RFR: 8340398: [JVMCI] Unintuitive behavior of UseJVMCICompiler option [v2]

[Todd V. Jonker]

On Mon, 23 Sep 2024 07:31:10 GMT, Tomáš Zezula <duke at openjdk.org> wrote:

>> Disabling the JVMCI compiler with `-XX:-UseJVMCICompiler` not only deactivates JVMCI-based CompileBroker compilations but also prevents the loading of the libjvmci compiler. While this works as expected for CompileBroker compilations, it poses issues for the Truffle compiler. When `-XX:-UseJVMCICompiler` is used, Truffle falls back to the jargraal compiler, if available. This behavior may be confusing for Truffle users.
>> 
>> Expected behavior:
>> 
>> With `-XX:+UseGraalJIT`, both CompileBroker compilations and Truffle compilations should utilize the libjvmci compiler, if available.
>> With `-XX:+EnableJVMCI`, CompileBroker compilations should use the C2 compiler, while only Truffle compilations should leverage the libjvmci compiler, if available.
>
> Tomáš Zezula has updated the pull request incrementally with one additional commit since the last revision:
> 
>   JDK-8340398: Fixed EnableJVMCI handling.

If I'm reading things correctly, the doc-string for `UseJVMCINativeLibrary` in `jvmci_globals.hpp` needs updating.  That [currently states](https://github.com/openjdk/jdk/blob/78f576192e815f957db93f5f8cb3763a35474381/src/hotspot/share/jvmci/jvmci_globals.hpp#L140-L144) "Defaults to true if EnableJVMCIProduct is  true and a JVMCI native library is available" but looks like it default to true if `EnableJVMCI` is true, regardless of the `EnableJVMCIProduct` setting.

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PR Comment: https://git.openjdk.org/jdk/pull/21069#issuecomment-2369922039