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Musings on Tracing in PyPy
Last summer, Shriram Krishnamurthi asked on Twitter: "I'm curious what the current state of tracing JITs is. They used to be all the rage for a while, then I though I heard they weren't so effective,
Tracing allows for doing very aggressive partial inlining, Following just the hot path through lots of layers of abstraction is obviously often really useful for generating fast code. Our allocation removal, the way PyPy's JIT can reason about the heap, about dictionary accesses, about properties of functions of the runtime, about the range and known bits of integer variables is all quite solid. There are also some classes of programs that tend to generally perform quite poorly when they are executed by a tracing JIT, bytecode interpreters in particularly, and other extremely unpredictably branchy code.
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