Re: [RFC] LKMM: Add volatile_if()

[Segher Boessenkool]

On Mon, Jun 07, 2021 at 11:01:39AM +0300, Alexander Monakov wrote:
> Uhh... I was not talking about some (non-existent) "optimizing linker".
> LTO works by relaunching the compiler from the linker and letting it
> consume multiple translation units (which are fully preprocessed by that
> point). So the very thing you wanted to avoid -- such barriers appearing
> in close proximity where they can be deduplicated -- may arise after a
> little bit of cross-unit inlining.
> 
> My main point here is that using __COUNTER__ that way (making things
> "unique" for the compiler) does not work in general when LTO enters the
> picture. As long as that is remembered, I'm happy.

Yup.  Exactly the same issue as using this in any function that may end
up inlined.

> > In the case of "volatile_if()", we actually would like to have not a
> > memory clobber, but a "memory read". IOW, it would be a barrier for
> > any writes taking place, but reads can move around it.
> > 
> > I don't know of any way to express that to the compiler. We've used
> > hacks for it before (in gcc, BLKmode reads turn into that kind of
> > barrier in practice, so you can do something like make the memory
> > input to the asm be a big array). But that turned out to be fairly
> > unreliable, so now we use memory clobbers even if we just mean "reads
> > random memory".
> 
> 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.

It will do nothing for relaxed ordering, and do blockage for everything
else.  Can it do anything weaker than that?


Segher