removes unnecessary units from the knowledge base (#1427)

The Project.create function were creating a separate unit in order to
compute a target from the available image specification. It was also
using Toplevel.eval for that, which computes all properties of an
object. As a result, we had all expensive unit-based computations
triggered twice per each project.

An ideal solution would be to get rid of this call, but, as an
immediate remediation, we can just use the same compilation unit as
for the main program and compute only the target slot.

lib/bap/bap_project.ml

@@ -123,12 +123,17 @@ module State = struct
     KB.Domain.flat ~empty ~equal "disassembly" ~inspect
 
   module Toplevel = struct
-    let compute_target spec =
-      Toplevel.eval Theory.Unit.target @@begin
-        KB.Object.create Theory.Unit.cls >>= fun unit ->
-        KB.provide Image.Spec.slot unit spec >>| fun () ->
-        unit
-      end
+    let compute_target ?file spec : Theory.target =
+      let result = Toplevel.var "target" in
+      let create_unit = match file with
+        | None | Some "" -> KB.Object.create Theory.Unit.cls
+        | Some file -> Theory.Unit.for_file file in
+      Toplevel.put result begin
+        let* unit = create_unit in
+        KB.provide Image.Spec.slot unit spec >>= fun () ->
+        KB.collect Theory.Unit.target unit
+      end;
+      Toplevel.get result
 
     let run spec target ~code ~memory file k =
       let result = Toplevel.var "disassembly-result" in
@@ -204,7 +209,7 @@ module Input = struct
       | #Arch.unknown -> Ogre.Doc.empty
       | arch -> Image.Spec.from_arch arch in {
       arch; file; code; data; finish;
-      target = State.Toplevel.compute_target spec;
+      target = State.Toplevel.compute_target ~file spec;
       memory = union_memory code data;
       spec;
     }
@@ -222,8 +227,8 @@ module Input = struct
     | None -> false
     | Some s -> not (Image.Segment.is_executable s)
 
-  let compute_target ?(target=Theory.Target.unknown) spec =
-    let target' = State.Toplevel.compute_target spec in
+  let compute_target ?file ?(target=Theory.Target.unknown) spec =
+    let target' = State.Toplevel.compute_target ?file spec in
     match (Theory.Target.order target target' : KB.Order.partial) with
     | LT | EQ -> target'
     | GT -> target
@@ -240,7 +245,7 @@ module Input = struct
     file;
     finish;
     spec = Image.spec img;
-    target = compute_target ?target (Image.spec img);
+    target = compute_target ~file ?target (Image.spec img);
   }
 
   let of_image ?target ?loader filename =