Hacker News Reader

Comment by beaconstudios | Hacker News Reader

V8 is a JIT compiler, which incorporates a runtime and a compiler.

> V8 is a JIT compiler, which incorporates a runtime and a compiler.

So it's a compiler which incorporates a compiler? Compilerception much?

Seriously though: Can I tell V8 to compile my JS and tell me any errors found before I deploy it to production and runtime? Yes or no?

Just because V8 internally JIT-compiles the JS-code to something eventually executable which the machine can run doesn't change the fact that it's interpreter and execution-engine (also known as a "runtime") for Javascript code.

Just like Python and all those other platforms you listed as not being "compilers" (in which case you're absolutely right).

> So it's a compiler which incorporates a compiler? Compilerception much?

No, what I meant is that the concept of a JIT compiler incorporates both a runtime and a compiler.

> Seriously though: Can I tell V8 to compile my JS and tell me any errors found before I deploy it to production and runtime? Yes or no?

That's not the job of a compiler. It's a feature of compilation, but a compiler does not by definition have to compile your code before deployment. What would you be compiling the JS to? If you're wanting to check your syntax, your IDE should be able to do that, or you can use a compile-to-JS language like TypeScript. Yes, it's dumb I know - that's one of the many reasons I use TypeScript.

> Just because V8 internally JIT-compiles the JS-code to something eventually executable which the machine can run doesn't change the fact that it's interpreter and execution-engine (also known as a "runtime") for Javascript code.

You seem to be hung up on the idea of a compiler as something that checks your code is correct. It isn't - that functionality is incidental to the compiler's job, because it can't translate invalid syntax into whatever destination language it compiles to.

> Just like Python and all those other platforms you listed as not being "compilers" (in which case you're absolutely right).

I did no such thing. That was someone else. Any language which internally uses a bytecode representation is a JIT compiler, including Python, Java, and PHP. You could argue I'm being nitpicky, given that pretty much all performant interpreted languages use a bytecode representation - it's true in a sense, but I'm just trying to highlight that you seem to have a misunderstanding of what a compiler's job is.

"but a compiler does not by definition have to compile your code before deployment" Huh?

Not sure many experienced developers would agree with you a JIT Compiler is quite different to a real compiled language C Fortran Etc

a compiled language, yes. But the function of a compiler, between a compiled language and an interpreted language, remains the same. It translates code from one representation to another.

This is really simple. Do you use V8 to compile your code or to run it?

Just because V8 has a compiler, doesn't make it a compiler. Do you see the difference?

V8 compiles code into bytecode and runs that. This is really simple.

V8 is a JIT compiler. It both compiles code and runs it. For your original complaint about not being able to compile JS up front, you're talking about Ahead-Of-Time (AOT) compilation. You don't seem very familiar with the relevant terminology.

In fact, I just looked it up and apparently V8 compiles JS directly to machine code rather than bytecode: https://en.wikipedia.org/wiki/Chrome_V8.

V8 compiles to native code with the exception of low memory devices (512M or less), for which it has a (relatively recently introduced) bytecode compiler, so it uses an interpreter in that one case.

Syntax errors will be detected prior to runtime because the application will not be able to be compiled into whatever format it compiles down to (raw ASM, some IR, etc.). A fully AOT compiler will generally check more than a JIT compiler, but there is still compilation and it is still a "compiler" in a technical sense.

And for the record, AOT languages also have "runtimes" provided by a platform. If someone combined gcc and glibc, would the combined product no longer be considered a "compiler"?

It is my understanding that V8 provides you with the capability to interpret your code and run it, while gcc produces a resulting binary which be run independently of gcc.

Can V8 be used to compile js to a artifact/binary which can later be executed independently of V8? If no, then it's not a compiler. It's as simple as that.

Executables from gcc can't be run independent of a binary-compatible C runtime.

The difference is just that V8 includes the runtime and compiler in the same program. If they split it into v8c and v8lib without making any changes to the way that JavaScript is converted into something that your CPU processes, would it then be a "real compiler"?

Are mcs or javac real compilers? They both leave behind an "executable", but they require a separate runtime to be installed actually execute it. What about GCJ? What about anything that compiles down to LLVM IR and depends on LLVM to convert it to machine code? Is that not a real compiler either? Is CPython a compiler? It leaves junk on my disk in the form of pyc files.

I'm really curious why you insist that something isn't performing compilation if it doesn't write its output to disk. When is the process of compiling from one target into another not compilation?

Perhaps you're suggesting that only compilers that emit appropriate machine code are "real compilers". That's silly, but I could understand if that's what you mean.

However, I think you'll be disappointed to learn that V8 does directly emit machine code. [0]

[0] https://github.com/v8/v8/tree/master/src/compiler

There is no interpretation involved; V8 has an unoptimized base compiler for compiling JS to machine code one function at a time, and a pair of optimizing compilers that work based on runtime profiling. V8 also has a bytecode interpreter for low memory devices, but it'd mainly be used on very low end mobile devices.

Reply on news.ycombinator.com