Can I run GCC as a daemon (or use it as a library)?

Question

I would like to use GCC kind of as a JIT compiler, where I just compile short snippets of code every now and then. While I could of course fork a GCC process for each function I want to compile, I find that GCC's startup overhead is too large for that (it seems to be about 50 ms on my computer, which would make it take 50 seconds to compile 1000 functions). Therefore, I'm wondering if it's possible to run GCC as a daemon or use it as a library or something similar, so that I can just submit a function for compilation without the startup overhead.

In case you're wondering, the reason I'm not considering using an actual JIT library is because I haven't found one that supports all the features I want, which include at least good knowledge of the ABI so that it can handle struct arguments (lacking in GNU Lightning), nested functions with closure (lacking in libjit) and having a C-only interface (lacking in LLVM; I also think LLVM lacks nested functions).

And no, I don't think I can batch functions together for compilation; half the point is that I'd like to compile them only once they're actually called for the first time.

I've noticed libgccjit , but from what I can tell, it seems very experimental.

Answer 1

My answer is "No (you can't run GCC as a daemon process, or use it as a library)", assuming you are trying to use the standard GCC compiler code. I see at least two problems:

1. The C compiler deals in complete translation units, and once it has finished reading the source, compiles it and exits. You'd have to rejig the code (the compiler driver program) to stick around after reading each file. Since it runs multiple sub-processes, I'm not sure that you'll save all that much time with it, anyway.

2. You won't be able to call the functions you create as if they were normal statically compiled and linked functions. At the least you will have to load them (using dlopen() and its kin, or writing code to do the mapping yourself) and then call them via the function pointer.

The first objection deals with the direct question; the second addresses a question raised in the comments.

Answer 2

I'm late to the party, but others may find this useful.

There exists a REPL (read–eval–print loop) for c++ called Cling , which is based on the Clang compiler. A big part of what it does is JIT for c & c++. As such you may be able to use Cling to get what you want done.

The even better news is that Cling is undergoing an attempt to upstream a lot of the Cling infrastructure into Clang and LLVM.

@acorn pointed out that you'd ruled out LLVM and co. for lack of ac API, but Clang itself does have one which is the only one they guarantee stability for: https://clang.llvm.org/doxygen/group__CINDEX.html