[avr-gcc-list] Re: Stack use - possible bug

[David Brown]

On 09/06/2010 15:26, Dale wrote:
> Hi Eric,>@2010.06.09_15:16:08_+0200
>
> > You need to use the "noinline" function attribute on the initialise() function.
>
> Sure, that's one solution. Another is to simply remove static from
> initialise(). Its easy if you know which functions are affected.

No, that's not a solution - trying to lie to the compiler is never 
(well, /almost/ never) the best idea.  Once avr-gcc moves over to gcc 
4.5 and people start using link-time optimisation, the compiler will 
figure out by itself that the function is only called once even without 
the "static".

> However, its not always obvious what the compiler is going to do. And
> therefore where to do this type of thing. Its the kind of issue that can
> catch you unawares.
>
> The optimising is effectively creating stack-space for variables but not
> destroying it. The boundary for where the variables are no longer used,
> should be known.

The compiler /does/ know when variables are used or not.  But the usual 
way to allocate frame space is to allocate the maximum needed space on 
function entry, and free it on function exit.  There are odd occasions 
like this one in which this is not the behaviour you want, but usually 
that produces the best code.  Similarly, the usual behaviour is to 
automatically inline functions that are known to be used only once.

In a one-off case like this, Eric's recommendation of a "noinline" 
function attribute is clear and simple to use.  If you want a more 
general solution, the compiler lets you fiddle with all sorts of 
parameters that control this sort of thing.  In particular, there are 
the "large-stack-frame" and "large-stack-frame-growth" parameters than 
you can play with.  Something like "--param large-stack-frame=20" might 
be more appropriate for your use than the standard, which is to allow a 
stack frame growth of 256 bytes.

See <http://gcc.gnu.org/onlinedocs/gcc-4.3.5/gcc/Optimize-Options.html>

mvh.,

David

> > See here:
> > <http://gcc.gnu.org/onlinedocs/gcc-4.3.5/gcc/Function-Attributes.html#Function-Attributes>
> >
> > Eric
> >
> > > -----Original Message-----
> > > From:
> > > address@hidden
> > > [mailto:address@hidden
> > > org] On Behalf Of Dale
> > > Sent: Wednesday, June 09, 2010 6:43 AM
> > > To: address@hidden
> > > Subject: [avr-gcc-list] Stack use - possible bug
> > >
> > > Hi
> > >
> > > Stumbled onto this due to dire shortage of ram and monitoring stack
> > > usage. To get an idea of whether it was enough, etc.
> > >
> > > I'll try and put together sample code which replicates it but the
> > > scenario for now is:
> > >
> > > static void initialise(void)
> > > {
> > >         char    buf[50];        // some data
> > >
> > >         // call a bunch of functions
> > > }
> > >
> > > int main(void)
> > > {
> > >         initialise();
> > >         
> > >         // bunch of stuff
> > > }
> > >
> > > Now, initialise() is only ever called once and therefore is optimised
> > > and included inline into main().
> > >
> > > What I find is that space used by buf[] is never released from the
> > > stack. There may in fact be more since initialise() calls a
> > > whole bunch
> > > of other functions, some of which may or may not have variables on the
> > > stack and/or be one-off functions and as a result 'rolled-into' code
> > > that then forms main().
> > >
> > > Given my constrained setup, its critical that this stack
> > > space is freed.
> > >
> > > Anyone come across this? Is it a known issue?
> > >
> > > --
> > >
> > > Cheers,
> > >
> > > Dale.
> > >
> > > _______________________________________________
> > > AVR-GCC-list mailing list
> > > address@hidden
> > > http://lists.nongnu.org/mailman/listinfo/avr-gcc-list