RFR: 8154096: Extend WhiteBox API with methods which retrieve from VM information about available GC

[Kim Barrett]

> On Apr 29, 2016, at 7:34 AM, Dmitry Fazunenko <dmitry.fazunenko at oracle.com> wrote:
> 
> Hello,
> 
> To address the offline comments from Igor I completely reimplemented the fix.
> The main change is introduction a new class sun.hotspot.gc.GC which could be used by tests to:
> http://cr.openjdk.java.net/~dfazunen/8154096/webrev.02/test/raw_files/new/test/lib/sun/hotspot/gc/GC.java
>  - get the currently used GC
>  - check if the current GC was selected by ergo or set explicitly
>  - get list of supported GC
> 
> The new version also minimize the changes to the WhiteBox API
> 
> So reviewers are still very welcome!
> 
> http://cr.openjdk.java.net/~dfazunen/8154096/webrev.02/
> https://bugs.openjdk.java.net/browse/JDK-8154096

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hotspot/src/share/vm/prims/whitebox.cpp
 289   } else if (UseParallelGC | UseParallelOldGC) {

Use || rather than | with booleans.

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hotspot/src/share/vm/prims/whitebox.cpp 
 289   } else if (UseParallelGC | UseParallelOldGC) {
 290     return parallel_code;
 291   } else if (UseParallelOldGC) {
 292     return FLAG_IS_ERGO(UseParallelOldGC);

The test on line 291 will never be true, because that case is already
handled by line 289.

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hotspot/src/share/vm/prims/whitebox.cpp 
 298   assert(false, "No GC selected yet");
and
 322   assert(false, "No GC selected yet");

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hotspot/src/share/vm/prims/whitebox.cpp 
 299   return false;
and
 323   return false;

Don't use boolean false as an empty bit mask; return 0.

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Looking at the JDK-8154096 and the CRs it is blocking, I'm failing to
understand the motivation for the "selected by ergonomics" predicate.
Why is ergo-selection more interesting than command-line or default
selection?  And for the described use-case (jtreg @requires
filtering), why would any of those distinctions be interesting?

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