[GHC] #15317: GHCi panic when trying to avoid GHC_OPTIONS -O warning

27 Jun 2018

      #15317: GHCi panic when trying to avoid GHC_OPTIONS -O warning
-------------------------------------+-------------------------------------
           Reporter:  ChaiTRex       |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  7.10.4
          Component:  GHCi           |           Version:  7.10.3
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Following the advice on [https://stackoverflow.com/a/27881726/7208029 an
 answer to "How can I load optimized code in GHCI?"], I get a GHCi panic.
 Running `ghc Luhn` succeeds just fine. On running `ghci Luhn`, I get:
 {{{
 $ ghci Luhn
 GHCi, version 7.10.3: http://www.haskell.org/ghc/  :? for help
 [1 of 1] Compiling Luhn             ( Luhn.hs, interpreted )
 ghc: panic! (the 'impossible' happened)
   (GHC version 7.10.3 for x86_64-unknown-linux):
         floatExpr tick break<15>()

 Please report this as a GHC bug:  http://www.haskell.org/ghc/reportabug
 }}}

 Contents of `Luhn.hs`:
 {{{#!hs
 {-# OPTIONS_GHC -fobject-code -O3 #-}

 module Luhn (checkLuhn) where

 import Data.Bits  (shiftL)
 import Data.Char  (digitToInt)
 import Data.List  (foldl')

 -- Quickly gets a list of digits from a nonnegative Integer
 -- Gives error for negative inputs
 -- Uses GMP's show for greatly-improved speed over GMP's div and mod
 toDigits :: Integer -> [Int]
 {-# INLINE toDigits #-}
 toDigits = map digitToInt . show

 -- Quickly gets the same result as iteratively getting the digit sum of a
 nonnegative Int until the digit sum is only one digit long
 -- Gives an erroneous value for negative inputs
 repeatedDigitSum :: Int -> Int
 {-# INLINE repeatedDigitSum #-}
 repeatedDigitSum n = (n - 1) `rem` 9 + 1

 -- Gets the Luhn sum, which is zero for valid inputs, of a list of digits
 -- Uses Data.Bits.shiftL to quickly double
 luhnSum :: [Int] -> Int
 {-# INLINE luhnSum #-}
 luhnSum = fromInteger . flip rem 10 . foldl' (+) 0 . zipWith ($) (cycle
 [toInteger, toInteger . repeatedDigitSum . flip shiftL 1])

 -- Checks whether a nonnegative Integer passes the Luhn algorithm
 -- Negative inputs are False, since the Luhn algorithm is intended for
 unsigned inputs
 checkLuhn :: Integer -> Bool
 {-# INLINABLE checkLuhn #-}
 checkLuhn n = (n >= 0) && ((== 0) . luhnSum . reverse . toDigits) n
 }}}

 Strangely, `ghci -fobject-code -O3 Luhn` works just great, so apparently
 it's not a problem with the switches?

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15317
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