RFR: 8205607: Use oop_iterate instead of oop_iterate_no_header

[Kim Barrett]

> On Jun 25, 2018, at 8:25 AM, Stefan Karlsson <stefan.karlsson at oracle.com> wrote:
> 
> Hi all,
> 
> Please review this patch to use oop_iterate instead of oop_iterate_no_headers.
> 
> http://cr.openjdk.java.net/~stefank/8205607/webrev.01/
> https://bugs.openjdk.java.net/browse/JDK-8205607
> 
> oop_iterate_no_header is a convenience function that allows developers to pass in sub classes of OopClosures, instead of sub classes of OopIterateClosure, to the oop_iterate machinery.
> 
> I propose that we remove this function, and change the few closures where this is used, to inherit from OopIterateClosure instead of OopClosure.
> 
> Thanks,
> StefanK

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src/hotspot/share/gc/cms/compactibleFreeListSpace.cpp
2438 class VerifyAllOopsClosure: public BasicOopIterateClosure {

Seeing this here and elsewhere, I wish BasicOopIterateClosure were
called something like OopIterateNoMetadataClosure or
NoMetadataOopIterateClosure. "Basic" doesn't really tell me much.

If you agree, such a name change can be another RFE.

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src/hotspot/share/gc/cms/compactibleFreeListSpace.cpp
2527     // Iterate over all oops in the heap. Uses the _no_header version
2528     // since we are not interested in following the klass pointers.
2529     CMSHeap::heap()->oop_iterate(&cl);

Comment needs updating; there isn't a _no_header version anymore.

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Some places that used to do oop_iterate_no_header now do oop_iterate
using some arbitrary (based on signature type) OopIterateClosure. 

MutableSpace
GenCollectedHeap
PsOldSpace

It's only by knowing the closure is a BasicOopIterateClosure that we
know the metadata won't be processed. And that may require tracing
through several levels of calls. [There may also be a (small)
performance cost associated with that in some cases, since we'll be
calling the virtual do_metadata function for each of the objects being
processed to discover the metadata should be skipped.  But I'm mostly
concerned about knowing that we're dealing with a no-header iteration.]

It might be clearer if the oop_iterate_no_header names were retained
here, with the argument type being changed to BasicOopIterateClosure*.

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