4 ms·

That has been the dream for a long time. When Java introduced JIT (Hotspot?) a lot of people claimed that Java was faster than C. Lately people are telling me t

That has been the dream for a long time. When Java introduced JIT (Hotspot?) a lot of people claimed that Java was faster than C. Lately people are telling me that JavaScript "can" run faster than C. Usually they present some well written code in their favorite language and then compare it to horribly written C. Usually I tell them that most likely the runtime of their language is written in C so you can always do as well in C.

saagarjha 8y ago
> Usually I tell them that most likely the runtime of their language is written in C so you can always do as well in C. The point is that with a JIT the runtime is executing as little as possible, with the rest being dynamically optimized machine code. So this isn’t really a fair thing to say…
- gameswithgo 8y ago
  I've never seen in practice any jit do better than hand tuned AOT compiled code.
  - saagarjha 8y ago
    It's rare, but it can happen occasionally. A just-in-time compiler has access to tracing and profiling that a ahead-of-time compiler may not, which means that it can do things like inline a virtual method call that is actually monomorphic at runtime.
    - maxxxxx 8y ago
      “occasionally” That’s the keyword here.
      - saagarjha 8y ago
        I was responding to a comment that had the word "never" in it.
  - barrkel 8y ago
    I've seen lambdas and virtuals inlined dynamically. AOT would need to be statically typed and instantiated all the way to do the same thing. I could create a sample program that would be hard to make as fast in C or C++, with the constraint that the code needs to be dynamically loaded at runtime. In practice, it's easier to modify a more dynamic program and be sure you're not breaking much. Highly tuned C tends to bake in assumptions about memory lifetime, layouts, call graph, etc. The flexibility to modify is what gives you the ability to target higher level improvements in a shorter amount of time. Given any concrete implementation, after it's been designed in a more dynamic language, you can convert it to something fast in something like C. But starting from C is probably not a great idea. And not everything can be written multiple times economically. E.g. the .net GC was prototyped in Lisp, and initially converted to C++ with a translator.
  - pjmlp 8y ago
    I have, try to make an AOT compiler remove dynamic dispatch across shared libraries and convert it into either direct calls or even inline them.

saagarjha 8y ago

> Usually I tell them that most likely the runtime of their language is written in C so you can always do as well in C. The point is that with a JIT the runtime is executing as little as possible, with the rest being dynamically optimized machine code. So this isn’t really a fair thing to say…

gameswithgo 8y ago
I've never seen in practice any jit do better than hand tuned AOT compiled code.
- saagarjha 8y ago
  It's rare, but it can happen occasionally. A just-in-time compiler has access to tracing and profiling that a ahead-of-time compiler may not, which means that it can do things like inline a virtual method call that is actually monomorphic at runtime.
  - maxxxxx 8y ago
    “occasionally” That’s the keyword here.
    - saagarjha 8y ago
      I was responding to a comment that had the word "never" in it.
- barrkel 8y ago
  I've seen lambdas and virtuals inlined dynamically. AOT would need to be statically typed and instantiated all the way to do the same thing. I could create a sample program that would be hard to make as fast in C or C++, with the constraint that the code needs to be dynamically loaded at runtime. In practice, it's easier to modify a more dynamic program and be sure you're not breaking much. Highly tuned C tends to bake in assumptions about memory lifetime, layouts, call graph, etc. The flexibility to modify is what gives you the ability to target higher level improvements in a shorter amount of time. Given any concrete implementation, after it's been designed in a more dynamic language, you can convert it to something fast in something like C. But starting from C is probably not a great idea. And not everything can be written multiple times economically. E.g. the .net GC was prototyped in Lisp, and initially converted to C++ with a translator.
- pjmlp 8y ago
  I have, try to make an AOT compiler remove dynamic dispatch across shared libraries and convert it into either direct calls or even inline them.