suspicious piece of code in RTS(Profiling.c) code

[Edward Z. Yang]

Hello Ömer,

That does seem pretty suspicious. My interpretation is that
ccs should just never be EMPTY_STACK.  If we look at the calling
codepaths, the only case that a NULL could be passed is if
pushCostCentre is called with NULL ccs, but we only create these
invocations in code generation and never pass a NULL in such situations.

What I am not sure is if there is some special treatment for empty
stacks that should be happening, but is not, at the moment.

Edward

Excerpts from Ömer Sinan Ağacan's message of 2014-06-02 08:19:21 -0700:
> Hi all,
> 
> I recently found a suspicious piece of code in RTS code.
> 
> * https://github.com/ghc/ghc/blob/master/rts/Profiling.c#L622 here it's
> checking that ccs is not EMPTY_STACK.
> * https://github.com/ghc/ghc/blob/master/includes/rts/prof/CCS.h#L96 here
> EMPTY_STACK is defined as NULL.
> 
> Now weird part is that before the if-statement checks for `ccs != NULL` ccs
> is used to reach some of it's fields(for example, line 601 and 602). As far
> as I can see either we have a bug here or the if-statement is not necessary
> and we can move it's body to top level.
> 
> Am I missing anything here?
> 
> Thanks.
> 
> ---
> Ömer Sinan Ağacan
> http://osa1.net