[Cost] Do not flag variations to/from zero-costing functions

Reviewed By: ezgicicek

Differential Revision: D13504638

fbshipit-source-id: 9f07b83d7

infer/src/backend/Differential.ml

@@ -217,12 +217,20 @@ let of_costs ~(current_costs : Jsonbug_t.costs_report) ~(previous_costs : Jsonbu
         in
         let curr_cost_info, curr_cost = max_degree_polynomial current in
         let _, prev_cost = max_degree_polynomial previous in
+        if
+          Config.filtering
+          && (CostDomain.BasicCost.is_one curr_cost || CostDomain.BasicCost.is_one prev_cost)
+        then
+          (* transitions to/from zero costs are obvious, no need to flag them *)
+          (left, both, right)
+        else
           let cmp = CostDomain.BasicCost.compare_by_degree curr_cost prev_cost in
           let concat_opt l v = match v with Some v' -> v' :: l | None -> l in
           if cmp > 0 then
             (* introduced *)
             let left' =
-            issue_of_cost curr_cost_info ~delta:`Increased ~prev_cost ~curr_cost |> concat_opt left
+              issue_of_cost curr_cost_info ~delta:`Increased ~prev_cost ~curr_cost
+              |> concat_opt left
             in
             (left', both, right)
           else if cmp < 0 then

infer/src/bufferoverrun/polynomials.ml

@@ -348,6 +348,8 @@ module NonNegativePolynomial = struct
 
   let is_zero = function NonTop p when NonNegativeNonTopPolynomial.is_zero p -> true | _ -> false
 
+  let is_one = function NonTop p when NonNegativeNonTopPolynomial.is_one p -> true | _ -> false
+
   let top_lifted_increasing ~f p1 p2 =
     match (p1, p2) with Top, _ | _, Top -> Top | NonTop p1, NonTop p2 -> NonTop (f p1 p2)
 

infer/src/bufferoverrun/polynomials.mli

@@ -33,6 +33,8 @@ module NonNegativePolynomial : sig
 
   val is_zero : t -> bool
 
+  val is_one : t -> bool
+
   val of_non_negative_bound : Bounds.NonNegativeBound.t -> t
 
   val plus : t -> t -> t

infer/tests/build_systems/differential_of_costs_report/fixed.exp

@@ -1 +0,0 @@
-PERFORMANCE_VARIATION, no_bucket, src/DiffExample.java, DiffExample.f2(int):void, 0