Stack overflow due to unsafePerformIO

The following program, extracted from a utf8 decoder for ByteStrings, overflows the stack when run with rules on. Speaking to SimonM, this is due to the stack squeezing optimisation not working for unsafePerformIO, meaning the resulting loop actually uses up stack.

import qualified Data.ByteString.Char8 as S 

main = putStrLn $ filter (\x -> enc x /= [x]) list
  where list = ['\0'..'\x10ffff'] ++ ['\0'..'\10']

{-# NOINLINE enc #-}
enc :: Char -> String
enc x = S.singleton x `seq` [x]

After fusing and optimising, S.singleton here is allocated using unsafePerformIO to wrap the allocation, and seems to run out of stack.

However, I've confirmed this code does work if its changed to use unsafeDupablePerformIO.

I'm filing this ticket to get documentation on what is going on here, so we can have something to point to if it comes up again, and to clarify that unsafeDupablePerformIO is the correct fix.

SimonM, could you summarise what the issue is?

Trac metadata

Trac field	Value
Version	6.8.2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	dons@galois.com
Operating system	Unknown
Architecture	Unknown