[jdk19] RFR: 8290250: Shenandoah: disable Loom for iu mode

[Zhengyu Gu]

On Wed, 13 Jul 2022 18:39:51 GMT, Zhengyu Gu <zgu at openjdk.org> wrote:

> Please review this trivial patch to disable Loom for Shenandoah iu mode (an experimental feature).
> 
> Test:
> - [x] hotspot_gc_shenandoah
> - [x] jdk_loom with Shenandoah + iu
> - [x] hotspot_loom with Shenandoah + iu

The problem is beyond C2, `Skynet` fails with -`Xint`:


Referenced from:
  interior location: 0x00000000e4de6df0
  0x00000000e4de6dc8 - klass 0x0000000800036ec0 jdk.internal.vm.StackChunk
        allocated after mark start
    not after update watermark
        marked strong
        marked weak
    not in collection set
  mark: mark(is_neutral no_hash age=0)
  region: | 1335|R  |BTE     e4dc0000,     e4e00000,     e4e00000|TAMS     e4dc0000|UWM     e4e00000|U   256K|T   256K|G     0B|S     0B|L   256K|CP   0

Object:
  0x00000000e4c2e2b0 - klass 0x0000000800036ec0 jdk.internal.vm.StackChunk
    not allocated after mark start
    not after update watermark
    not marked strong
    not marked weak
    not in collection set
  mark: mark(is_neutral no_hash age=0)
  region: | 1328|R  |BTE     e4c00000,     e4c40000,     e4c40000|TAMS     e4c40000|UWM     e4c40000|U   256K|T   256K|G     0B|S     0B|L 93264B|CP   0

Forwardee:
  (the object itself)


So, it seems that IU mode either misses iu barrier on load or we need to treat new object as grey.

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PR: https://git.openjdk.org/jdk19/pull/140