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

[Tomáš Zezula]

On Sun, 22 Sep 2024 11:46:49 GMT, Doug Simon <dnsimon 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.
>
> src/hotspot/share/jvmci/jvmci_globals.cpp line 82:
> 
>> 80:   CHECK_NOT_SET(LibJVMCICompilerThreadHidden, UseJVMCICompiler)
>> 81: 
>> 82:   if (EnableJVMCI) {
> 
> This needs to be `EnableJVMCI || UseJVMCICompiler` (since deriving `EnableJVMCI` from `UseJVMCICompiler` is only done [below](https://github.com/openjdk/jdk/blob/ab06a878f888827026424530781f0af414a8a611/src/hotspot/share/jvmci/jvmci_globals.cpp#L96)).

I see, `FLAG_SET_DEFAULT(EnableJVMCI, true)` on [line 99](https://github.com/openjdk/jdk/blob/78f576192e815f957db93f5f8cb3763a35474381/src/hotspot/share/jvmci/jvmci_globals.cpp#L99).
Maybe moving this block 

if (!FLAG_IS_DEFAULT(EnableJVMCI) && !EnableJVMCI) {
      jio_fprintf(defaultStream::error_stream(),
          "Improperly specified VM option UseJVMCICompiler: EnableJVMCI cannot be disabled\n");
      return false;
    }
    FLAG_SET_DEFAULT(EnableJVMCI, true);

in front of my change  makes it more readable.

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PR Review Comment: https://git.openjdk.org/jdk/pull/21069#discussion_r1770881033