Re: [RFC] Bridging the gap between the Linux Kernel Memory Consistency Model (LKMM) and C11/C++11 atomics

[Mathieu Desnoyers]

On 7/5/23 03:05, Boqun Feng wrote:
> On Mon, Jul 03, 2023 at 03:20:31PM -0400, Olivier Dion wrote:
> [...]
> > NOTE: On x86-64, we found at least one corner case [7] with Clang where
> > a RELEASE exchange is optimized to a RELEASE store, when the returned
> > value of the exchange is unused, breaking the above expectations.
> > Although this type of optimization respect the standard "as-if"
> > statement, we question its pertinence since a user should simply do a
> > RELEASE store instead of an exchange in that case.  With the
> > introduction of these new primitives, these type of optimizations should
> > be revisited.
>
> FWIW, this is actually a LLVM bug:
>
> 	https://github.com/llvm/llvm-project/issues/60418

So it was more than a dubious optimization, it's actually broken as well.

I am worried about adding to the compiler's ability to optimize those 
atomics because of the subtle corner-cases/bugs that can creep up.

Thanks,

Mathieu


--
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com