Emscripten Compiler Gets Optimizations, Now Self-Hosting 60
Emscripten is an LLVM-based compiler from dozens of languages to JavaScript (previously demoed as a repl and used to port Doom to the browser), and some recent changes have made it a bit faster, and allowed it to compile itself. Some highlights include a redundant variable eliminator, parallelization of the optimizier and compiler, and a new relooper. From the developer's weblog: "With all of the emscripten optimization passes now in JavaScript, I then worked on parallelizing that. ... The speedup can be close to linear in the number of cores. ... For the LLVM to JS compiler, I made the emscripten compiler parallel as well: It splits up the LLVM IR into 3 main parts: type data, function data, and globals. The function data part is unsurprisingly by far the largest in all cases I checked (95% or so), and it can in principle be parallelized - so I did that. Like in the optimizer, we use a Python process pool which feeds chunks of function data to multiple JavaScript compiler instances. There is some overhead due to chunking, and the type data and globals phases are not parallelized, but overall this can be a close to linear speedup. ... [On the new relooper] Note that this update makes Emscripten a 'self-hosting compiler' in a sense: one of the major optimization passes must be compiled to JS from C++, using Emscripten itself. Since this is an optimization pass, there is no chicken-and-egg problem: We bootstrap the relooper by first compiling it without optimizations, which works because we don't need to reloop there. We then use that unoptimized build of the relooper (which reloops properly, but slowly since it itself is unoptimized) in Emscripten to compile the relooper once more, generating the final fully-optimized version of the relooper, or 'relooped relooper' if you will."
