[sr-dev] Compilation defaults for git code

[Andrei Pelinescu-Onciul]

On Oct 15, 2009 at 02:06, Jan Janak <jan at ryngle.com> wrote:
[...]
> 
> We have DBG_QM_MALLOC enabled by default in the git repository which I
> think is good, however, the value of memdbg is set to L_DBG and the
> value of debug is set to L_WARN by default. This means that unless you
> explicitly configure memdbg in your configuration file, you'll see all
> the memory debugging messages and it is a *a lot* of text.

No, that means by _default_ you don't see any malloc debug message
(debug=L_WARN and memdbg=L_DBG => memdbg>debug => you don't see them
unless you change debug to L_DBG or bigger).
> 
> You can disable memory debugging messages by setting the value of
> memdbg higher than debug, but I pretty much always forget to do it.
> Also the default configuration files are not consistent in this
> regard. The simple one disables it by setting debug to 2 (L_DBG is 3
> so memory debugging is disabled), but if you want to investigate a
> problem and set debug to 3 to do that then you end up having lots of
> traffic in syslog again and you have to use memdbg explicitly in the
> configuration file.
> The complex one, sip-router-oob.cfg, uses the defaults so you get all
> the messages.

In this case debug=L_WARN=0, so you won't see them.
> 
> How about changing the default for memdbg to a value that is higher
> than 3 (the debugging value)? Then we would always have memory
> debugging messages disabled and people (both users and developers) can
> easily turn them on by configuring a lower value in the configuration
> file.

Agreed.

> 
> The second default which I believe is not entirely correct for the
> code stored in the master branch in the git repository is the
> compilation mode. The makefile system generates code that is optimized
> for speed by default. We use -O9 which turns on pretty much all
> performance optimizations in gcc. That includes variables stored in
> CPU registers, however, code that uses such optimizations is difficult
> to debug because gdb cannot display values of variables stored in
> registers properly.
> 
> Here again, I pretty much always forget to add mode=debug when
> configuring the build and often have to recompile the code after
> trying to use gdb to debug something. Shouldn't we use mode=debug by
> default for the code in git master branch?

The problem with turning optimization off is that then you won't see any
warnings related to features which are enabled only at higher
optimizations levels (e.g. pointer aliasing, arrays overflows) and even
more importantly you won't see some bugs (e.g. forgetting a compiler
barrier won't have any ill effects since no vars are optimized in
registers).
I think it's better to find all the problems as early as possible and
not wait for a release to turn on the optimizations.


> 
> Or perhaps it is just me having these issues? Maybe others have secret
> techniques to share how to avoid such dumb problems?

In some cases optimizations debugging is harder, but statistically
that number of cases is small (we can figure most backtraces even with
all the optimizations).
I would rather have a fully optimized version and if I cannot figure a
crash, turn on debugging, rather then having a version which works
perfectly with debugging but sometimes crashes when optimized.


Andrei