RFR: 8344050: Shenandoah: Retire GC LABs concurrently [v2]

[William Kemper]

> Test results show a consistent reduction in the time stopped at `init-update-refs` (as expected). Other metrics and benchmark scores are not significantly affected. This change itself does not completely eliminate the safepoint.
> 
> Results comparing `-XX:ShenandoahGCMode=satb` to `shenandoah/master` on `aarch64` (results for generational mode and other platforms are similar):
> 
> -51.91% sunflow/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.020ms (+/-  0.00ms)         70
>   Test:         0.013ms (+/-  0.00ms)         20
> 
> -50.70% luindex/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.019ms (+/-  0.00ms)         70
>   Test:         0.013ms (+/-  0.00ms)         20
> 
> -50.50% avrora/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.017ms (+/-  0.00ms)        144
>   Test:         0.012ms (+/-  0.00ms)         42
> 
> -49.79% fop/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.022ms (+/-  0.00ms)        572
>   Test:         0.015ms (+/-  0.00ms)        164
> 
> -47.99% batik/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.024ms (+/-  0.01ms)        420
>   Test:         0.016ms (+/-  0.00ms)        120
> 
> -42.54% graphchi/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.020ms (+/-  0.00ms)         70
>   Test:         0.014ms (+/-  0.00ms)         20
> 
> -42.36% xalan/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.017ms (+/-  0.00ms)        213
>   Test:         0.012ms (+/-  0.00ms)         60
> 
> -23.63% scimark.lu.large/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.045ms (+/-  0.00ms)         54
>   Test:         0.037ms (+/-  0.00ms)         12
> 
> -20.66% serial/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.041ms (+/-  0.00ms)        140
>   Test:         0.034ms (+/-  0.00ms)         40
> 
> -20.31% crypto.aes/shenandoahinitupdaterefs_stopped p=0.00000
>   Control:      0.041ms (+/-  0.00ms)         69
>   Test:         0.034ms (+/-  0.01ms)         20

William Kemper has updated the pull request incrementally with one additional commit since the last revision:

  Fix builds

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

Changes:
  - all: https://git.openjdk.org/shenandoah/pull/535/files
  - new: https://git.openjdk.org/shenandoah/pull/535/files/8297f4d3..64b71fe7

Webrevs:
 - full: https://webrevs.openjdk.org/?repo=shenandoah&pr=535&range=01
 - incr: https://webrevs.openjdk.org/?repo=shenandoah&pr=535&range=00-01

  Stats: 3 lines in 1 file changed: 0 ins; 1 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/535.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/535/head:pull/535

PR: https://git.openjdk.org/shenandoah/pull/535