On Sat, Jan 27, 2007 at 09:53:54AM -0800, Nishanth Aravamudan wrote:
> On 27.01.2007 [23:37:16 +1100], David Gibson wrote:
> > On Fri, Jan 26, 2007 at 02:44:59PM -0800, Nishanth Aravamudan wrote:
> > > On 26.01.2007 [10:37:03 +1100], David Gibson wrote:
> > > > On Thu, Jan 25, 2007 at 03:24:56PM -0600, Adam Litke wrote:
> > > > > Hmm. On x86, sometimes gcc will optimize out the alloca() call. This
> > > > > patch fixes it up nicely. Please apply.
> > > > >
> > > > > Signed-off-by: Adam Litke <[EMAIL PROTECTED]>
> > > > >
> > > > > diff --git a/tests/stack_grow_into_huge.c
> > > > > b/tests/stack_grow_into_huge.c
> > > > > index fc8a030..7112428 100644
> > > > > --- a/tests/stack_grow_into_huge.c
> > > > > +++ b/tests/stack_grow_into_huge.c
> > > > > @@ -49,6 +49,7 @@ void do_child()
> > > > > {
> > > > > while (1) {
> > > > > alloca(16*1024*1024);
> > > > > + sleep(0); /* Defeat the gcc optimizer */
> > > > > }
> > > > > }
> > > >
> > > > Hrm, that's a little strangely magical. I'd suggest instead doing:
> > > > volatile int *x;
> > > > x = alloca(lots);
> > > > *x = 1;
> > > >
> > > > Then it's clear that the compiler is forbidden from eliding the
> > > > alloca().
> > >
> > > Agreed, and I prefer this to the barrier() suggestion from Dave. I'll
> > > make this change locally and drop Adam's fix-up patch.
> >
> > Oh, did realise the only problem with that approach is that it will
> > really truly allocate rather a lot of memory, which might be a problem
> > in some conditions. That can be addressed by making the alloc() block
> > size larger.
>
> How much larger do you think it should be than 16M?
Duh, brain failure. I was reading that as 16k. 16M should be fine.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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