RFR: 8303252: G1: Return early from Full-GC if no regions are selected for compaction.

[Thomas Schatzl]

On Mon, 27 Feb 2023 15:21:00 GMT, Ivan Walulya <iwalulya at openjdk.org> wrote:

> Please review this change to return early from full-gc in case no regions have been selected as targets for compaction. 
> To skip  `phase3_adjust_pointers` and `phase4_do_compaction()`, we need to decouple cleanup work that was piggybacked onto these phases. Thus, the `phase5_` is introduced to do the cleanup.
> 
> Testing: Tier 1-3.

Changes requested by tschatzl (Reviewer).

src/hotspot/share/gc/g1/g1FullCollector.cpp line 334:

> 332: 
> 333:   if (!has_compaction_targets()) {
> 334:     return;

Maybe it is useful to add a log message here that no compaction targets were found before exiting. Something like:

log_debug(gc, phases) debug("Phase 2: No compaction targetes");

So that people looking at logs have a better understanding why the other phases are skipped and the next log message will be about phase 5.

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PR: https://git.openjdk.org/jdk/pull/12768