RFR: 8278482: G1: Improve HeapRegion::block_is_obj

[Thomas Schatzl]

On Thu, 9 Dec 2021 11:19:39 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Adding assertions to `HeapRegion::block_is_obj` and its callers, and removing some dead code based on the assertions.
> 
> Test: tier1-5

Changes requested by tschatzl (Reviewer).

src/hotspot/share/gc/g1/heapRegion.cpp line 661:

> 659:     // We cast p to an oop, so region-bottom must be an obj-start.
> 660:     assert(is_starts_humongous(), "invariant");
> 661:   }

I would prefer an `assert(!is_region_humongous || is_starts_humongous(), "....");` instead of the if. The change can keep the comment.

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PR: https://git.openjdk.java.net/jdk/pull/6781