atMost in GHC.Event.PSQ causes seqfault or bus error

[Kazu Yamamoto ( 山本和彦 )]

Hi,

After a long fight, I found that this is a bug of the "-O2" option of
GHC head on 32bit machine. Now, it is easy to reproduce this
segfault. Please read and try this:

	https://github.com/kazu-yamamoto/buggy-psq

Note that this segfault does not occur when "-O' is specified.

--Kazu

> Hi,
> 
> threadDelay of GHC head causes seqfault or bus error on 32bit machine
> including Mac and Linux. This bug does not exist as least in GHC
> 7.4.2.
> 
> This bug is due to atMost of GHC.Event.PSQ. After modifying
> libraries/base/base.cabal to export GHC.Event and building GHC head,
> the following code (compiled by the GHC head with -threaded) causes
> segfault or bus error:
> 
> main :: IO ()
> main = do
>     s <- newSource
>     ents <- replicateM 100 (entry s)
>     let q = fold ents
>     print $ Q.atMost 1.5 q
> 
> fold :: [(Q.Key, Q.Prio)] -> Q.PSQ ()
> fold [] = Q.empty
> fold ((u,r):xs) = Q.insert u r () $ fold xs
> 
> entry :: UniqueSource -> IO (Q.Key, Q.Prio)
> entry s = do
>     u <- newUnique s
>     r <- randomIO
>     return (u,r)
> 
> But if I copy GHC.Event.PSQ and GHC.Event.Unique, modify their module
> names, and compile the above code with them, no segfault/bus error
> happens.
> 
> I have been tracking this for a long time but I have no idea why this
> happens. Any advice would be highly appreciated.
> 
> For more information, please refer to:
> 
> 	http://hackage.haskell.org/trac/ghc/ticket/7715
> 
> --Kazu