Compilers come in all shapes and sizes. Some are single pass like Turbo Pascal. They directly generate machine code while parsing (no AST!). Niklaus Wirth (Pas

Compilers come in all shapes and sizes. Some are single pass like Turbo Pascal. They directly generate machine code while parsing (no AST!). Niklaus Wirth (Pascal/Modula/Oberon guy) wrote a book following this approach [1]. Some have multiple passes like Chez Scheme. They have many simple passes, which they call a "nanopass". Andrew Keep has a great talk on this approach [2]. In practice, most compilers today are multi-pass, though probably not as many Chez. If we look at Rust, they go from AST -> HIR -> MIR -> LLVM IR -> machine code [3]. There are probably more things going on from LLVM to machine code, but I'm not knowledgeable enough to comment on it. I think the trade-off here is clear: less passes -> shorter compile times, more passes -> faster code generated, more modular compiler. Martin Odersky (Scala guy) has a paper attempting to get the best of both [4]. [1] http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf http://www.ethoberon.ethz.ch/WirthPubl/CBEAll.pdf [2] https://www.youtube.com/watch?v=Os7FE3J-U5Q https://www.youtube.com/watch?v=Os7FE3J-U5Q [3] https://blog.rust-lang.org/2016/04/19/MIR.html https://blog.rust-lang.org/2016/04/19/MIR.html [4] https://infoscience.epfl.ch/record/228518/files/paper.pdf https://infoscience.epfl.ch/record/228518/files/paper.pdf