Re: [PATCH v2 18/18] arm64: select ARCH_SUPPORTS_LTO_CLANG

From: Will Deacon
Date: Thu Nov 16 2017 - 13:45:12 EST

On Thu, Nov 16, 2017 at 10:39:51AM -0800, Paul E. McKenney wrote:
> On Thu, Nov 16, 2017 at 10:16:22AM -0800, Nick Desaulniers wrote:
> > > On Thu, Nov 16, 2017 at 06:34:17PM +0100, Peter Zijlstra wrote:
> > >> So the problem is that its very very hard (and painful) to find these
> > >> bugs. Getting the tools people to comment on these specific
> > >> optimizations would really help lots.
> >
> > No doubt; I do not disagree with you. Kernel developers have very
> > important use cases for the language.
> >
> > But the core point I'm trying to make is "do we need to block this
> > patch set until issues with the C standards body in regards to the
> > kernels memory model are resolved?" I would hope the two are
> > orthogonal and that we'd take them and then test them even more
> > extensively than the developer has in order to find out.
> Given that I have been working on getting the C and C++ standards to
> correctly handle rcu_dereference() for more than ten years, I recommend
> -against- waiting on standardization in the strongest possible terms.
> And if you think that ten years is bad, the Java standards community has
> been struggling with out-of-thin-air (OOTA) values for almost 20 years.
> And the C and C++ standards haven't solved OOTA, either.

The problem is, if we go ahead with this change, the compiler *will* break
some address dependencies and something will eventually go wrong. At that
point, what do we do? Turn off some random compiler optimisation? Add a
random barrier()?

We don't necessarily need standardisation, but we at least need guarantees
from the compiler implementation that LTO/PGO will respect source level
address dependencies. I don't think we have that today.