Re: [LTP] [PATCH] syscalls/mallinfo02: introduce LTP_VAR_USED to avoid optimization

[Cyril Hrubis <chrubis@suse.cz>]

Hi!
> > > gcc 15 is a bit more clever and noticed that 'buf' isn't used
> > > for anything so it optimized it out, including call to malloc.
> > > So, there's also no mmap() call behind it and test fails,
> > > because nothing was allocated.
> >
> > Huh, that sounds like the optimizations are getting more and more evil
> > over the time.
> 
> Also see the next patch for bpf.

Sigh...

> > > Introduce LTP_VAR_USED macro, that makes compiler aware of the
> > > variable and doesn't optimize it out.
> > >
> > > Signed-off-by: Jan Stancek <jstancek@redhat.com>
> > > ---
> > >  include/tst_common.h                               | 2 ++
> > >  testcases/kernel/syscalls/mallinfo2/mallinfo2_01.c | 2 +-
> > >  2 files changed, 3 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/tst_common.h b/include/tst_common.h
> > > index b14bbae04077..3de826acd0ec 100644
> > > --- a/include/tst_common.h
> > > +++ b/include/tst_common.h
> > > @@ -13,6 +13,8 @@
> > >  #define LTP_ATTRIBUTE_UNUSED         __attribute__((unused))
> > >  #define LTP_ATTRIBUTE_UNUSED_RESULT  __attribute__((warn_unused_result))
> > >
> > > +#define LTP_VAR_USED(p) asm volatile("" :: "m"(p)); p
> >
> > Shouldn't __attribute__((used)) suffice?
> 
> It's ignored for local variables. It does work for global ones.

Maybe mention that in the commit description...

Reviewed-by: Cyril Hrubis <chrubis@suse.cz>

-- 
Cyril Hrubis
chrubis@suse.cz

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