Thank you for the write-up, Brandt. This is an exciting development 
A few questions:
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LLVM: I am a bit confused about the statements related to LLVM. In the “Rational” section you write using LLVM would incur significant overhead and a heavy dependency. Then in the “Support” section you write that some platforms may not receive JIT support due to LLVM not having good support for them. Could you clarify whether you are using LLVM of not ? Other comments in the PEP suggest that you are using LLVM. If that’s the case, how do you manage to keep the overhead low ? (In that case, it may also make sense to change/update the “Rational” section to avoid such questions)
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Patents: Are you aware of any patents on the copy-and-patch approach or other advanced techniques you are using in the JIT, which may encumber the use of a JIT enabled CPython ?
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RAM usage: Does the JIT limit itself to using a certain percentage of available RAM (as a measure against DOS style attacks or bugs in the code) ? How quickly are less used JIT compiled parts garbage collected ? Can the JIT GC be controlled programmatically (e.g. by sys module calls) ?
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Dependencies: Assuming you are using LLVM, is it still possible to build CPython without JIT support on platforms where LLVM is not available/supported ?