tracing jni Get<Primitive>TypeFields calls

[Mark Wielaard]

Hi hotspot runtime devs,

Jon VanAlten wrote an extensive testsuite for using the static dtrace
markers with systemtap (see the icedtea distro-pkg-dev mailinglist) and
we found out that the JNI Get<PrimitiveType>Field probes only trigger
when disabling the UseFastJNIAccessors. This makes sense when looking
through the code. So we added documentation to the systemtap
hotspot_jni.stp tapset to document this fact.

Some questions:

- Should this be documented anywhere else?

- Would it make sense to enable -XX:-UseFastJNIAccessors when
  -XX:+ExtendedDTraceProbes is given?

- quicken_jni_functions() is only called once and so only checks
  preconditions for enabling them once at startup. I couldn't
  immediately see how one would deoptimize these when an
  enable/disable flag was dynamically toggled.
  Is there, or should there be such a mechanism?

Thanks,

Mark