| The article clearly admits that the LLVM compiler uses heuristics. You even quoted that part of the article. >WebAssembly is designed to perform well without relying on heuristic-based optimizations, avoiding the performance cliffs that come if code doesn’t meet those heuristics. It is expected that the compiler emitting the WebAssembly (in this case rustc and LLVM) already has sophisticated optimization infrastructure, that the engine is receiving WebAssembly code that has already had optimization passes applied, and that the WebAssembly is close to its final form. But really the point of the article is the last part. >that the engine is receiving WebAssembly code that has already had optimization passes applied, and that the WebAssembly is close to its final form. The compiler applies heuristics once during the compilation step and then never again. Compare this to JITs which are constantly changing and can pull the rug from under you. >Maybe it would also surprise the author to learn that their are JITs that beat the pants off LLVM AOT for dynamic languages like javascript (they just don't happen to be integrated into web browsers). Yes but it gets even better! If you limit yourself to a formalised subset of javascript called asm.js which gives you fine control over memory layout and allocation you can reach even the performance of C! Have you heard of it's successor? I think it's name was Web Assembly and every major browser has integrated it. It's a really cool technology that shows how sandboxed JITs can have the same performance characteristics as AOT compilers. |