RFR: 8349457: [leyden] Workaround Leyden problems with -XX:-TieredCompilation [v3]

[Aleksey Shipilev]

On Thu, 6 Feb 2025 21:45:20 GMT, Vladimir Ivanov <vlivanov at openjdk.org> wrote:

>> I tested different combinations of `{+|-}TieredCompilation` with training and production runs, and it works well. I believe the normal mechanisms make it right. I.e. there is no guarantee that lower-tiers code would always be present with `+TieredCompilation`. So, no C1 code? Fine, nothing loaded.
>> 
>> The loaders are also aware of the compilers that are present. So, tier{2,3} code in archive, but C1 is disabled? We are not loading it then.
>> 
>> From performance angle, there seem to be a slight performance edge in training with C2 only, and then running with C1+C2: no penalty on loading lots of C1 compiled code, and quicker transition from intepreter to "real" C1 in only a few places where it is needed! Compiler dynamics is weird. This PR gains us access to these kind of studies :)
>
> As I mentioned during the meeting, it looks weird to me to see `if (TieredCompilation)` checks in Precompiler code which is executed only during assembly phase. As a data point, regular CDS archive is used to be generated in interpreter-only mode (to make the process reproducible). 
> 
> Instead of supporting arbitrary JVM configurations during assembly phase, I'd prefer to see offending JVM parameters being overridden/dropped.

Right, I thought it matters for Precompiler not to attempt pre-compiling C1 classes with `-XX:-TieredCompilation`, but it does not. I see `runtime/cds` passing with/without TieredCompilation without these hunks, and CDS archive sizes are similar, and performance is still the same. So I have reverted Precompiler hunks as unnecessary.

Take a look again?

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PR Review Comment: https://git.openjdk.org/leyden/pull/32#discussion_r1948837477