Messages in this thread | | | Date | Wed, 7 Oct 2020 10:11:07 -0700 | From | "Paul E. McKenney" <> | Subject | Re: Control Dependencies vs C Compilers |
| |
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.
| |