On Tue, Dec 07, 2021 at 01:55:34PM +0000, Peter Maydell wrote:
> On Tue, 7 Dec 2021 at 13:53, Stefan Hajnoczi <stefa...@redhat.com> wrote:
> >
> > On Mon, Dec 06, 2021 at 02:34:45PM +0000, Peter Maydell wrote:
> > > On Mon, 6 Dec 2021 at 14:33, Stefan Hajnoczi <stefa...@redhat.com> wrote:
> > > >
> > > > v3:
> > > > - Added __attribute__((weak)) to get_ptr_*() [Florian]
> > >
> > > Do we really need it *only* on get_ptr_*() ? If we need to
> > > noinline the other two we probably also should use the same
> > > attribute weak to force no optimizations at all.
> >
> > The weak attribute can't be used on static functions, so I think we need
> > a different approach:
> >
> > In file included from ../util/async.c:35:
> > /builds/stefanha/qemu/include/qemu/coroutine-tls.h:201:11: error: weak
> > declaration of 'get_ptr_my_aiocontext' must be public
> > type *get_ptr_##var(void)
> > \
> > ^~~~~~~~
> > ../util/async.c:673:1: note: in expansion of macro
> > 'QEMU_DEFINE_STATIC_CO_TLS'
> > QEMU_DEFINE_STATIC_CO_TLS(AioContext *, my_aiocontext)
> > ^~~~~~~~~~~~~~~~~~~~~~~~~
> >
> > Adding asm volatile("") seems to work though:
> > https://godbolt.org/z/3hn8Gh41d
>
> You can see in the clang disassembly there that this isn't
> sufficient. The compiler puts in both calls, but it ignores
> the return results and always returns "true" from the function.
Specifying a input operand forces the compiler to evaluate the return
value of get_ptr_i():
static __attribute__((noinline)) long get_ptr_i()
{
long l = (long)&i;
asm volatile("" : "+rm" (l));
return l;
}
https://godbolt.org/z/e6ddGdPMv
Stefan
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