On Sat, 18 Jun 2016, Connor Lane Smith <c...@lubutu.com> wrote: > Hi all, > > I was wondering if others had an opinion on JIT. Suppose we don't need > anything fancy like adaptive optimisation, but just wanted to compile > a program at runtime. One possibility might be to generate C code and > store that in a file and then call the C compiler and then execute the > resulting binary... But that seems a bit unpleasant, prone to > compilation failure, and not particularly lightweight either. > > One solution could be simply to produce assembly code, but then that > is tied to a specific architecture, which is unfortunate. There's also > LLVM, but that is a very big and complicated dependency, which > shouldn't really be necessary if we're just jitting something quickly > and don't mind it being a little unoptimised for the sake of > simplicity and speed of compilation. We just want to portably generate > machine code and then run it. > > An ideal might be something like an abstract instruction set together > with a JIT for the abstract machine. To be honest a JIT might not even > be necessary so long as it is has very little interpretation overhead, > the instruction set is general purpose and fixed, and it plays well > with the C memory model. > > Does anyone have any ideas? > > Thanks, > cls
Hi Connor, Creating a simple and general-purpose VM shouldn't be hard! It used to be my favourite exercise for learning a new programming language. Probably much more difficult to get real-world performance; I wouldn't be surprised if the initial efforts resulted in a 1000x-slower-than-C execution speed for typical programs. With lots of test cases, tuning, benchmarks, and generally a lot of hard work, I can imagine you could bring it to the 10-100x-slower[1] class. [1]: https://benchmarksgame.alioth.debian.org/u64q/compare.php?lang=python3&lang2=gcc Of course this doesn't matter that much if your purpose is mostly scripting behavior (games), or IO-bound stuff (as in waiting for database - things like Snabb[2] actually do need some real power). Having good C interop via FFI can save you in many cases. [2]: https://github.com/snabbco/snabb Yes, JITing is inherently architecture-specific, but the bytecode can be designed with trade-offs between interpretation and compilation speed. These days supporting x86-64, ARM and MIPS probably covers >99% of the devices you'll ever encounter in the wild; the rest can run a bit slower until someone is motivated enough to write a JIT backend. I've never had a close look at any of the Big Name VMs, as most of that code must suck horribly. Some real-world VMs&JITs however remained relatively simple - I think there might be a lot to learn from Dis[3] and LuaJIT[4]. [3]: http://doc.cat-v.org/inferno/4th_edition/dis_VM_design [4]: http://luajit.org/ If you have some concrete applications in mind, please do share. I'd gladly give a shot at prototyping something in this area. <3,K.