RFR: 8355970: C2: Add command line option to print the compile phases

[Thomas Stuefe]

On Wed, 14 May 2025 14:08:40 GMT, Christian Hagedorn <chagedorn at openjdk.org> wrote:

> > > > Thanks for working on the Manuel, looks very useful! Have you considered using the Unified Logging (UL) instead of creating a new JVM flag for this? We already have `-Xlog:jit+compilation` that seems related to this. You might print the compile phase information with e.g. `-Xlog:jit+compilation=trace`, or add a new UL tag if necessary.
> > > > We want to move towards using the UL framework in the JVM compiler components, now that the preparation work by @anton-seoane is completed.
> > > 
> > > 
> > > UL is definitely the long-term solution. But given that we have more levels with this new flag (-1 to 6) than UL provides (trace, info, etc.), how could we do it with UL?
> > 
> > 
> > Good point, I guess we would have to remap the IGV print levels to the (fewer) UL logging levels.
> 
> That's true that these are probably too many levels. It would just sad when we want to (re-)add another level but we already used all UL levels. But maybe with UL, we want to have different tags or something like that.
> 
> > But I am also OK with adding a new JVM flag in the context of this RFE and revisiting it when migrating to UL.
> 
> I agree with that.

Drive-by comment: we want to use UL (it has many benefits beside cutting down on the many switches) then UL should fit all our needs. If UL has too few tracing levels for all usages, then we need to add one. I also sometimes found four levels too few.

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PR Comment: https://git.openjdk.org/jdk/pull/25183#issuecomment-2882175296