RFR: 8253286: Use expand_exact() instead of expand_at() for fixed requests in G1

[Albert Mingkun Yang]

On Fri, 18 Sep 2020 06:59:34 GMT, Stefan Johansson <sjohanss at openjdk.org> wrote:

> Hi all,
> 
> Please review this small cleanup that uses `expand_exact()` rather than `expand_at()` when requesting a specific
> region. This will ensure that we get exactly the requested region.
> Testing:
> tier1-2 alone and 3-5 with other changes.

src/hotspot/share/gc/g1/heapRegionManager.cpp line 466:

> 464:     HeapRegion *hr = _regions.get_by_index(curr);
> 465:     if (hr == NULL || !is_available(curr)) {
> 466:       expand_exact(curr, 1, NULL);

It seems that the original code handles the expansion failure case, but the new one doesn't. Better to have comments
explaining why expansion should always success here.

It's not covered in this PR, but the documentation for `expand_exact` function is quite abstract, which could also be
addressed in this PR, if you feel like it.

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PR: https://git.openjdk.java.net/jdk/pull/235