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

[Albert Mingkun Yang]

On Fri, 7 May 2021 17:26:17 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

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

Thanks for pointing this out. A fix at https://github.com/openjdk/jdk/pull/3936

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