profiling,-O in 6.12.1

[Simon Marlow]

On 03/02/2010 17:59, Serge D. Mechveliani wrote:
> To my
>
>>> Dear GHC team,
>>>
>>> It looks like  ghc-6.12.1  reports erroneous time profiling --
>>> when the Main module of the project is made under  -O.
>>> [..]
>>> This is for  ghc-6.12.1  made from source for Debian Linux and
>>> i386-like.
>>>
>>> Main.main  calls for  Complete.complete,  `complete' calls for
>>> eLoop  inside its source.
>>> eLoop  must take almost all the time.
>>> My whole user library is made under  -O -prof,  and
>>>                                       --enable-library-profiling.
>>> Main  is compiled by
>>>                           ghc $dmCpOpt -prof --make Main
>>> and run by               ./Main +RTS -M400m -pT -RTS
>>> For this key, the profiling report Main.prof looks natural and shows
>>>                                                     eLoop -- 97%.
>>>
>>> But for        ghc $dmCpOpt  -O  -prof --make Make,
>>>
>>> it shows a different thing:  zero  for  eLoop  and  99%  for `main'.
>>
>
>
> On Wed, Feb 03, 2010 at 05:38:36PM +0100, Daniel Fischer wrote:
>
>> Could be that eLoop is inlined with -O.
>
> Thank you.
> I also thought about this. But the question still looks difficult.
>
>
>> Try
>>
>> ghc $dmCpOpt -O -prof -auto-all --make
>>
>> That should show eLoop (if that's a top-level declaration, otherwise you'd
>> have to insert a pragma {-# SCC "eLoop" #-} manually).
>>
>
> eLoop  is not a top-level declaration, and I do set {-# SCC "eLoop" #-}.
> The key combination
>                         ghc $dmCpOpt -prof --make Main
> shows  95% for  eLoop,
> and adding  -O  to this line shows  0%  for  eLoop,  independently on
> presence of  -auto-all  in this line
> (the whole library is made under  -O -prof).

There are known bugs in this area, see e.g.

http://hackage.haskell.org/trac/ghc/ticket/2552

> Yes, I recall that the effect may be of inlining.

Inlining should not in general affect the shape of the profile.  I can't 
say much else here without seeing the specific code.

Cheers,
	Simon