RFR: 8340733: Add scope for relaxing constraint on JavaCalls from CompilerThread [v6]

[Doug Simon]

On Thu, 31 Oct 2024 12:38:17 GMT, Tomáš Zezula <duke at openjdk.org> wrote:

>> [JDK-8318694](https://bugs.openjdk.org/browse/JDK-8318694) limited the ability for JVMCI CompilerThreads to make Java upcalls. This is to mitigate against deadlock when an upcall does class loading. Class loading can easily create deadlock situations in -Xcomp or -Xbatch mode.
>> 
>> However, for Truffle, upcalls are unavoidable if Truffle partial evaluation occurs as part of JIT compilation inlining. This occurs when the Graal inliner sees a constant Truffle AST node which allows a Truffle-specific inlining extension to perform Truffle partial evaluation (PE) on the constant. Such PE involves upcalls to the Truffle runtime (running in Java).
>> 
>> This PR provides the escape hatch such that Truffle specific logic can put a compiler thread into "allow Java upcall" mode during the scope of the Truffle logic.
>
> Tomáš Zezula has updated the pull request incrementally with one additional commit since the last revision:
> 
>   Improved a comment in CompilerThread.

Still look good to me.

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Marked as reviewed by dnsimon (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21285#pullrequestreview-2407850562