
   let pp_id f id = F.pp_print_int f id

infer/src/backend/preanal.ml

@@ -74,7 +74,7 @@ module NullifyTransferFunctions = struct
   let last_instr_in_node node =
     let get_last_instr () =
       let instrs = CFG.instrs node in
-      match List.rev instrs with instr :: _ -> instr | [] -> Sil.skip_instr
+      List.last instrs |> Option.value ~default:Sil.skip_instr
     in
     if phys_equal node !cache_node then !cache_instr
     else
@@ -135,9 +135,10 @@ let add_nullify_instrs pdesc tenv liveness_inv_map =
   let node_add_nullify_instructions node pvars =
     let loc = Procdesc.Node.get_last_loc node in
     let nullify_instrs =
-      List.filter ~f:is_local pvars |> List.map ~f:(fun pvar -> Sil.Nullify (pvar, loc))
+      List.rev_filter_map pvars ~f:(fun pvar ->
+          if is_local pvar then Some (Sil.Nullify (pvar, loc)) else None )
     in
-    if nullify_instrs <> [] then Procdesc.Node.append_instrs node (List.rev nullify_instrs)
+    if nullify_instrs <> [] then Procdesc.Node.append_instrs node nullify_instrs
   in
   let node_add_removetmps_instructions node ids =
     if ids <> [] then