RFR: 8309271: A way to align already compiled methods with compiler directives [v31]

[Serguei Spitsyn]

On Wed, 13 Mar 2024 17:31:32 GMT, Dmitry Chuyko <dchuyko at openjdk.org> wrote:

>> Compiler Control (https://openjdk.org/jeps/165) provides method-context dependent control of the JVM compilers (C1 and C2). The active directive stack is built from the directive files passed with the `-XX:CompilerDirectivesFile` diagnostic command-line option and the Compiler.add_directives diagnostic command. It is also possible to clear all directives or remove the top from the stack.
>> 
>> A matching directive will be applied at method compilation time when such compilation is started. If directives are added or changed, but compilation does not start, then the state of compiled methods doesn't correspond to the rules. This is not an error, and it happens in long running applications when directives are added or removed after compilation of methods that could be matched. For example, the user decides that C2 compilation needs to be disabled for some method due to a compiler bug, issues such a directive but this does not affect the application behavior. In such case, the target application needs to be restarted, and such an operation can have high costs and risks. Another goal is testing/debugging compilers.
>> 
>> It would be convenient to optionally reconcile at least existing matching nmethods to the current stack of compiler directives (so bypass inlined methods).
>> 
>> Natural way to eliminate the discrepancy between the result of compilation and the broken rule is to discard the compilation result, i.e. deoptimization. Prior to that we can try to re-compile the method letting compile broker to perform it taking new directives stack into account. Re-compilation helps to prevent hot methods from execution in the interpreter.
>> 
>> A new flag `-r` has beed introduced for some directives related to compile commands: `Compiler.add_directives`, `Compiler.remove_directives`, `Compiler.clear_directives`. The default behavior has not changed (no flag). If the new flag is present, the command scans already compiled methods and puts methods that have any active non-default matching compiler directives to re-compilation if possible, otherwise marks them for deoptimization. There is currently no distinction which directives are found. In particular, this means that if there are rules for inlining into some method, it will be refreshed. On the other hand, if there are rules for a method and it was inlined, top-level methods won't be refreshed, but this can be achieved by having rules for them.
>> 
>> In addition, a new diagnostic command `Compiler.replace_directives...
>
> Dmitry Chuyko has updated the pull request incrementally with one additional commit since the last revision:
> 
>   No dots in -r descriptions

The fix looks good. But I do not have an expertise in the compiler-specific part.
So, a review from the Compiler team is still required.

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Marked as reviewed by sspitsyn (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/14111#pullrequestreview-1935188358