Don't create local copies of vars in tail loops unless closures are c…

…reated.

The whole reason for creating local copies in the first place is to make sure
that those variables keep referring to the same thing they referred to when
they were closed over. There's no point in imposing that extra overhead if
there are no closures to refer to them.

src/CodeGen/Javascript/StgGen.hs

@@ -31,6 +31,7 @@ import CodeGen.Javascript.Optimize
 import CodeGen.Javascript.Errors
 import CodeGen.Javascript.Config
 import CodeGen.Javascript.Replace
+import CodeGen.Javascript.Traverse
 import Data.Int
 import Data.Word
 import Data.Maybe (isJust)
@@ -160,18 +161,44 @@ genRhs tailpos _ (StgRhsClosure _ _ _ upd _ args body) = do
     thunk [] l@(Lit _) = l
     thunk stmts expr   = Thunk stmts expr
 
-    loop deps | tailpos   = (:[]) . While (litN 1) . Block . copyVars deps
+    loop deps | tailpos   = (:[]) . While (litN 1) . Block . maybeCopyVars deps
               | otherwise = id
-    copyVars vars fnBody =
-      let locals = localCopies vars
-          -- First replace all vars with their local copies, then replace all
-          -- locals on the LHS with their external counterparts again.
-          -- Slow and stupid, but slightly simpler than doing one smart
-          -- replacement pass. Replace ASAP!
-          body' = replace (repVarsLHS (zip locals vars))
-                $ replace (repVars (zip vars locals)) fnBody
-      in return $ LocalCopy locals vars body'
-
+
+    -- Only copy vars if closures are created in the loop. This frees tight,
+    -- unboxed loops from the performance penalty of an extra compare and call
+    -- each iteration.
+    maybeCopyVars vars fnBody
+      | createsClosures (Block fnBody) =
+        let locals = localCopies vars
+            -- First replace all vars with their local copies, then replace all
+            -- locals on the LHS with their external counterparts again.
+            -- Slow and stupid, but slightly simpler than doing one smart
+            -- replacement pass. Replace ASAP!
+            body' = replace (repVarsLHS (zip locals vars))
+                  $ replace (repVars (zip vars locals)) fnBody
+        in return $ LocalCopy locals vars body'
+      | otherwise =
+          replace returnNullWithContinue fnBody
+
+    returnNullWithContinue = repNothing {
+        repStm = \e -> case e of
+           Ret Null -> Continue
+           _        -> e
+      }
+
+    -- Does the given code create any closures?
+    -- This is a bit too coarse-grained; we could eliminate the local copy for
+    -- a few more cases by only counting closures that reference any of the
+    -- tail-looped arguments.
+    createsClosures code =
+      case traverse isClosure False code of
+        Just x -> x
+        _      -> False
+
+    isClosure (Exp (Fun _ _))   _ = done True
+    isClosure (Exp (Thunk _ _)) _ = done True
+    isClosure _ acc               = return acc
+
 -- | Create local copies of the given vars.
 localCopies :: [JSVar] -> [JSVar]
 localCopies = map $ fmap (";n_" ++)