[GHC] #11533: Stack check not optimized out even if it could be

3 Feb 2016


      #11533: Stack check not optimized out even if it could be
-------------------------------------+-------------------------------------
           Reporter:  jscholl        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.3
  (CodeGen)                          |
           Keywords:  cmm            |  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:
-------------------------------------+-------------------------------------
 CmmLayoutStack tries to remove a stack check if a function uses no stack
 space. It knows that Sp < SpLim is always false, but not that Sp >= SpLim
 is always true. However, the latter can arise when GHC flips the
 comparison (which it does sometimes).

 An example would be the worker function generated for {{{go}}}:

 {{{
 countDown :: Int -> Int
 countDown = go 0
     where
         go acc n | n > 0 = go (acc + 1) (n - 1)
                  | otherwise = acc + n
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11533
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