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> . I don't see an architectural path from a cut-and-paste JIT to something optimizing. One approach used in V8 is to have a dumb-but-very-fast JIT (ie. this),

> . I don't see an architectural path from a cut-and-paste JIT to something optimizing. One approach used in V8 is to have a dumb-but-very-fast JIT (ie. this), and keep counters of how often each block of code runs (perhaps actual counters, perhaps using CPU sampling features), and then any block of code running more than a few thousand times run through a far more complex yet slower optimizing jit. That has the benefit that the 0.2% of your code which uses 95% of the runtime is the only part that has to undergo the expensive optimization passes.

Sesse__ 3y ago
Note that V8 didn't have a dumb-but-very-fast JIT (Sparkplug) until 2021; the interpreter (Ignition) did that block counting and sent it straight to the optimizing JIT (TurboFan). V8 pre-2021 (i.e., only Ignition+TurboFan) was significantly faster than current CPython is, and the full current four-tier bundle (Ignition+Sparkplug+Maglev+TurboFan) only scores roughly twice as good on Speedometer as pure Ignition does. (Ignition+Sparkplug is about 40% faster than Ignition alone; compare that “dumbness” with CPython's 2–9%.) The relevant lesson should be that things like very carefully designed value representation and IR is a much more important piece of the puzzle than having as many tiers of compilation as possible.
- uluyol 3y ago
  In case anyone is interested, V8 pre-ignition/TurboFan had different tiers [1]: full-codegen (dumb and fast) and crankshaft (optimizing). It's interesting to see how these things change over time. [1]: https://v8.dev/blog/ignition-interpreter https://v8.dev/blog/ignition-interpreter
dataangel 3y ago
> keep counters of how often each block of code runs ... and then any block of code running more than a few thousand times run through a far more complex yet slower optimizing jit. That's just all JITs. Sometimes its counters for going from interpreter -> JIT rather than levels of JITs, but this idea is as old as JITs.

Sesse__ 3y ago

Note that V8 didn't have a dumb-but-very-fast JIT (Sparkplug) until 2021; the interpreter (Ignition) did that block counting and sent it straight to the optimizing JIT (TurboFan). V8 pre-2021 (i.e., only Ignition+TurboFan) was significantly faster than current CPython is, and the full current four-tier bundle (Ignition+Sparkplug+Maglev+TurboFan) only scores roughly twice as good on Speedometer as pure Ignition does. (Ignition+Sparkplug is about 40% faster than Ignition alone; compare that “dumbness” with CPython's 2–9%.) The relevant lesson should be that things like very carefully designed value representation and IR is a much more important piece of the puzzle than having as many tiers of compilation as possible.

uluyol 3y ago
In case anyone is interested, V8 pre-ignition/TurboFan had different tiers [1]: full-codegen (dumb and fast) and crankshaft (optimizing). It's interesting to see how these things change over time. [1]: https://v8.dev/blog/ignition-interpreter https://v8.dev/blog/ignition-interpreter

dataangel 3y ago

> keep counters of how often each block of code runs ... and then any block of code running more than a few thousand times run through a far more complex yet slower optimizing jit. That's just all JITs. Sometimes its counters for going from interpreter -> JIT rather than levels of JITs, but this idea is as old as JITs.