RFR: 8189685: need PerfMemory class update and a volatile_static_field support in VMStructs [v3]

[David Holmes]

On Wed, 23 Aug 2023 18:09:24 GMT, Chris Plummer <cjplummer at openjdk.org> wrote:

>> The intent was to generalize it but the comments also need changing, and now I'm not clear what this is actually doing.
>
> This change wasn't in the original patch, but the `GENERATE_STATIC_PTR_VOLATILE_VM_STRUCT_ENTRY` change above it was.  I found it when I noticed the use of `GENERATE_STATIC_PTR_VOLATILE_VM_STRUCT_ENTRY` and thought it should be renamed too. I was also doing "static volatile" -> "volatile stack" at the time so that is also part of this macros rename. 
> 
> As David points out, the purpose is to make it more general purpose, although for this macro only the name change is needed (`GENERATE_STATIC_PTR_VOLATILE_VM_STRUCT_ENTRY also has a minor implementation change that was needed).
> 
> The comments for both of these macros references "static pointer volatile" entries. That should changed to "volatile static". The type no longer needs to be a pointer type. 
> 
> I'm not certain what this CHECK macro does, but it was pre-existing, and there is also the `CHECK_STATIC_VM_STRUCT_ENTRY` macro above it which is identical except of the absence of "volatile" in the name and implementation. I think it verifies that the type  specified in the vmstructs definition of the field is actually the same as the field itself.  The macro will generate a compiler error if a cast was needed. I think this type of check can only be done with static fields. You would need an instance of the class/struct to do a similar check on an instance field. Thus CHECK_NONSTATIC_VM_STRUCT_ENTRY is much more complex and seems to be a runtime check that will assert if it fails.

I can't convince myself that `volatile` is in the right place in `type volatile * dummy`. But `volatile * dummy` means that the pointer is volatile as opposed to the type being pointed to, which would match with the original macro name's use of `PTR_VOLATILE`.

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PR Review Comment: https://git.openjdk.org/jdk/pull/15373#discussion_r1303612740