RFR: 8306836: Remove pinned tag for G1 heap regions

[Albert Mingkun Yang]

On Tue, 25 Apr 2023 13:49:05 GMT, Thomas Schatzl <tschatzl at openjdk.org> wrote:

> Hi all,
> 
>   please review this change that removes the pinned tag from `HeapRegion`.
> 
> So that "pinned" tag for G1 heap regions indicates that the region should not move during (young) gc. This applies to now removed archive regions and humongous objects/regions.
> 
> With "real" g1 region pinning to deal with gclocker in g1 once and for all upcoming we need a refcount, a single bit is not sufficient anymore. Further there will be a naming conflict as this kind of "pinning" is different to g1 region pinning "pinning". The former indicates "contents can not be moved, but can be reclaimed", while the latter means "contents can not be moved and not reclaimed".
> 
> The (current) pinned flag is surprisingly little used, only for policy decisions.
> 
> The suggestion this change implements is to remove the "pinned" tag as it is, and reserve it for future g1 region pinning (that needs to store the pinning attribute differently as a refcount anyway).
> 
> Testing: tier1-3, gha
> 
> Thanks,
>   Thomas

The name, `is_young_gc_movable`, seems to suggest it is context sensitive. Maybe it's clearer if it's not defined as an API of heap-region, like other region-type-predicates.

For example, it's odd to see them on the same abstraction level, when the two APIs refer to sth quite different.


  assert(hr->is_young_gc_movable(), "Should only be movable region in compaction queue");
  assert(!hr->is_humongous(), "Should be no humongous regions in compaction queue");


Additionally, it's confusing to see sth named "*young_gc*" in full-gc code (`g1FullGCPrepareTask.inline.hpp`).

For this particular PR, could one just use `!hr->is_humongous()` where `is_young_gc_movable` is used? (Essentially, inlining the new API.)

src/hotspot/share/gc/g1/g1HeapVerifier.cpp line 414:

> 412:       // There are no other valid region types. Check for one invalid
> 413:       // one we can identify before crashing: non-movable.
> 414:       assert(hr->is_young_gc_movable(), "Heap region %u is non-movable.", hr->hrm_index());

Given this branch is unreachable, can one just use `fatal(...)` without a predicate?

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PR Review: https://git.openjdk.org/jdk/pull/13643#pullrequestreview-1400139894
PR Review Comment: https://git.openjdk.org/jdk/pull/13643#discussion_r1176660073