[kscarlet]

It seems that JIT is overloaded with at least 2 meaning.

The canonical definition of JIT is "compilation during execution of a program". Usually, a program is being interpreted first, then switches to compiled code in the middle of execution. This is not what this article does.

What this article does is sometimes called on-the-fly AOT, or just on-the-fly compilation. I'd prefer not overloading the term "JIT".

[dataflow]

> Usually, a program is being interpreted first, then switches to compiled code in the middle of execution.

I agree what they have isn't JIT compilation, but not for that reason. Tiered execution was never a central part of JIT compilation either. It was a fairly new invention in comparison.

The reason what they describe isn't JIT compilation is IMO fairly boring: it's not compiling the input program in any meaningful way, but simply writing hard-coded logic into executable memory that it already knows the program intended to perform. Sure there's a small degree of freedom based on the particular arithmetic operations being mentioned, but that's... very little. When your compiler already knows the high-level source code logic before it's even read the source code, it's... not a compiler. It's just a dynamic code emitter.

As to the actual difference between JIT vs. AOT... it may just come down to accounting. That is, on whether you can always exclude the compilation time/cost from the overall program execution time/cost or not. If so, you're compiling ahead of (execution) time. If not, you're compiling during execution time.

[kscarlet]

> As to the actual difference between JIT vs. AOT... it may just come down to accounting. That is, on whether you can always exclude the compilation time/cost from the overall program execution time/cost or not. If so, you're compiling ahead of (execution) time. If not, you're compiling during execution time.

Well, this includes what I refer to as "on-the-fly" AOT, like SBCL, CCL, Chez Scheme... Even ECL can be configured to work this way. As I mentioned in another comment, people in those circles do not refer to these as "JIT" at all, instead saying "I wish my implementation was JIT instead of on-the-fly AOT"!

[ordu]

> it's not compiling the input program in any meaningful way, but simply writing hard-coded logic into executable memory that it already knows the program intended to perform.

The program reads the logic from stdin and translates it into machine instructions. I can agree that there is not a lot of a freedom in what can be done, but I think it just means that the source language is not Turing complete. I don't believe that compiler needs to deal with a Turing complete language to claim the title "JIT compiler".

[dataflow]

> The program reads the logic from stdin and translates it into machine instructions. I can agree that there is not a lot of a freedom in what can be done, but I think it just means that the source language is not Turing complete. I don't believe that compiler needs to deal with a Turing complete language to claim the title "JIT compiler".

"Not Turing-complete" is quite the understatement.

A "compiler" is software that translates computer code from one programming language into another language. Not just any software that reads input and produces output.

The input language here is... not even a programming language to begin with. Literally all it can express is linear functions. My fixed-function calculator is more powerful than that! If this is a programming language then I guess everyone who ever typed on a calculator is a programmer too.

[ordu]

> A "compiler" is software that translates computer code from one programming language into another language.

Yes, of course.

> Literally all it can express is linear functions.

And why a language to express linear functions can't be a programming language?

> My fixed-function calculator is more powerful than that!

But it isn't compiling, it is interpreting, is it? So your fixed-function calculator is not a compiler, it is an interpreter. It is irrelevant how powerful it is. There are much even more powerful interpreters and less powerful compilers.

The example we see gets computer code in one language and translates it into machine instructions. Talking about 'understatement' you are adding to this definition more constraints and these are very fuzzy constraints on what counts as a programming language.

[tgv]

A compiler takes some language and translates it into something close(r) to the hardware. And that's what the OP does. And since it compiles in process and executed it too, it's JIT, as opposed to AOT.

These terms are not related to the complexity of the problem. The first compilers could only translate for formulas, hence FORTRAN.

[dataflow]

I'm not sure where you got your definition, but I basically copied Wikipedia's.

[Brian_K_White]

That's not compilation merely substitution.

[orthoxerox]

I think if your program starts to execute machine code that wasn't present in the binary, then it counts as having a JIT compiler.

Of course, there are edge cases like embedding libtcc, but I think it's a reasonable definition.

[ignoramous]

> This is not what this article does.

Discounting books, many other well written articles on JIT have been shared on HN over the years [0][1][2]; the one I particularly liked as it introduces the trinity in a concise way: Compiler, Interpreter, JIT https://nickdesaulniers.github.io/blog/2015/05/25/interprete... / https://archive.vn/HaFlQ (2015).

[0] How to JIT - an introduction, https://eli.thegreenplace.net/2013/11/05/how-to-jit-an-intro... (2013).

[1] Bytecode compilers and interpreters, https://bernsteinbear.com/blog/bytecode-interpreters/ (2019).

[2] Let's build a Simple Interp, https://ruslanspivak.com/lsbasi-part1/ (2015).

[MobiusHorizons]

I always took the distinction to be the executable memory bit as opposed to writing an executable and then launching it in the usual way. Of course high performance runtimes that contain JITs do typically interpret first in order to get type information and reduce startup latency, but that’s more a function of the language not being efficiently AOT compileable rather than fundamental to the concept of a JIT

[throwaway17_17]

Can you explain what mean by a “language not being efficiently AOT compileable”? I am guessing you are referring to languages that emphasize REPL based development or languages that are ‘image’ based (I think Smalltalk is the most common example).

Like the guesses above, I can understand difficulty with AOT compilation in conjunction with certain use cases; however, I can not think of a language that based on its definition would be less amenable to AOT compilation.

[Nevermark]

The more context is narrowed down, the more optimizations that can applied during compilation.

AOT situations where a lot of context is missing:

• Loosly typed languages. Code can be very general. Much more general than how it is actually used in any given situation, but without knowing what the full situation is, all that generality must be complied.

• Increment AOT compilation. If modules have been compiled separately, useful context wasn't available during optimization.

• Code whose structure is very sensitive to data statistics or other conditional runtime information. This is the prime advantage of JIT over AOT. Unless the AOT compiler is working in conjunction with representative data and a profiler.

Those are all cases where JIT has advantages.

A language where JIT is optimal, is by definition, less amenable to AOT compilation.

[MobiusHorizons]

I should probably have said, not compilable to efficient code. rather than not efficiently compilable. Basically I was referring to dynamic typing. Typically such languages are interpreted, although there is probably a way to compile the same sequence the interpreter would take as an AOT compiler. When types are not known until runtime, the compiler can't emit efficient code, and instead has to emit code that checks the type on each function invocation which is basically exactly what the interpreter was already doing. This is where JITs really shine, since they can produce one or more compilations of the same function specialized to types that have been observed to be used frequently. the interpreter can then call those specialized versions of the functions directly when appropriate.

[MobiusHorizons]

What I meant was dynamic typing primarily. Think JavaScript or lua. These are both languages where the choice is between interpreter and JIT rather than JIT or AOT. VM based languages like Java also fall in that category, not because of dynamic typing so much as because of shipping bytecode to the client for portability reasons.

[kscarlet]

In that sense almost every compiled Lisp/Scheme implementation, GHC, etc. or any other interactive programming system, count as JIT. But virtually nobody in those circles refer to such implementations as JIT. Instead, people says "I wish our implementation was JIT to benefit from all those optimizations it enables"!

[MobiusHorizons]

Do they generate machine code in ram and jump to it? Or do they interpret byte code?

EDIT: at least GHC seems to be a traditional AOT compiler.

[kscarlet]

They generate native machine code.