RFR(XS): 8233416: JFR: assert((((((klass)->trace_id()) & (((1 << 1) << 8) | (JfrTraceIdEpoch::in_use_this_epoch_bit()))) != 0))) failed: invariant

[Markus Gronlund]

Greetings,

Please review this changeset:

Bug: https://bugs.openjdk.java.net/browse/JDK-8233416 
Webrev: http://cr.openjdk.java.net/~mgronlun/8233416/webrev01/
Testing: jdk_jfr

Summary:
In debug builds, an assertion located in the code involved in traceid tagging, USED_THIS_EPOCH(klass), gives intermittent false positives.
The reason is that the traceid bits are untagged at the point of the initial load (SHOULD_TAG(klass)), and a subsequent load as part of setting updated bits finds the traceid bits to be tagged already.
The artifact is correctly tagged but the absence of a load barrier allows the bit pattern from the initial load to be compared against the expectation (that is, not loading it again), and the assertion traps.
More details in the bug.

As we now have a top level predicate SHOULD_TAG(type) which did not exist pre-streaming, instead of adding a DEBUG_ONLY(OrderAccess::loadload();), we remove the check in the set_bits(), delegating this to SHOULD_TAG.

Thanks
Markus