Integrated: 8354954: Typed static memory for late initialization of static class members in Hotspot

[Johan Sjölen]

On Wed, 16 Apr 2025 12:36:45 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.

This pull request has now been integrated.

Changeset: 604225fb
Author:    Johan Sjölen <jsjolen at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/604225fb0c5f6bf2128a305d09649d76c43dedc9
Stats:     91 lines in 5 files changed: 81 ins; 3 del; 7 mod

8354954: Typed static memory for late initialization of static class members in Hotspot

Reviewed-by: qamai, kbarrett, jvernee

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PR: https://git.openjdk.org/jdk/pull/24689