RFR: 8255231: Avoid upcalls when initializing the statSampler [v2]

[Ioi Lam]

On Thu, 22 Oct 2020 11:28:22 GMT, Claes Redestad <redestad at openjdk.org> wrote:

>> Current implementation of the statSampler does upcalls to System.getProperty to collect values for a number of properties that are all provided by the VM itself. And since the sampling starts before any user code run then no property can have changed.
>> 
>> I suggest refactoring the code so that no upcalls are made normally - while asserting this invariant holds using assert-only upcalls. 
>> 
>> This is a small startup optimization - reducing the startup sequence by approx. 300k instructions and 70k branches in my linux-x64 setup.
>
> Claes Redestad has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Revert unrelated changes to perfData

Looks good to me. A nice clean up.

src/hotspot/share/runtime/statSampler.cpp line 217:

> 215:  */
> 216: 
> 217: // stable interface, supported counters in the JAVA_PROPERTY name space

* The list of System Properties that have corresponding PerfData
 * string instrumentation created by retrieving the named property's
 * value from System.getProperty() and unconditionally creating a
 * PerfStringConstant object initialized to the retrieved value. This
 * is not an exhaustive list of Java properties with corresponding string
 * instrumentation as the create_system_property_instrumentation() method
 * creates other property based instrumentation conditionally.
I found the above comment unreadable. Is it possible to clarify it?

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Marked as reviewed by iklam (Reviewer).

PR: https://git.openjdk.java.net/jdk/pull/802