Help me grok addFinalizer

[Ian Lynagh]

On Thu, Feb 16, 2012 at 02:55:13PM -0600, Austin Seipp wrote:
> 64-bit GHC on OS X gives me this:
> 
> $ ghc -fforce-recomp -threaded finalizer
> [1 of 1] Compiling Main             ( finalizer.hs, finalizer.o )
> Linking finalizer ...
> $ ./finalizer
> waiting ...
> done!
> waiting ...
> running finalizer
> done!
> 
> However, it's a different story when `-O2` is specified:
> 
> $ ghc -O2 -fforce-recomp -threaded finalizer
> [1 of 1] Compiling Main             ( finalizer.hs, finalizer.o )
> Linking finalizer ...
> $ ./finalizer
> waiting ...
> running finalizer
> done!
> waiting ...
> done!
> 
> This smells like a bug. The stranger thing is that the GC will run the
> finalizer, but it doesn't reclaim the object? I'd think `readIORef`
> going after an invalidated pointer the GC reclaimed would almost
> certainly crash.

The finalizer is attached to the Thing, not the IORef. I haven't
checked, but I assume that ioref gets inlined, so effectively (ioref x)
is evaluated early. If you change it to

    readIORef (ioref' x) >>= \ix -> ix `seq` return ()

and define

    {-# NOINLINE ioref' #-}
    ioref' :: Thing -> IORef Bool
    ioref' = ioref

then you'll get the sort of output you expect.

Finalizers are tricky things, especially when combined with some of
GHC's optimisations.


Thanks
Ian