RFR: 8349457: [leyden] Workaround Leyden problems with -XX:-TieredCompilation

[Aleksey Shipilev]

On Wed, 5 Feb 2025 12:14:56 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Currently, supplying `-XX:-TieredCompilation` would break with Leyden workflows, because the archive-backed code cache heap would only be created if `SegmentedCodeCache` is enabled. With default compilation policy, this happens normally when tiered compilation is enabled, but not without it.
> 
> There are other minor peculiarities that we need to handle, which are exposed with `runtime/cds` + `-XX:-TieredCompilation` runs.
> 
> This prevents us from testing C2-only Leyden configs and studying compilation dynamics without tiered policy getting in the way.
> 
> Additional testing:
>  - [x] Ad-hoc Leyden experiments now work with `-XX:-TieredCompilation`: the C2-only code is being stored and loaded successfully from the CDS archive
>  - [x] `runtime/cds` now passes with `-XX:-TieredCompilation`
>  - [x] `runtime/cds` still passes by default (i.e. with tiered compilation)

Taking the rest of the week off, so feel free to integrate without me.

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PR Comment: https://git.openjdk.org/leyden/pull/32#issuecomment-2640939991