RFR: Specialize ShenandoahGenerationType to avoid generational check penalty on fast paths [v2]

[Aleksey Shipilev]

> The target for this PR is to resolve the performance TODO at `ShenandoahMark::mark_through_ref`.
> The same can probably be used in native barrier code too, if we decide to specialize those with `GENERATION`.
> 
> With sample HyperAlloc test that is configured to stress the marking code:
> 
> 
> $ java -XX:+UseShenandoahGC -XX:+UnlockDiagnosticVMOptions -XX:ShenandoahGCMode=passive -XX:-ShenandoahDegeneratedGC -Xlog:gc+stats -Xms1g -Xmx1g -XX:+AlwaysPreTouch -XX:ParallelGCThreads=1 -jar HyperAlloc-1.0.jar -d 30 -s 800 -t 1
> 
> 
> We have consistent improvements in mark times for about 2.5%, look at average times below:
> 
> 
> # Baseline
> 
> [31.009s][info][gc,stats] Pause Full GC (G)              =   25.546 s (a =   532201 us) (n =    48) (lvls, us =   203125,   539062,   542969,   544922,   559177)
> [31.009s][info][gc,stats] Pause Full GC (N)              =   25.543 s (a =   532136 us) (n =    48) (lvls, us =   203125,   539062,   542969,   544922,   559061)
> [31.009s][info][gc,stats]   Mark                         =   10.076 s (a =   209922 us) (n =    48) (lvls, us =    74609,   212891,   212891,   212891,   215692)
> 
> [31.065s][info][gc,stats] Pause Full GC (G)              =   25.618 s (a =   533715 us) (n =    48) (lvls, us =   230469,   541016,   542969,   544922,   556460)
> [31.065s][info][gc,stats] Pause Full GC (N)              =   25.615 s (a =   533650 us) (n =    48) (lvls, us =   230469,   539062,   542969,   544922,   556338)
> [31.065s][info][gc,stats]   Mark                         =   10.090 s (a =   210202 us) (n =    48) (lvls, us =    83398,   212891,   212891,   214844,   217067)
> 
> [30.975s][info][gc,stats] Pause Full GC (G)              =   25.613 s (a =   533606 us) (n =    48) (lvls, us =   201172,   539062,   544922,   546875,   558696)
> [30.975s][info][gc,stats] Pause Full GC (N)              =   25.611 s (a =   533566 us) (n =    48) (lvls, us =   201172,   539062,   544922,   546875,   558576)
> [30.975s][info][gc,stats]   Mark                         =   10.152 s (a =   211491 us) (n =    48) (lvls, us =    73633,   214844,   214844,   214844,   218065)
> 
> # Patched
> 
> [30.607s][info][gc,stats] Pause Full GC (G)              =   25.224 s (a =   525492 us) (n =    48) (lvls, us =   216797,   531250,   537109,   539062,   541888)
> [30.607s][info][gc,stats] Pause Full GC (N)              =   25.220 s (a =   525411 us) (n =    48) (lvls, us =   216797,   531250,   537109,   539062,   541788)
> [30.607s][info][gc,stats]   Mark                         =    9.841 s (a =   205013 us) (n =    48) (lvls, us =    77734,   207031,   208984,   208984,   211176)
> 
> [30.647s][info][gc,stats] Pause Full GC (G)              =   25.293 s (a =   526937 us) (n =    48) (lvls, us =   287109,   531250,   535156,   539062,   545759)
> [30.647s][info][gc,stats] Pause Full GC (N)              =   25.290 s (a =   526868 us) (n =    48) (lvls, us =   287109,   531250,   535156,   539062,   545660)
> [30.647s][info][gc,stats]   Mark                         =    9.833 s (a =   204856 us) (n =    48) (lvls, us =   101562,   207031,   207031,   207031,   212495)
> 
> [30.640s][info][gc,stats] Pause Full GC (G)              =   25.193 s (a =   524860 us) (n =    48) (lvls, us =   210938,   531250,   535156,   537109,   540524)
> [30.640s][info][gc,stats] Pause Full GC (N)              =   25.190 s (a =   524792 us) (n =    48) (lvls, us =   210938,   531250,   535156,   537109,   540420)
> [30.640s][info][gc,stats]   Mark                         =    9.830 s (a =   204788 us) (n =    48) (lvls, us =    76367,   207031,   207031,   208984,   210008)

Aleksey Shipilev has updated the pull request with a new target base due to a merge or a rebase. The incremental webrev excludes the unrelated changes brought in by the merge/rebase. The pull request contains eight additional commits since the last revision:

 - Handle more omissions
 - Add a performance blurb
 - Merge branch 'master' into gentype-non-generational
 - More work
 - Micro-optimize in_generation
 - Touchups
 - Cleanups
 - Initial work

-------------

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/254/files
  - new: https://git.openjdk.org/shenandoah/pull/254/files/9fa68289..c09f5782

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=254&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=254&range=00-01

  Stats: 14995 lines in 490 files changed: 7528 ins; 6264 del; 1203 mod
  Patch: https://git.openjdk.org/shenandoah/pull/254.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/254/head:pull/254

PR: https://git.openjdk.org/shenandoah/pull/254