Re: [GHC] #13425: Semantically-equivalent expressions evaluating at different values with -O1

17 Mar 2017

      #13425: Semantically-equivalent expressions evaluating at different values with -O1
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        Reporter:  rbasso            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Incorrect result  |  Unknown/Multiple
  at runtime                         |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 Reid thinks he's found the ultimate source, in an optimization he made
 intended to help with tag bit checks. Here's a greatly reduced
 reproduction case:

 {{{#!hs
 import Data.Bits ((.&.))

 flags :: Int -> Int
 flags x
   | x .&. 128 > 0 = 12
   | otherwise = 13
 {-# NOINLINE flags #-}

 main :: IO ()
 main = print (flags 255)
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13425#comment:4
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