[GHC] #16111: Inconsistent behavior of Data.Bits.shiftL with different optimization levels and -fllvm

[GHC]

#16111: Inconsistent behavior of Data.Bits.shiftL with different optimization
levels and -fllvm
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.6.3
           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:
-------------------------------------+-------------------------------------
 Consider this program:

 {{{#!hs
 module Main (main) where

 import Data.Bits
 import Data.Word

 main :: IO ()
 main = print $ toInteger (shiftL 1 hm :: Word64)
             == toInteger (shiftL 1 hm :: Word64)

 hm :: Int
 hm = -1
 {-# NOINLINE hm #-}
 }}}

 The result of this program depends greatly on what optimization levels are
 used, and whether `-fllvm` is used:

 {{{
 $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O0 Bug.hs && ./Bug
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 Linking Bug ...
 True

 $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O1 Bug.hs && ./Bug
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 Linking Bug ...
 True

 $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O2 Bug.hs && ./Bug
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 Linking Bug ...
 Bug: Bad shift length-1

 $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O0 -fllvm Bug.hs && ./Bug
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 Linking Bug ...
 True

 $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O1 -fllvm Bug.hs && ./Bug
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 Linking Bug ...
 False

 $ /opt/ghc/8.6.3/bin/ghc -fforce-recomp -O2 -fllvm Bug.hs && ./Bug
 [1 of 1] Compiling Main             ( Bug.hs, Bug.o )
 Linking Bug ...
 Bug: Bad shift length-1
 }}}

 This program manages to return all of `True`, `False`, and `Bad shift
 length-1` depending on which flags are used!

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16111>
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