[foreign-memaccess+abi] RFR: 8287516: Implement fallback Linker [v2]

[Jorn Vernee]

On Thu, 19 Jan 2023 12:09:35 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

>> Jorn Vernee has updated the pull request incrementally with one additional commit since the last revision:
>> 
>>   remove TypeClass
>
> src/java.base/share/classes/jdk/internal/foreign/abi/fallback/FallbackLinker.java line 218:
> 
>> 216:     private static void writeValue(Object arg, MemoryLayout layout, MemorySegment argSeg,
>> 217:                                    Consumer<MemorySegment> acquireCallback) {
>> 218:         if (layout instanceof ValueLayout.OfBoolean bl) {
> 
> Do you think we lose a lot of performance if we don't switch? E.g. use the underlying var handle (which is stored in the value layout) and access it in an inexact mode?

I considered that, but thought inexact access would be a lot worse in terms of performance since then we lose all inlining, and have to go through `asType` a lot of the time (since we probably won't see the same layout type all the time).

I think what I ended up doing is a nice balance between performance and simplicity, and looks kind of like a 'template' for using type switch in the future.

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PR: https://git.openjdk.org/panama-foreign/pull/770