[GHC] #11795: Performance issues with replicateM_

6 Apr 2016

      #11795: Performance issues with replicateM_
-------------------------------------+-------------------------------------
           Reporter:  snoyberg       |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:                 |           Version:  7.10.3
  libraries/base                     |
           Keywords:                 |  Operating System:  MacOS X
       Architecture:  x86_64         |   Type of failure:  Runtime
  (amd64)                            |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 When working on optimizing a program by minimizing allocations, I can into
 an issue with `replicateM_`. Consider the following code

 {{{#!hs
 import Control.Monad (replicateM_)
 import Foreign.C.String (withCString)
 import Foreign.Storable (peek)

 main :: IO ()
 main = withCString "foo" $ replicateM_ 10000000 . peek
 }}}

 When I run this program, I get:

 160,042,656 bytes allocated in the heap

 The result is the same whether I compile with `-O0`, `-O`, or `-O2`. And
 as expected, the total allocation increases or decreases based on the
 numbers of times I replicate the action. On the other hand, replacing
 `replicateM_` with a hand-written version makes the total allocations for
 the program only 42KB, and does not increase with the numbers of
 replications.

 {{{#!hs
 replicateM_ :: Monad m => Int -> m a -> m ()
 replicateM_ cnt0 f =
     loop cnt0
   where
     loop cnt
         | cnt <= 0  = return ()
         | otherwise = f >> loop (cnt - 1)
 }}}

 By contrast, `Control.Monad.replicateM_` looks like:

 {{{#!hs
 replicateM_       :: (Monad m) => Int -> m a -> m ()
 {-# INLINEABLE replicateM_ #-}
 {-# SPECIALISE replicateM_ :: Int -> IO a -> IO () #-}
 {-# SPECIALISE replicateM_ :: Int -> Maybe a -> Maybe () #-}
 replicateM_ n x   = sequence_ (replicate n x)
 }}}

 I can't see an advantage to this implementation over the more direct
 implementation I've provided above. Unless there are objections, I'll send
 a patch to switch the implementation. (Since master already uses
 `Applicative`, I'll make the relevant updates to generalize the function
 signature too.)

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11795
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