Phil Endecott writes:
> Andrew Haley wrote:
> > Andrew Haley writes:
> > > Phil Endecott writes:
> > > > Dear Experts,
> > > >
> > > > I believe that ARM's swap instruction can be used to implement the
> > > > __sync_lock_test_and_set builtin, yet as far as I can see it is not
> > > > implemented. Can anyone please confirm if (a) it is implemented but
> > > > I'm doing something wrong, or (b) there's some reason why it can't be
> > > > done? FWIW here is some asm that I am currently using for this:
> > > >
> > > > asm volatile ("swp\t%0, %1, [%2]"
> > > > :"=r" (oldval)
> > > > :"r" (newval),
> > > > "r" (&mem)
> > > > :"memory");
> > >
> > > Looks fine to me.
> >
> > I forgot: SWP is deprecated on ARM. Use LDREX and STREX instead.
> >
> > See http://lkml.org/lkml/2006/12/6/180
>
> Yes, some ARMs have LDREX and STREX, but currently these still provide
> SWP. I'm coding for an XScale which only has SWP.
>
> But I don't believe that gcc implements __sync builtins that expand to
> the LDREX or STREX instructions either, does it?
No, it doesn't.
> This raises another issue, which is whether there is any way for code
> to determine which of the atomic builtins are provided, i.e. I'd like
> to write:
>
> #if HAS__sync_lock_test_and_set
> .. locking code using __sync_lock_test_and_set
> #elseif HAS__sync_something_else
> .. locking code using builtins that expand to LDREX and STREX
> #else
> #warn "no atomic ops, falling back to pthread mutex"
> ...
> #endif
What OS are you talking about? If you're using Linux, just #include
asm/atomic.h.
Andrew.
