When performing a downcall with the foreign functions API, one can link with a "captured call state". As of JDK 26, this will provide a memory segment for accessing thread-local execution state like errno once the downcall has been completed. JDK-8378559 extends this memory segment with the ability to set thread-locals before downcalls.

This RFE proposes to introduce a new JMH benchmark which will measure execution time for downcalls with and without call state capturing. This will give a pragmatic indication of the overhead that captured call states introduce.

As a one-off, it is also a goal to quantify the performance regression (trade-off) that JDK-8378559 introduces.