RFR: 8293117: Add atomic bitset functions

[Coleen Phillimore]

On Fri, 28 Apr 2023 07:10:40 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

> Please review this change that adds and/or/xor bitops to Atomic.
> 
> Some offline discussion reached consensus on the following API (with T being a
> template parameter that must be an integral type):
>   T fetch_then_and(volatile T* dest, T bits, atomic_memory_order order)
>   T fetch_then_or(volatile T* dest, T bits, atomic_memory_order order)
>   T fetch_then_xor(volatile T* dest, T bits, atomic_memory_order order)
>   T and_then_fetch(volatile T* dest, T bits, atomic_memory_order order)
>   T or_then_fetch(volatile T* dest, T bits, atomic_memory_order order)
>   T xor_then_fetch(volatile T* dest, T bits, atomic_memory_order order)
> with order defaulting to memory_order_conservative.
> 
> I'm hoping there won't be more bike-shedding on the names.
> 
> This naming convention differs from what exists for add/sub, where we have
> fetch_and_add, add_and_fetch, and the like.  fetch_and_and and and_and_fetch
> just looked too weird.  For consistency we should probably rename the add/sub
> operations to use "then".
> 
> A default implementation is provided, using a CAS loop for all operations.
> That implementation can be overridden on a per-platform basis.  Currently
> there aren't any platform-specific implementations; such will be added by
> followup RFEs.  We'll want to override for most (if not all) platforms, since
> the CAS loop implementation is generally less than optimal (in some cases
> rather comically so).
> 
> For some platforms a CAS loop is necessary to obtain either the old or the new
> value, but have a better implementation available if neither value is needed.
> However, we can't easily make use of such.  We would need to either add
> functions returning void to the API or change the API to return expression
> templates so we can detect how the result is used.  However, we may be able to
> use compiler intrinsics for some platforms, which may be able to specialize
> for the usage context.  If that turns out to be a problem then we can expand
> the API accordingly later.
> 
> Testing:
> mach5 tier1, including new gtests for the new functionality.

Thank you for doing this.  I'll file a follow up to convert some duplicated code to use these after this is pushed.

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Marked as reviewed by coleenp (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/13711#pullrequestreview-1406388949