[foreign-memaccess+abi] RFR: 8294970: Add linker option for saving thread-locals that the VM can overwrite

[Maurizio Cimadamore]

On Tue, 25 Oct 2022 17:14:56 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> This patch adds a new linker option, called `preserveValue`, that can be used to save thread-local values such as `errno` and `GetLastError` that the VM might overwrite. It takes a list of names of the values to save. Values are saved into a MemorySegment passed by the user, that is allocated according to the layout associated with the returned `PreserveValue` instance.
>> 
>> The API for this is somewhat convoluted, as we have to account for:
>>  1. a different JVM implementation might not overwrite the same thread-locals, or non at all. Such JVM implementations should be free to support a different set of names to save, or none at all.
>>  2. it should be possible to save multiple values during a single call.
>> 
>> But, this API is also flexible enough for an implementation to choose any layout it wants for the MemorySegment into which the values are saved. In the current implementation it's just a set of ints, but this could be changed as well.
>> 
>> ---------
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>
> src/java.base/share/classes/java/lang/foreign/Linker.java line 326:
> 
>> 324:          * @see PreserveValue#supported()
>> 325:          */
>> 326:         static PreserveValue preserveValue(String... preservedValues) {
> 
> I think I prefer `saveValue` - it's simpler, but means more or less the same thing.
> Also, I think we should use the plural "saveValues" both in the factory and in the option interface.

I also find it a bit odd that I can create a LinkerOption whose preservedValue is unrelated from the supported ones? E.g. I'd like to get an exception early, while it seems like the implementation will only give me an exception when I'm creating the downcall, which seems a tad late.

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PR: https://git.openjdk.org/panama-foreign/pull/742