Re: [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
      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:                    |
-------------------------------------+-------------------------------------
Description changed by hsyl20:

Old description:
...
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"
}}}
New description:

 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

 main :: IO ()
 main = do
    let x = I#
 (0xfffffffffffffffffffffffffffffffffffffffffffffffffffffffffffffff#)
    if (x > maxBound)
       then print "Oups"
       else print "Ok"
...
ghc TestLitOverflow.hs -Wall -O0
./TestLitOverflow
 "Ok"
...
ghc TestLitOverflow.hs -Wall -O2
./TestLitOverflow
 "Oups"
 }}}
--

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15460#comment:1
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