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

[Andrew Dinn]

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)

src/hotspot/share/compiler/precompiler.cpp line 243:

> 241:     }
> 242: 
> 243:     if (TieredCompilation) {

How does this work as far as consistency between training/assembly run and production run is concerned? Do we need to enforce the same setting across these runs or can we e.g. train and then (assuming we fork) assemble with -TieredCompilation but run in production with +TieredCompilation -- or vice versa?

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PR Review Comment: https://git.openjdk.org/leyden/pull/32#discussion_r1942921125