id,summary,reporter,owner,description,type,status,priority,milestone,component,version,resolution,keywords,cc,os,architecture,failure,difficulty,testcase,blockedby,blocking,related
2154,Stack overflow due to unsafePerformIO,dons,igloo,"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?",merge,closed,normal,6.8.3,Runtime System,6.8.2,fixed,,dons@…,Unknown/Multiple,Unknown/Multiple,,Unknown,,,,