RFR: 8316961: Fallback implementations for 64-bit Atomic::{add,xchg} on 32-bit platforms [v3]

[Volker Simonis]

On Mon, 23 Oct 2023 09:47:21 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Throughout, Atomic::PlatformMumble is intended to be used in the implementation of Atomic::Mumble, and isn't intended to be used elsewhere. I think intent is even adhered to currently. This proposed change violates that in a bunch of places.

@kimbarrett, do you suggest to use `Atomic::cmpxchg()` instead of `PlatformCmpxchg()` in `Atomic::XchgUsingCmpxchg<byte_size>::operator()` and `Atomic::AddUsingCmpxchg::fetch_then_add()`? I think that would be reasonable. Otherwise I don't understand your comment as this PR follows the same pattern you've used when introducing [atomic bitset functions](https://bugs.openjdk.org/browse/JDK-8293117). Or am I missing something?

-------------

PR Comment: https://git.openjdk.org/jdk/pull/16252#issuecomment-1774911476