RFR (S): 8153439: do not install an empty SpeculationLog in an nmethod

Mon Apr 4 21:50:40 UTC 2016

Thanks for the quick turnaround.  Looks good.

> On Apr 4, 2016, at 11:30 AM, Doug Simon <doug.simon at oracle.com> wrote:
> 
> The JVMCI speculation log mechanism might not be used by a specific JVMCI compiler or JVMCI compiler configuration. In this case, the nmethod::_speculation_log field should be initialized to NULL to reduce unnecessary processing of the field by the GC.
> 
> https://bugs.openjdk.java.net/browse/JDK-8153439
> http://cr.openjdk.java.net/~dnsimon/8153439