RFR: 8260046: Assert left >= right in pointer_delta() methods

[Kim Barrett]

On Thu, 6 May 2021 14:02:13 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Adding the assertion, and fixing a benign violation.
> 
> Test: tier1-7; no violation of this assertion.

Changes requested by kbarrett (Reviewer).

src/hotspot/share/gc/g1/g1Allocator.cpp line 460:

> 458:   // chunk if we don't yet cross the boundary, but the remainder would be too
> 459:   // small to fill.
> 460:   HeapWord* new_top = old_top + word_size;

Pre-existing but related to the change - What, if anything, prevents the calculation of new_top from overflowing or detects that happened.  The usual way we do such calculations safely involves first checking that `(_max - old_top) <= word_size` or something similar.  I don't see anything to prevent that, and if nothing does then the various checks and calculations involving new_top are suspect.

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PR: https://git.openjdk.java.net/jdk/pull/3900