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by xapata 3192 days ago
Yes. A jit compiler can compile based on both code and data, and can compile all the dependencies. In contrast an aot compiler must only use the code and links to other pre-compiled binaries.

Further, a jit can compile to specific hardware. An aot must anticipate what hardware is possible. If you aot compile to several possible hardware configs, then you'll bloat the binary.
2 comments
MaxBarraclough 3192 days ago
> In contrast an aot compiler must only use the code

Not since the development of "profile-guided optimisation", no.

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xapata 3191 days ago
Ok, then you've got the code and the profile. The actual data and hardware it happens to be using is still better information.
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MaxBarraclough 3187 days ago
> The actual data and hardware it happens to be using is still better information.

That's certainly what HotSpot's promoters want us to believe, but I'm not convinced there's a lot of extra value in having detailed data on the current run, compared to 'static' profile-guided optimisation. I'm not sure if there's been any serious study of this question.

As to the specific hardware: this might matter for things like SIMD, yes, but often won't make any appreciable difference. Some Linux distros have the user compile almost everything locally, but aren't all that much faster for it.

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Sean1708 3191 days ago
Which is great in theory, but what was intending to ask was are there any actual examples of JITed programs which beat C by a large margin? Because, as I say, I've only ever seen ones which are comparable to C.
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xapata 3190 days ago
Trouble is, you don't tend to write a big program in two languages the exact same way. The rewrite nearly always wins because of algorithm improvements and cleaning cruft. So it's hard to benchmark.

If two small programs are about the same speed in C vs JITlang, then the theory says the JITted one should take over as the program gets bigger.

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