[cost] Fix nb_node_exec

Summary: D19496263 had refactored cost to its own subdirectory. In this process, it introduced a perf degradation by bundling of collection and computation of constraints which relies on imperative union find. This diff fixes that by separating them.

Reviewed By: ngorogiannis

Differential Revision: D19598610

fbshipit-source-id: 0e466522d

infer/src/cost/constraintSolver.ml

@@ -181,9 +181,7 @@ let compute_costs ~debug bound_map equalities =
   debug.f "[ConstraintSolver][CImpr] %a@\n" Equalities.pp_costs equalities
 
 
-let get_node_nb_exec ~debug node_cfg bound_map node_id =
-  let equalities = collect_constraints ~debug node_cfg in
-  let () = compute_costs ~debug bound_map equalities in
+let get_node_nb_exec equalities node_id =
   let set =
     node_id |> ControlFlowCost.make_node |> Equalities.find equalities
     |> Equalities.find_set equalities

infer/src/cost/constraintSolver.mli

@@ -12,7 +12,16 @@ module Node = ProcCfg.DefaultNode
 
 type debug = {f: 'a. ('a, F.formatter, unit, unit) IStd.format4 -> 'a} [@@unboxed]
 
-val get_node_nb_exec :
-  debug:debug -> Procdesc.t -> BasicCost.t Node.IdMap.t -> Node.id -> BasicCost.t
+module Equalities : sig
+  type t
+end
+
+val compute_costs : debug:debug -> BasicCost.t Node.IdMap.t -> Equalities.t -> unit
+(** repeatedly improve the costs given the constraints *)
+
+val get_node_nb_exec : Equalities.t -> Node.id -> BasicCost.t
 (** compute the number of times a node is executed by taking into account the program structural
     (e.g. control-flow) constraints *)
+
+val collect_constraints : debug:debug -> Procdesc.t -> Equalities.t
+(** collect initial constraints for a CFG *)

infer/src/cost/cost.ml

@@ -303,7 +303,10 @@ let compute_get_node_nb_exec node_cfg bound_map : get_node_nb_exec =
   let start_node = NodeCFG.start_node node_cfg in
   NodePrinter.with_session start_node
     ~pp_name:(fun fmt -> F.pp_print_string fmt "cost(constraints)")
-    ~f:(fun () -> ConstraintSolver.get_node_nb_exec ~debug node_cfg bound_map)
+    ~f:(fun () ->
+      let equalities = ConstraintSolver.collect_constraints ~debug node_cfg in
+      let () = ConstraintSolver.compute_costs ~debug bound_map equalities in
+      ConstraintSolver.get_node_nb_exec equalities )
 
 
 let compute_worst_case_cost tenv integer_type_widths get_callee_summary_and_formals instr_cfg_wto