RFR: 8283710: JVMTI: Use BitSet for object marking [v6]

[Thomas Stuefe]

On Fri, 8 Apr 2022 17:34:57 GMT, Roman Kennke <rkennke at openjdk.org> wrote:

> Yes, I get that. But the fragments and fragment-table are themselves inner classes that take a MEMFLAGS template. I could (perhaps) either use a constexpr MEMFLAGS arg and pass this through, or do at some point a switch like:
> 
> ```
> switch (_flags) {
>   case mtServiceability:
>   ... new BitMapFragmentTable<mtServiceability>(); break;
>   case mtServiceability:
>   ... new BitMapFragmentTable<mtServiceability>(); break;
>   default: ShouldNotReachHere();
> }
> ```
> 
> Which seems kinda-ugly but would work (I think), and avoid making the outer class template-ized.

I see what you mean. This MEMFLAGS template parameter is deeply interwoven into everything. I'd just live with the current solution. It uses established pattern, so at least nobody is surprised.

I think the basic problem is that CHeapObj itself is a template class. Rethinking MEMFLAGS seems worthwhile for a future RFE. As I wrote, one approach could be to make them a property of the current thread, and switchable and stackable via a Mark class. That way, everything allocated within a given range of frames would count toward a given category. No need to decide on a fine-granular basis. No need for templates. Maybe no need even to have a MEMFLAGS argument for every allocation.

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PR: https://git.openjdk.java.net/jdk/pull/7964