RFR: 8258382: Fix optimization-unstable code involving pointer overflow

[Hao Sun]

On Thu, 24 Dec 2020 17:57:44 GMT, Kim Barrett <kbarrett at openjdk.org> wrote:

>> src/hotspot/share/gc/parallel/psPromotionLAB.hpp line 122:
>> 
>>> 120:     HeapWord* obj = top();
>>> 121:     // Pointer overflow check is needed here.
>>> 122:     if (end() >= obj && size <= (size_t) (end() - obj)) {
>> 
>> `top() <= end()` is always true by design and construction, so the
>> `end() >= obj` test is unnecessary and undesirable. An assertion could
>> be added, but there are lots of places where that relation is assumed
>> to hold and that don't bother asserting, so I wouldn't bother here
>> either.
>
> Instead of `(size_t)(end() - obj)` use `pointer_delta(end(), obj)`.

Cool. Will update soon. Thanks.

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PR: https://git.openjdk.java.net/jdk/pull/1886