[GHC] #15127: Unbox around runRW#

6 May 2018

      #15127: Unbox around runRW#
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  low            |         Milestone:  8.8.1
          Component:  Compiler       |           Version:  8.4.2
           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:
-------------------------------------+-------------------------------------
 Because `runRW#` inlines so late, we don't end up unboxing around it. For
 example, suppose we write

 {{{#!hs
 f x = runST $ do
   mar <- newArray 100 x
   unsafeFreeze mar
 }}}

 This will compile to something like

 {{{#!hs
 f x = case runRW# (\s ->
         case newArray# 100# x s of { (# s', mar #) ->
         case unsafeFreezeArray# mar s' of { (# s'', ar# #) ->
           (# s'', Array ar# #) }}) of
          (# _, ar #) -> ar
 }}}

 We generally want to pull the constructor application out of the `runRW#`
 argument:

 {{{#!hs
 f x = case runRW# (\s ->
         case newArray# 100# x s of { (# s', mar #) ->
           unsafeFreezeArray# mar s' }) of
         (# _, ar# #) -> Array ar#
 }}}

 This exposes the `Array` constructor to case-of-case.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15127
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler