RFR (XS): JDK-6563994 - assert(wf.check_method_context(ctxk, m), "proper context") failed

[S. Bharadwaj Yadavalli]

I would like to get reviews for a small fix for the occasional nightly 
failures reported in https://bugs.openjdk.java.net/browse/JDK-6563994

The assertion is observed to be triggered while looking up for a 
monomorphic call target. It happens in a rare situation, where the 
method holder class is redefined at safepoints *after* resolution of the 
target method's class (in find_monomorphic_target() - 
src/share/vm/ci/ciMethod.cpp:694) but *before* the call of 
find_unique_concrete_method() - src/share/vm/ci/ciMethod.cpp:733). This 
change returns NULL for a such a redefined method which would be marked 
as old, indicating that candidate call target may not be inlined, since 
all we have is a pointer to an old method that has been redefined. The 
subsequent assertion of check_method_context for old methods is not 
applicable and hence not done for such methods.

Testing:

Without the fix: Was able to trigger the assertion upon running the test 
nsk.jvmti.RetransformClasses.retransform001 a few hundred times.

With the fix: Was able to run the test for over 3000 iterations 
continuously for more than 3 days without triggering the assertion.

Ran JPRT tests successfully

webrev: http://cr.openjdk.java.net/~bharadwaj/6563994/webrev_0/
bug: https://bugs.openjdk.java.net/browse/JDK-6563994

Thanks,

Bharadwaj