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

Mathieu Desnoyers
2023-07-05T13:16:28Z

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.