[GHC] #6166: Performance regression in mwc-random since 7.0.x

[GHC]

#6166: Performance regression in mwc-random since 7.0.x
-------------------------------------+-------------------------------------
        Reporter:  bos               |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.4.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:  x86_64
 Type of failure:  Runtime           |  (amd64)
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------
Description changed by bgamari:

Old description:

> I've had a report that the performance of the mwc-random package has
> regressed seriously after upgrading from GHC 7.0 to 7.4. It turns out
> that 7.2 also has the regression.
>
> Here's a sample program.
>
> {{{
> import qualified Data.Vector.Unboxed as U
>
> import qualified System.Random.MWC as R
> import System.Random.MWC.Distributions (standard)
>
> count = 1000 * 1000
>
> fast gen = standard gen
>
> slow gen = standard gen >>= return
>
> -- Edit this to choose fast or slow.
> which gen = slow gen
>
> main = do
>   gen <- R.create
>   v <- U.replicateM count (which gen)
>   print (U.last v)
> }}}
>
> With GHC 7.0.3 -O2, this runs in 0.294 sec, regardless of whether `fast`
> or `slow` is used.
>
> Under 7.4, `fast` runs in 0.062 sec (a nice speedup!), but `slow` now
> takes 9.2 sec (yikes!).
>
> Roman suggested compiling the `slow` version with `-fno-state-hack`,
> which brings performance back up to 0.062 sec.

New description:

 I've had a report that the performance of the mwc-random package has
 regressed seriously after upgrading from GHC 7.0 to 7.4. It turns out that
 7.2 also has the regression.

 Here's a sample program.

 {{{#!hs
 import qualified Data.Vector.Unboxed as U

 import qualified System.Random.MWC as R
 import System.Random.MWC.Distributions (standard)

 count = 1000 * 1000

 fast gen = standard gen

 slow gen = standard gen >>= return

 -- Edit this to choose fast or slow.
 which gen = slow gen

 main = do
   gen <- R.create
   v <- U.replicateM count (which gen)
   print (U.last v)
 }}}

 With GHC 7.0.3 -O2, this runs in 0.294 sec, regardless of whether `fast`
 or `slow` is used.

 Under 7.4, `fast` runs in 0.062 sec (a nice speedup!), but `slow` now
 takes 9.2 sec (yikes!).

 Roman suggested compiling the `slow` version with `-fno-state-hack`, which
 brings performance back up to 0.062 sec.

--

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/6166#comment:16>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler