Andrew Haley wrote:
Phil Endecott writes:
> David Daney wrote:
> > Andrew Haley wrote:
> >> Phil Endecott writes:
> >> > Andrew Haley wrote:
> >> > > this stuff really should be done by the compiler.
> >> >
> >> > Yes. I've filed a bug asking for a __sync_lock_test_and_set builtin:
> >> >
> >> > http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33413
> >>
> >> Surer, but the problem is that for most of the things we want to do
> >> (lightweight locks, for example) __sync_lock_test_and_set() doesn't
> >> really do what we need: we need compare_and_swap(). That's why the
> >> kernel helper is so useful, because it's robust even if we are on
> >> pre-ARMv6 hardware.
> >
> > Probably the enhancement request should be expanded to include *all* the
> > __sync_* atomic memory primitives which includes compare_and_swap.
>
> No, because none of the others can be implemented without kernel
> support or other magic.
Yeah, I think that's right. Even in the case of post-v6 -- where it
actually is possible to do this safely in userland -- if it's
necessary to execute a bunch of instructions you might as well call a
subroutine.
There is nothing that says that __sync_compar_and_swap() cannot expand
to a libcall. If a libcall is the more efficient, then it *should* be
generated.
The reason I suggest it is that I would like to see as much knowledge as
possible about how to generate optimal code put into the compiler.
Ideally a GCC user would not have to write target specific assembly for
this.
David Daney
