[GHC] #15460: Literals overflow

31 Jul 2018

      #15460: Literals overflow
-------------------------------------+-------------------------------------
           Reporter:  hsyl20         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.4.3
           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 the following example:

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 import GHC.Int

 main :: IO ()
 main = do
    let x = I#
 (0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff#)
    print x
 }}}

 It gets desugared into:
 {{{#!hs
 main
   = print
       @ Int
       GHC.Show.$fShowInt
       (GHC.Types.I#
 7237005577332262213973186563042994240829374041602535252466099000494570602495#)
 }}}

 Problem: the literal value isn't rounded and there is no overflow warning.

 It breaks the invariant that literal values in Core have to be in range.
 Bad things can happen when we break this invariant:

 {{{#!hs
 {-# LANGUAGE MagicHash #-}
 import GHC.Int
 import Control.Monad

 main :: IO ()
 main = do
    let x = I#
 (0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff#)
    when (x > maxBound) $ do
       print "Oups"
...
ghc TestLitOverflow.hs -Wall -O2
./TestLitOverflow
 "Oups"
 }}}
-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15460
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