only Up is needed, optimize a special case of Upto

Author: bobzhang

jscomp/core/js_dump.ml

@@ -267,7 +267,6 @@ let pp_direction f (direction : J.for_direction) =
   match direction with
   | Up
   | Upto -> P.string f L.plus_plus
-  | Down
   | Downto -> P.string f L.minus_minus 
 
 let return_sp f = 
@@ -1181,10 +1180,6 @@ and statement_desc top cxt f (s : J.statement_desc) : cxt =
                       let (_,_,right) = Js_op_util.op_prec Ge in
                       P.string f L.ge ;
                       right
-                    | Down ->
-                      let (_,_,right) = Js_op_util.op_prec Gt in
-                      P.string f L.gt ;
-                      right                      
                   in
                   P.space f ;
                   let cxt  =

jscomp/core/js_op.ml

@@ -188,7 +188,7 @@ type direction_flag =
   | Upto
   | Downto
   | Up
-  | Down
+
 (* 
     {[
     let rec x = 1 :: y 

jscomp/core/lam_compile.ml

@@ -1606,11 +1606,11 @@ and compile_lambda
     | Lwhile(p,body) ->
       compile_while p body lambda_cxt
     | Lfor (id,start,finish,direction,body) ->  
-      begin match direction with 
-        | Upto ->                 
-          compile_for id start finish Upto body lambda_cxt
-        | Downto -> 
-          compile_for id start finish Downto body lambda_cxt
+      begin match direction,finish with 
+        | Upto, Lprim {primitive = Psubint ; args = [ new_finish ; Lconst (Const_int 1) ]} ->
+          compile_for id start new_finish Up body lambda_cxt
+        | _ -> 
+          compile_for id start finish (if direction = Upto then Upto else Downto) body lambda_cxt
       end
     | Lassign(id,lambda) ->
       compile_assign id lambda lambda_cxt