[tach4n]

Multi-stage JITs are indeed fascinating and give some nice wins, but as a "user" (that is, a developer) I find them frustrating at times because they make it more difficult to reason about the performance and behaviour of my code.

Not only a priori, but in terms of testing actual code and designing benchmarks as well - you have to make sure the right sections of code are hot, hitting the right levels, etc. It's extra mental load and work. (and if you're a JS developer, you may have to do this across multiple platforms). And oh, regressions and updates.

I'm not entirely convinced it's all worth it, but so far I'm begrudgingly accepting it.

[pjmlp]

It is no different than using multiple AOT compilers (for any standardized language) across multiple platforms.

Even the same compiler can behave differently just by changing the target CPU.

[lmm]

Sure, but with most AOT compliers you only get one machine-code version of your code, which is easier to benchmark than worrying about the multiple levels of a JIT.

[pjmlp]

Actually I think many developers aren't aware of the right tools for this type of job.

JITs are easy to benchmark when tools like Oracle Studio, Java Flight Recorder, dotTrace and so forth are used.

The problem is trying to benchmark them using old style concepts, e.g. outputting time differences to a text file.

[RussianCow]

Those tools don't exist for all JITed platforms. Hence the problem.