Re: [RFC] LKMM: Add volatile_if()

[Marco Elver <elver@google.com>]

On Mon, 7 Jun 2021 at 19:04, Marco Elver <elver@google.com> wrote:
[...]
> > > > So the barrier which is a compiler barrier but not a machine barrier is
> > > > __atomic_signal_fence(model), but internally GCC will not treat it smarter
> > > > than an asm-with-memory-clobber today.
> > >
> > > FWIW, Clang seems to be cleverer about it, and seems to do the optimal
> > > thing if I use a __atomic_signal_fence(__ATOMIC_RELEASE):
> > > https://godbolt.org/z/4v5xojqaY
> >
> > Indeed it does!  But I don't know of a guarantee for that helpful
> > behavior.
>
> Is there a way we can interpret the standard in such a way that it
> should be guaranteed?

I figured out why it works, and unfortunately it's suboptimal codegen.
In LLVM __atomic_signal_fence() turns into a real IR instruction,
which when lowered to asm just doesn't emit anything. But most
optimizations happen before in IR, and a "fence" cannot be removed.
Essentially imagine there's an invisible instruction, which explains
why it does what it does. Sadly we can't rely on that.

> The jumping-through-hoops variant would probably be asking for a
> __builtin primitive that allows constructing volatile_if() (if we can't
> bend existing primitives to do what we want).

I had a think about this. I think if we ask for some primitive
compiler support, "volatile if" as the target is suboptimal design,
because it somewhat limits composability (and of course make it hard
to get as an extension). That primitive should probably also support
for/while/switch. But "volatile if" would also preclude us from
limiting the scope of the source of forced dependency, e.g. say we
have "if (A && B)", but we only care about A.

The cleaner approach would be an expression wrapper, e.g. "if
(ctrl_depends(A) && B) { ... }".

I imagine syntactically it'd be similar to __builtin_expect(..). I
think that's also easier to request an extension for, say
__builtin_ctrl_depends(expr). (If that is appealing, we can try and
propose it as std::ctrl_depends() along with std::dependent_ptr<>.)

Thoughts?

Thanks,
-- Marco