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

[Jorn Vernee]

On Tue, 25 Oct 2022 17:19:47 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.
>> 
>> ---------
>> ### Progress
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>> 
>> 
>> 
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>
> src/java.base/share/classes/jdk/internal/foreign/abi/AbstractLinker.java line 57:
> 
>> 55:         checkHasNaturalAlignment(function);
>> 56:         LinkerOptions optionSet = LinkerOptions.of(options);
>> 57:         optionSet.validateForDowncall(function);
> 
> Does it make sense to have a validation step that is disjoint from the creation of the LinkerOptions? E.g. should LinkerOptions factory just take the descriptor and perform all the required validation in one go?

I don't really like that idea, because it requires the descriptor to be specified twice, which opens up the possibility of them going out of sync.

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