RFR: 8354954: Typed static memory for late initialization of static class members in Hotspot [v9]

[Kim Barrett]

On Mon, 28 Apr 2025 11:37:35 GMT, Johan Sjölen <jsjolen at openjdk.org> wrote:

>> Hi,
>> 
>> This PR introduces a `StableValue<T>` which is sized and aligned identically to a `T`, with the difference that a `StableValue<T>` needs to be explicitly instantiated.
>> 
>> Dynamic static initalization in C++ leads to unpredictable bugs as there is no defined order in which objects will be initialized, to the degree that 'static initialization fiasco' is a term used. In the code I've worked on in Hotspot we resolve this by having an initialization function, and instead of having static members of `T` we have `T*` instead and use `malloc` in order to gain the memory for the objects. This is workable, but is unnecessary.
>> 
>> That's why I'd like to have `StableValue<T>`. It let's you avoid the whole `malloc` thing, and we overload `->` to make it behave as if it is actually a `T`. We add in a simple checker in debug mode that checks whether the memory has been initialized before using it.
>> 
>> In the code I've switched two members to be of `StableValue` instead. One is the malloc case above, the second (MemBaseline) is one where I got a bug while developing. The bug occurred because I  changed the initializer of `MemBaseline` without knowing that it was dynamic-static-allocated, and the exact change I made caused weird crashes (because of initialization order issues).
>> 
>> This solution is quite practical to me, but I wanted to know what others think.
>
> Johan Sjölen has updated the pull request incrementally with one additional commit since the last revision:
> 
>   More documentation, rename ptr to get

Changes requested by kbarrett (Reviewer).

src/hotspot/share/utilities/stableValue.hpp line 61:

> 59: 
> 60:   T& operator*() {
> 61:     return *get();

Why not just `return _t;`?

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PR Review: https://git.openjdk.org/jdk/pull/24689#pullrequestreview-2800108031
PR Review Comment: https://git.openjdk.org/jdk/pull/24689#discussion_r2064155286