Limit CSE to pure expressions over constant variables

CSE performs some aggressive code motion, as it moves commoned
expressions up to the start of the code snippet. To simplify
reasoning, limit it to cases for which code motion is trivial.

libASL/transforms.ml

@@ -1465,7 +1465,19 @@ module CommonSubExprElim = struct
   *)
   exception CSEError of string
 
-  class gather_expressions = object
+  class gather_consts = object
+    inherit nopAslVisitor
+    val mutable consts : IdentSet.t = IdentSet.empty
+    method! vstmt s =
+      (match s with
+      | Stmt_ConstDecl(_, n, _, _) ->
+          consts <- IdentSet.add n consts;
+          SkipChildren
+      | _ -> DoChildren)
+    method get_info = consts
+  end
+
+  class gather_expressions consts = object
     inherit Asl_visitor.nopAslVisitor
 
     val mutable exprs: expr list = ([]: expr list);
@@ -1481,7 +1493,7 @@ module CommonSubExprElim = struct
       let () = match e with
       (* For now, only gather TApply's that we've seen more than once
          See eval_prim in value.ml for the list of what that covers. *)
-      | Expr_TApply(f,_,_) when List.mem f (Symbolic.prims_pure ()) ->
+      | Expr_TApply(f,_,_) when Symbolic.is_pure e && IdentSet.subset (fv_expr e) consts ->
           (match infer_type e with
           | Some (Type_Bits _) ->
               if (List.mem e cand_exprs) && not (List.mem e exprs) then
@@ -1585,7 +1597,10 @@ module CommonSubExprElim = struct
     )
 
   let do_transform (xs: stmt list): stmt list =
-    let expression_visitor = new gather_expressions in
+    let const_visitor = new gather_consts in
+    let _ = visit_stmts const_visitor xs in
+
+    let expression_visitor = new gather_expressions (const_visitor#get_info) in
     let expression_replacer = new replace_all_instances in
 
     let xs = visit_stmts expression_visitor xs in