GHCi produces incorrect results when evaluating with compiled code

When GHCi evaluates in non-interpreted mode, it sometimes produces incorrect results. The following example---extracted from a much larger program---illustrates the problem:

import Lex

num  = 100000
txt1 = "aaa,aaa"

main :: IO ()
main = print $ sum $ map (length . haskellLex) $ replicate num txt1

This program lexes the same string 100000 times, and prints the total number of tokens. Since we are lexing the same string, we'd expect to always get the same result, but due to this bug, this is not the case! We repeat the process 100000 times so that we can encounter the bug somewhat reliably---the problem does not occur every time.

To reproduce the problem, we need to load Lex in compiled form, not interpreted and with some optimizations:

ghc -c -O1 -dynamic Lex.hs

The source code for the "lexer" is as follows:

module Lex (haskellLex) where

cclass :: Char -> Int
cclass c =
  case c of
    'a' -> 10
    ',' -> 11

    'A' -> 0
    'B' -> 0
    'C' -> 0
    'D' -> 0
    'E' -> 0
    'F' -> 0

haskellLex :: String -> [()]
haskellLex [] = []
haskellLex (i:is) =
  case cclass i of
    10 -> haskellLex62 is
    11 -> () : haskellLex is

haskellLex62 :: String -> [()]
haskellLex62 [] = [()]
haskellLex62 (i:is) =
  case cclass i of
    0   -> [()]
    1   -> [()]
    2   -> [()]
    3   -> [()]
    4   -> [()]

    10  -> haskellLex62 is
    11  -> () : haskellLex (i:is)

    x   -> error ("[GHC BUG] cclass returned: " ++ show (i,x)

This is a minimized version from a full lexer. As odd as it looks, removing pretty much anything seems to cause the bug to go away. This is what happens when we run the program:

runhaskell test.hs
test.hs: [GHC BUG] cclass returned: (',',-556)
CallStack (from HasCallStack):
  error, called at Lex.hs:36:12 in main:Lex

The problem is that after many evaluations, the function cclass returned -556 for character ,, when it should have returned 11.

I've been able to reproduce this on two Linux machines, both running 64-bit Ubuntu (16.04). The issue does not seem to happen on Mac OS.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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