Make join/smart_join/widen/smart_widen preserve equal inputs.

Issue #1091 was caused by the join/widen (and corresponding smart operations)
for unions in ValueDomain not return the same element.
The optimization in PMap that returns the unchanged map when two
physically identical maps were joined was thus yielding a different value in the
from-scratch run compared to the incremental run.

src/cdomains/valueDomain.ml

@@ -545,9 +545,7 @@ struct
         | None -> AD.join y AD.top_ptr)
     | (Address x, Address y) -> Address (AD.join x y)
     | (Struct x, Struct y) -> Struct (Structs.join x y)
-    | (Union (f,x), Union (g,y)) -> Union (match UnionDomain.Field.join f g with
-        | `Lifted f -> (`Lifted f, join x y) (* f = g *)
-        | x -> (x, Top)) (* f <> g *)
+    | (Union x, Union y) -> Union (Unions.join x y)
     | (Array x, Array y) -> Array (CArrays.join x y)
     | (Blob x, Blob y) -> Blob (Blobs.join x y)
     | Blob (x,s,o), y
@@ -582,9 +580,7 @@ struct
         | None -> AD.widen y (AD.join y AD.top_ptr))
     | (Address x, Address y) -> Address (AD.widen x y)
     | (Struct x, Struct y) -> Struct (Structs.widen x y)
-    | (Union (f,x), Union (g,y)) -> Union (match UnionDomain.Field.widen f g with
-        | `Lifted f -> (`Lifted f, widen x y) (* f = g *)
-        | x -> (x, Top))
+    | (Union x, Union y) -> Union (Unions.widen x y)
     | (Array x, Array y) -> Array (CArrays.widen x y)
     | (Blob x, Blob y) -> Blob (Blobs.widen x y)
     | (Thread x, Thread y) -> Thread (Threads.widen x y)
@@ -605,19 +601,21 @@ struct
     let join_elem: (t -> t -> t) = smart_join x_eval_int y_eval_int in  (* does not compile without type annotation *)
     match (x,y) with
     | (Struct x, Struct y) -> Struct (Structs.join_with_fct join_elem x y)
-    | (Union (f,x), Union (g,y)) -> Union (match UnionDomain.Field.join f g with
-        | `Lifted f -> (`Lifted f, join_elem x y) (* f = g *)
-        | x -> (x, Top)) (* f <> g *)
+    | (Union (f,x), Union (g,y)) ->
+      let field = UnionDomain.Field.join f g in
+      let value = join_elem x y in
+      Union (field, value)
     | (Array x, Array y) -> Array (CArrays.smart_join x_eval_int y_eval_int x y)
     | _ -> join x y  (* Others can not contain array -> normal join  *)
 
   let rec smart_widen x_eval_int y_eval_int x y:t =
     let widen_elem: (t -> t -> t) = smart_widen x_eval_int y_eval_int in (* does not compile without type annotation *)
     match (x,y) with
     | (Struct x, Struct y) -> Struct (Structs.widen_with_fct widen_elem x y)
-    | (Union (f,x), Union (g,y)) -> Union (match UnionDomain.Field.widen f g with
-        | `Lifted f -> `Lifted f, widen_elem x y  (* f = g *)
-        | x -> x, Top) (* f <> g *)
+    | (Union (f,x), Union (g,y)) ->
+      let field = UnionDomain.Field.widen f g in
+      let value = widen_elem x y in
+      Union (field, value)
     | (Array x, Array y) -> Array (CArrays.smart_widen x_eval_int y_eval_int x y)
     | _ -> widen x y  (* Others can not contain array -> normal widen  *)