Re: Control Dependencies vs C Compilers

[Paul E. McKenney]

On Wed, Oct 07, 2020 at 01:50:54PM +0200, Peter Zijlstra wrote:
> On Wed, Oct 07, 2020 at 12:20:41PM +0200, Florian Weimer wrote:
> > * Peter Zijlstra:

[ . . . ]

> > >> I think in GCC, they are called __atomic_load_n(foo, __ATOMIC_RELAXED)
> > >> and __atomic_store_n(foo, __ATOMIC_RELAXED).  GCC can't optimize relaxed
> > >> MO loads and stores because the C memory model is defective and does not
> > >> actually guarantee the absence of out-of-thin-air values (a property it
> > >> was supposed to have).
> > >
> > > AFAIK people want to get that flaw in the C memory model fixed (which to
> > > me seemd like a very good idea).
> > 
> > It's been a long time since people realized that this problem exists,
> > with several standard releases since then.
> 
> I've been given to believe it is a hard problem. Personally I hold the
> opinion that prohibiting store speculation (of all kinds) is both
> necesary and sufficient to avoid OOTA. But I have 0 proof for that.

There are proofs for some definitions of store speculation, for example,
as proposed by Demsky and Boehm [1] and as prototyped by Demsky's student,
Peizhao Ou [2].  But these require marking all accesses and end up being
optimized variants of acquire load and release store.  One optimization
is that if you have a bunch of loads followed by a bunch of stores,
the compiler can emit a single memory-barrier instruction between the
last load and the first store.

I am not a fan of this approach.

Challenges include:

o	Unmarked accesses.  Compilers are quite aggressive about
	moving normal code.

o	Separately compiled code.  For example, does the compiler have
	unfortunatel optimization opportunities when "volatile if" 
	appears in one translation unit and the dependent stores in
	some other translation unit?

o	LTO, as has already been mentioned in this thread.

Probably other issues as well, but a starting point.

							Thanx, Paul

[1]	https://dl.acm.org/doi/10.1145/2618128.2618134
	"Outlawing ghosts: avoiding out-of-thin-air results"
	Hans-J. Boehm and Brian Demsky.

[2]	https://escholarship.org/uc/item/2vm546k1
	"An Initial Study of Two Approaches to Eliminating Out-of-Thin-Air
	Results" Peizhao Ou.