On Thu, 2007-01-25 at 21:16 -0800, Nishanth Aravamudan wrote:
> On 25.01.2007 [19:35:15 -0800], Dave Hansen wrote:
> > On Thu, 2007-01-25 at 15:24 -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 */
> > > }
> > > }
> >
> > Aren't barriers the generally accepted (kernel-style) way of doing
> > this?
> >
> > #define barrier() __asm__ __volatile__("": : :"memory")
>
> Yes, but this isn't the kernel. Alternatively, we could probably just
> not optimize this one testcase, if it's really gcc causing the problem.
Oh, I was just saying that the kernel has a pretty good way of doing it.
And, if you use the kernel way, your application will break only as
often as the kernel in this regard. I can see a future compiler
optimizing away a sleep(0), but I can't see one optimizing away a
kernel-style barrier.
> > Can you do it in alloca() itself?
>
> Well, for __GNUC__, we get:
>
> # define alloca(size) __builtin_alloca (size)
>
> And even for non-__GNUC__, the function is inlined.
>
> So you want us to modify a GCC builtin, or a glibc, function for a
> simple testcase?
Heh. I've never used alloca(), I thought it was a libhuge thing. Why
don't you just distribute a custom version of glibc with libhuge? That
should be pretty easy.
-- Dave
;)
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