RFR: 8334137: Marlin: replace sun.misc.Unsafe memory access methods with FFM

[Laurent Bourgès]

On Mon, 9 Jun 2025 22:09:44 GMT, Kevin Rushforth <kcr at openjdk.org> wrote:

> > > I'd looked at that but it was not obvious. And it definitely is not obvious that it will be the same thread that initialises the MaskMarlinAlphaConsumer.java class
> > 
> > Yes, I see what you are saying. The `ALPHA_MASK_XXX` arrays are static fields of `MaskMarlinAlphaConsumer`, and the renderer context class has an instance field of `MaskMarlinAlphaConsumer`. So it does seem possible that `MaskMarlinAlphaConsumer` could be initialized by a thread other than the prism renderer thread. Hmmm.
> 
> Fortunately, the presence of following field in RendererConext does not cause the MaskMarlinAlphaConsumer class to be initialized:
> 
> ```
>     public MaskMarlinAlphaConsumer consumer = null;
> ```
> 
> I instrumented the code, adding something that _did_ force initialization and was able to provoke a WrongThreadException. Without my modification, MaskMarlinAlphaConsumer doesn't get initialized until rendering.
> 
> This seems _very_ fragile, though. At best, presuming we can prove that initialization and access to MaskMarlinAlphaConsumer always happens on the Prism rendering thread, this is an accident waiting to happen.
> 
> @bourgesl Do you have any thoughts on this? Using the shared arena is definitely safer, but there might be a performance penalty.

These alpha masks are constant (static), so reused in marlin java2d where multiple threads & rendering contexts can work in parallel.
With FFM, the global arena could be used, but the current patch is good enough as javafx uses only 1 thread to perform rendering.
I hope using the shared arena does not hurt performance in this case (1 thread).

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PR Comment: https://git.openjdk.org/jfx/pull/1814#issuecomment-2959886137