RFR: 8372528: Unify atomic exchange and compare exchange

[Kim Barrett]

On Tue, 25 Nov 2025 18:28:19 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> AtomicAccess::xchg is only required to support `4` bytes and `sizeof(intptr_t)` size.
> This restriction added a lot of extra logic to the Atomic implementation because
> we have a set of features we must support (including compare exchange) for `1`, `4` and `8` byte atomics on all platforms. We have some checks for unsupported `8` byte compare exchange (`VM_Version::supports_cx8()`), but the Atomic class does not try to handle these for generating its supported functions. On such a platform we would more than likely get a linking error.
> 
> I propose we change requirement for exchange to `1`, `4` and `8` bytes to achieve parity with compare exchange. Initially by implementing exchange via the `AtomicAccess::XchgUsingCmpxch`. And have follow up RFEs for each applicable platform where we specialize `AtomicAccess::PlatformXchg<1>`.
> 
> This enhancement both simplifies the Atomic implementation and provides exchange capabilities for types like `bool` and enums represented by a byte.
> 
> _It is a little unclear how we deal with `VM_Version::supports_cx8()`. Its existence makes it impossible to use `compare_exchange` on `int64_t` in general code. Currently the `Atomic` implementation assumes that `exchange` can always be used on `8` byte integers (at least going by the gtest). Even though `AtomicAccess` only specifies `4` bytes and the platform size. This PR changes this to `1`, `4` and `8` bytes. But not sure if the previous behaviour / implicit requirements is an oversight a similar property to `VM_Version::supports_cx8()` should apply here for `exchange`._
> 
> * Testing
>   * Extended gtest / (no other users of Atomic byte with exchange exists.
>   * GHA
>   * Running Tier 1-5 on Oracle supported platforms

`VM_Version::supports_cx8()` is vestigial, and is required to return true on
all platforms:
https://github.com/openjdk/jdk/blob/275cb9f28799081878e0a7c53ce1c0450f4e963e/src/hotspot/share/runtime/vm_version.cpp#L32
https://github.com/openjdk/jdk/blob/275cb9f28799081878e0a7c53ce1c0450f4e963e/src/hotspot/share/runtime/atomicAccess.hpp#L58-L64
See https://bugs.openjdk.org/browse/JDK-8318776
"Require `supports_cx8` to always be true"
I'm not sure why we haven't nuked `supports_cx8()`; maybe because nobody has
collected sufficient 'tuits. @dholmes-ora might remember.

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PR Comment: https://git.openjdk.org/jdk/pull/28498#issuecomment-3580388565