RFR(M): 8227680: FastJNIAccessors: Check for JVMTI field access event requests at runtime

[Doerr, Martin]

Hi,

the current implementation of FastJNIAccessors ignores the flag -XX:+UseFastJNIAccessors when the JVMTI capability "can_post_field_access" is enabled.
This is an unnecessary restriction which makes field accesses (Get<Type>Field) from native code slower when a JVMTI agent is attached which enables this capability.
A better implementation would check at runtime if an agent actually wants to receive field access events.

Note that the bytecode interpreter already uses this better implementation by checking if field access watch events were requested (JvmtiExport::_field_access_count != 0).

I have implemented such a runtime check on all platforms which currently support FastJNIAccessors.

My new jtreg test runtime/jni/FastGetField/FastGetField.java contains a micro benchmark:
test-support/jtreg_test_hotspot_jtreg_runtime_jni_FastGetField/runtime/jni/FastGetField/FastGetField.jtr
shows the duration of 10000 iterations with and without UseFastJNIAccessors (JVMTI agent gets attached in both runs).
My Intel(R) Xeon(R) CPU E5-2660 v3 @ 2.60GHz needed 4.7ms with FastJNIAccessors and 11.2ms without it.

Webrev:
http://cr.openjdk.java.net/~mdoerr/8227680_FastJNIAccessors/webrev.00/

We have run the test on 64 bit x86 platforms, SPARC and aarch64.
(FastJNIAccessors are not yet available on PPC64 and s390. I'll contribute them later.)
My webrev contains 32 bit implementations for x86 and arm, but completely untested. It'd be great if somebody could volunteer to review and test these platforms.

Please review.

Best regards,
Martin