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

[Jan Stancek]

On Thu, Feb 6, 2025 at 1:45 PM Cyril Hrubis <chrubis@suse.cz> wrote:
>
> 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.

>
> > 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.

>
> >  #ifndef ARRAY_SIZE
> >  # define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]))
> >  #endif
> > diff --git a/testcases/kernel/syscalls/mallinfo2/mallinfo2_01.c b/testcases/kernel/syscalls/mallinfo2/mallinfo2_01.c
> > index 90cf4fcb3b89..51783fc22b80 100644
> > --- a/testcases/kernel/syscalls/mallinfo2/mallinfo2_01.c
> > +++ b/testcases/kernel/syscalls/mallinfo2/mallinfo2_01.c
> > @@ -26,7 +26,7 @@ void test_mallinfo2(void)
> >       char *buf;
> >       size_t size = 2UL * 1024UL * 1024UL * 1024UL;
> >
> > -     buf = malloc(size);
> > +     LTP_VAR_USED(buf) = malloc(size);
> >
> >       if (!buf)
> >               tst_brk(TCONF, "Current system can not malloc 2G space, skip it");
> > --
> > 2.43.0
> >
> >
> > --
> > Mailing list info: https://lists.linux.it/listinfo/ltp
>
> --
> Cyril Hrubis
> chrubis@suse.cz
>