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> Fascinating stuff, though I do not what IRs are used by the modern > compilers like LLVM, GHC, Java, .Net?

There are a couple of things going on with CPS as implemented in this style of compilers that make it particularly nice for writing optimization passes:

1) There are only function calls ("throws") and no returns. Though there's no theoretical reduction in the stuff you have to track (since what is a direct-style return is now a throw to the rest of the program starting at the return point), there's a huge reduction in in the complexity of your optimizations because you're not tracking the extra thing. For some examples, you can look in the Manticore codebase where even simple optimizations like contraction and let-floating are implemented in both our early direct-style IR (BOM) and our CPS-style IR (CPS). The latter is gobs more readable, and there's no way at all I'd be willing to port most of the harder optimizations like reflow-informed higher-order inlining or useless variable elimination to BOM.

2) IRs are cheap in languages like lisp and ML. So you write optimizations as a tree-to-tree transformation (micro-optimization passes). This style makes it much easier to enforce invariants. If you look at the internals of most compilers written in C++, you'll see far fewer copies of the IR made and a whole bunch of staged state in the object that's only valid at certain points in the compilation process (e.g., symbols fully resolved only after phase 2b, but possibly invalid for a short time during optimization 3d unless you call function F3d_resolve_symbol...). Just CPS'ing the representation of a C++ compiler without also making it easy to write your optimization passes as efficient tree-to-tree transformations will not buy you much, IMO.

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