[GHC] #16040: Unboxing-Related Performance Issue with Polymorphic Functions

12 Dec 2018

      #16040: Unboxing-Related Performance Issue with Polymorphic Functions
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           Reporter:  _recursion     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.2
           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:
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 My team has observed a 2x performance degradation in code that makes use
 of `StateT` that appears to be related to strictness and unboxing, even
 when built with `-O2`. Our code makes heavy use of the state monad, and
 when GHC fails to optimise this performance

 We've managed to minimise the behaviour to the following reproducer (that
 ignores state entirely), consisting of two files. It depends on
 `criterion`

 `Lib.hs`:

 {{{#!hs
 {-# LANGUAGE BangPatterns #-}

 module Lib where

 -- A type to take the place of state
 data X a = X { runX :: !a }

 test1 :: Int -> Int
 test1 = \(!i) -> go i where
     go = \(!i) -> if i > 0
         then go $! i - 1
         else i
 {-# NOINLINE test1 #-}

 test2 :: Int -> Int
 test2 = \(!i) -> runX (go i) where
     go = \(!i) -> if i > 0
         then go $! i - 1
         else X i
 {-# NOINLINE test2 #-}
 }}}

 `Main.hs`:

 {{{#!hs
 {-# LANGUAGE Strict #-}
 module Main where

 import Lib
 import Criterion
 import Criterion.Main

 main :: IO ()
 main = defaultMain
     [ bgroup "main"
         [ bgroup "small"
             [ bench "without state" $ whnf test1 100000000
             , bench "with state"    $ whnf test2 100000000
             ]
         ]
     ]
 }}}

 Run as above, the code takes twice as long to execute `test2` as it does
 `test1`. However, when the signature for `runX` is changed to `runX ::
 !Int`, the code in `test2` exhibits identical performance to `test1`.

 It has been reproduced across multiple linux64 machines, but not tested on
 any other architecture or operating system.

 Please find the full (stack - yes, I know) project as an attachment. You
 can simply `stack run` to observe the issue. If you have any further
 queries please let me know.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/16040
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