How copying an int made my code 11 times faster https://medium.com/@robertgrosse/how-copying-an-int-made-my-code-11-times-faster-f76c66312e0f (JITs are criticised as black boxes, but LLVM can be subtle too)
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It's strange how AOT compilers usually have different modes for optimising time or space usage, but JITs rarely have modes like "prefer fast startup".
The vast majority of JITs seem to focus on runtime of hot code after warming up.
This comparison of OpenSmalltalkVM and TruffleSqueak is a really effective demonstration of JITs.
TruffleSqueak initially has a lower framerate, but eventually exceeds OpenSmalltalkVM. As soon as the GUI is used (at 3:30), the framerate drops again!
JITs are blackboxes that can make it hard to reason about performance in production: