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Runtime-Compiled C++

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45 pointsluu8 comments

In case anyone is wondering how the magic works, I am guessing it is using shared libraries (but on top of that a lot of syntactic sugar). Also a good teaser video of it here (although I think it is older): http://vimeo.com/26224634

Also, you can find the author here, who is also working on some interesting stuff outside of RCC: https://twitter.com/dougbinks

"The runtime-modifiable classes use a macro that registers the files themselves to be monitored. Whenever one of those files changes, we pick it up and recompile it into a DLL. By linking that DLL we can switch over to the new code, leaving the original EXE file untouched. This way we avoid any interpreter and use native compiled C++ throughout."

(from http://www.runtimecompiledcplusplus.blogspot.co.uk/2011/07/t...)

Is that really necessary? I mean, we have JIT compilers nowadays, and they all produce native code at runtime, without such tricks.

Yes, there are JIT compilers, see (not just first answer): http://programmers.stackexchange.com/questions/29344/jit-com...

I'm not sure why these solutions have not caught on yet (seemingly). Performance? Code complexity (i.e. invalidated pointers from recompile)?

I think the biggest challenge is compilation time. With the lack of incremental compilation modifying more complex classes can result in long compilation time. Same is true for modifying templates that are widely used within the code base.

If you look for example at runtime compilation of queries in database systems, compiling can take significant amount of time, at least enough so it doesn't feel instant anymore.

To get incremental compilation, one would need fully dynamic dispatched invocation which would be probably horrible slow.

This said, I'm a strong believer in runtime code generation, just with a reduced scope rather than full c++.

A lot of the complexity of the RCC++ solution over others (I listed those I can find on the wiki https://github.com/RuntimeCompiledCPlusPlus/RuntimeCompiledC...) is about compiling the minimum possible code quickly in a platform independent way. I'd love to see C++ get modules in the spec, along with introspection to make this easier.

Compiling C++ code at runtime is less about end-user code than about improving developer workflow for those who use C++ and want rapid iteration (game developers for example), so avoiding extra languages and VMs is one of the goals.

As for full dynamic dispatch, RCC++ does use virtual functions. I advocate using a 'data oriented approach' so the function table pointer lookup should be amortized by having expensive functions process multiple data items. In practice in my own code I see no detectable overhead.

really cool; would love it more if it was just 1 or 2 files. i think it needs a bit more docs too.

Yes, I do need to add more docs - work is getting in the way a bit at the moment though I'm using RCC++ for that so am maintaining and improving it as I go.

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