RFR: 8372564: Convert StringDedup to use Atomic<T>

[Kim Barrett]

On Tue, 2 Dec 2025 08:36:40 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

>> Please review this change to StringDedup to use Atomic rather than directly
>> applying AtomicAccess to volatile members.
>> 
>> Testing: mach5 tier1-5
>
> src/hotspot/share/gc/shared/stringdedup/stringDedupProcessor.hpp line 56:
> 
>> 54: 
>> 55:   static OopStorage* _storages[2];
>> 56:   static DeferredStatic<Atomic<StorageUse*>> _storage_for_requests;
> 
> What is the reason for the `DeferredStatic`?
> 
> Is it just for the init once semantics and asserts?
> 
> _Is this a style we should adapt more broadly for our init once static variables?_

The Style Guide says

"Variables with static storage duration and _dynamic initialization_
[C++14 3.6.2](https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2014/n4296.pdf)).
should be avoided, unless an implementation is permitted to perform the
initialization as a static initialization."

It may not be immediately obvious whether dynamic initialization is required.
(Currently, a non-local Atomic has dynamic initialization that is likely
reducible to static initialization when the ctor argument is a constant.
Making the Atomic ctors constexpr would make most such static. Translated
Atomics might also need the decay function to be constexpr.)

So the safe thing to do in this respect is to use DeferredStatic for a
non-local variable of class type. Of course, then you need to find a good
place to put the initialization. That already existed in the case at hand.

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PR Review Comment: https://git.openjdk.org/jdk/pull/28524#discussion_r2581291100