[Haskell-cafe] How to debug GHC

Frederik Eaton frederik at a5.repetae.net
Wed Sep 14 16:22:34 EDT 2005

On Wed, Sep 14, 2005 at 02:44:11PM +0100, Simon Marlow wrote:
> On 10 September 2005 21:15, Frederik Eaton wrote:
> > On Fri, Sep 02, 2005 at 04:40:05PM +0400, Bulat Ziganshin wrote:
> >> Hello Nils,
> >> 
> >> Friday, September 02, 2005, 10:47:05 AM, you wrote:
> >> 
> >>>     Compile your program with -prof -auto-all (make sure you have
> >>> the 
> >> 
> >>> I tried this out under GHC 6.4/Linux and got a segmentation fault
> >>> instead of a stack trace. Under GHC 6.2.2 it seemed to work, though.
> >> 
> >> this error is already fixed in current pre-6.4.1 version
> > 
> > I'm using a 2005/9/3 version of 6.4.1 and running into situations
> > where the "stack trace" has function A calling function B, where when
> > I look at the code, A never calls B. Is this normal? Is it some
> > side-effect of laziness? It sure makes the traces a lot less useful.
> It could be a bug - can you reduce the example and report it?
> GHC's profiler tries to overlay a lexical call graph on to the dynamic
> execution of the program.  It does this more or less in the way you
> described before: every function gets an extra argument describing the
> call context.  However, there are some tricky areas: notably CAFs.  We
> don't as yet have a principled description of the mechanism, and I know
> of various cases where odd results are obtained with the current system.
> Also, the optimiser has to be careful not to change the shape of the
> call graph, and I suspect there are cases where it goes wrong.

I see. Well, I'm afraid I've lost the example, but I'll keep my eyes
open in case it happens again. It's good to know what the correct
behavior is supposed to be.



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