bug in compacting GC

[Simon Marlow]

Bulat Ziganshin wrote:
> Hello ,
> 
> yes, i found this famous "-c" bug! the humor of situation is what
> easiest way to got this bug is to run ghc itself! :)  it seems that
> noone except me tried this ever. i just found that any large enough
> module raises this error!!! that, for example, results of compiling
> Data.Map module:
> 
> 
>>ghc Map.hs  -cpp  +RTS -c
> 
> ghc.EXE: internal error: update_fwd: unknown/strange object  39004
>     Please report this as a bug to glasgow-haskell-bugs at haskell.org,
>     or http://www.sourceforge.net/projects/ghc/
> 
> 
> ghc 6.4.1/mingw32. the error is absolutely reproducible, so at least
> now we had case, even lot of cases :)
> 
> or you wanna a smaller program? :)

I think this may be a different bug - I can reproduce it, but not with 
the current STABLE branch.  It is probably the freeze/thaw bug that was 
fixed after 6.4.1.

However, I've still seen strange crashes in STABLE when using +RTS -c. 
We build stage 3 of the nightly build with +RTS -c every night, and 
right now we aren't getting any crashes, but a few weeks ago we were. 
When I looked into it, I couldn't get a deterministically repeatable 
crash - every tiny change made the crash go away.

Cheers,
	Simon