Context-Guided Splitting: poc in TruffleRuby

[sophie-kaleba]

Part of the implementation resides in my fork of graal: PR #

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Context-Guided Splitting is a variation of the current splitting heuristic used in GraalVM. It aims at reducing the amount of splits by favouring the reuse of previously split methods. Such a decision is guided by the notion of "context": each call is represented by a context. If a method was previously split under a context C, and is flagged for being split again, we check whether the current context and context C match. If they do, we dispatch to the previously split method, rather than triggering splitting again.

What does this PR do?

• For each call, pick the user-provided arguments and compute a context signature, ie. a hash of the arguments' types (see RubyDynamicObject and ContextSignatureUtils)
• If a RubyProc (block) is passed as an argument, we use the hashcode of its call-target rather than the plain hash of RubyProc's metaclass, to avoid false positives (see RubyProc)
• Each arguments' hashcode is multiplied by a prime number (see ContextSignatureUtils). On our biggest benchmarks, this leads to less than 0.2% of collisions

The remainder of the approach, notably how the contexts are used to avoid splitting, is implemented as part of the graal repository. The related PR is available here