#15717: Performance regression in for_ alternatives from GHC 8.2.2 to newer GHCs -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by nh2): * milestone: => 8.8.1 Comment: Is this something that could be tackled for 8.8? Some of the real-world applications we'd really like to upgrade past 8.2 appear to get slower from this. (Tentatively seting milestone) -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15717#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15717: Performance regression in for_ alternatives from GHC 8.2.2 to newer GHCs -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Comment (by nh2): Replying to [comment:3 simonpj]:

There are 9 tests in the repro case. I've lost track of what is what. If someone was able to distil out a simple before-and-after on one test

This part I can answer immediately: I use only `./TraverseMaybePerformance 8` in the issue description, so that's `test8` form the repro. But yes, a smaller test case will make things easier. Here it is: {{{ #!/usr/bin/env stack -- stack --resolver lts-11.22 script --optimize -- The above one is fast. Slow is: -- stack --resolver lts-12.11 script --optimize {-# OPTIONS_GHC -Wall #-} import Control.Concurrent (yield) import Control.Monad (when) import qualified Data.Text.Lazy as T import qualified Data.Foldable as F myfor_ :: (Foldable f, Applicative app) => f a -> (a -> app ()) -> app () myfor_ t f = case F.toList t of [] -> pure () x -> go x where go [x] = f x go (x:xs) = f x *> go xs printChars_myfor_ :: Int -> T.Text -> IO () printChars_myfor_ idx t = myfor_ (T.uncons t) $ \(c, t') -> do when (idx `mod` 100000 == 0) $ do -- Using putStrLn, I observe 2x more allocations with GHC 8.4.3 vs 8.2.2 (860 vs 460 M) --putStrLn $ "Character #" ++ show idx ++ ": " ++ show c -- Using yield, I observe 4x more allocations with GHC 8.4.3 vs 8.2.2 (860 vs 220 M) yield -- the putStrLn isn't necessary, this is enough to trigger the regression printChars_myfor_ (idx + 1) t' main :: IO () main = printChars_myfor_ 1 $ T.replicate 5000000 $ T.singleton 'x' }}} I even managed to make the regression 4x worse using `yield`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15717#comment:4 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler

#15717: Performance regression in for_ alternatives from GHC 8.2.2 to newer GHCs -------------------------------------+------------------------------------- Reporter: nh2 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.8.1 Component: Compiler | Version: 8.6.1 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: Runtime | Unknown/Multiple performance bug | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by nh2): * Attachment "TraverseMaybePerformanceSimplified-GHC-bug-15717-GHC-8.4.3 .dump-simpl" added. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15717 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler