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

[Jorn Vernee]

On Tue, 25 Oct 2022 17:28:14 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/jdk/internal/foreign/abi/PreservableValues.java line 43:
> 
>> 41:     PreservableValues(String valueName, ValueLayout layout, int mask) {
>> 42:         this.valueName = valueName;
>> 43:         this.layout = layout.withName(valueName);
> 
> Ok, layouts get the name attached - which can be used by developers to generate the correct getter. Another choice, I realize, would have been for the PreserveValue linker option to expose a static getter which takes a name. Then you don't need to expose a layout. Not sure which one is better. The current approach is probably cleaner in that if the layout doesn't have a certain named layout, then you cannot obtain a getter for it.

Besides getting, you'd also need to know the layout in order to allocate the segment to pass to a downcall. So, the layout is needed either way (or at least a byte size). A _static_ getter wouldn't work, since a layout can be different depending on the combination of values saved. An instance getter could be an option, but then I don't think the var handle underneath will be seen as constant.

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