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Up next - Markus Denker on AOSTA. Markus is doing this work for Squeak. We already have JIT systems for the commercial systems, and even an (unreleased) JIT for Squeak. What this is about is "hotspotting". This is all done at the Smalltalk level, using the profile information created by the JIT compiler. The system would compile to optimized bytecode, but get turned off for debugging purposes. The main areas:

• Profiled execution of (unoptimized) areas from the JIT
• Collect type information from the PIC (Polymorphic Inline Cache). Needs to be readable at the Smalltalk level
• Inlining (from the above)

Where is it? The design is done, but not tested, The bytecode transformations (to SSA) is done, and works in VisualWorks. The backend - a transformation out of SSA is being done now, with a simple code generator (In Squeak - works with some examples now). SSA stands for Static Simple Assignment.

Two steps:

• Deconstruct the SSA
• Generate Bytecode

The main problem that can crop up is that some optimizations end up with wrong results - you need to fix that up - which makes the algorithm more complex. There's an answer to this called the Phi-Congruency Method.

To do - lots - dynamic deoptimization (for debugging). Possible experiments:

• AOSTA for Squeak with a JIT
• Does it make sense for an interpreter?
• Exupery as a backend?

With a byte code to byte code translator, what else is possible?

• Enables more late binding
• An API for changing the language semantics and implementation at runtime (for example, the meaning of inheritance)
• Squeak has MetaClassTalk, but it's slow
• iVARS are accessed via offsets
• offsets calculated at compile time
• makes changes and experiments harder
• Make a MOP more practical
• Allows a much simpler system

Bottom line - could enable a lot of things that aren't (directly) related to performance.