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Comment by amir734jj | Hacker News Reader

What is the difference between MLIR and LLVM IR code that we used to?

LLVM IR is just one IR dialect in the MLIR ecosystem, and there are a bunch of (included) higher-level IR dialects that can be transformed (usually, automatically) into the LLVM IR dialect, and from there, the normal LLVM compiler can take over and produce runnable machine code.

Your own languages can target the higher-level IR dialects in MLIR, or directly target the LLVM IR dialect, or both: MLIR is unique in that multiple IR dialects are allowed to be "live" at any time in the compiler, there are no strict "phases" where one IR is lowered, one-shot, into a lower-lever IR, like most compilers require (and compiler books teach).

MLIR is a really, really neat bit of technology.

As I understand it, MLIR is the new subsystem that the LLVM project is transitioning to in the long term, and LLVM IR is the old.

As such, LLVM IR isn't a proper subset of MLIR. Rather, there is a LLVM "dialect" in the MLIR system which can be translated 1:1 to LLVM IR.

MLIR in its structure and textual syntax is a bit different. A "dialect" is more like a namespace for your ops than a different language, in my view.

In the transition, traditional LLVM IR isn’t being left behind. It’s simply a later step in the compilation process. All MLIR is (eventually) translated through LLLVM IR before machine code.

>As I understand it, MLIR is the new subsystem that the LLVM project is transitioning to in the long term, and LLVM IR is the old.

this is very much not a forgone conclusion and many people in LLVM would boo vociferously at the idea (see last year's LLVM US meeting where Johannes Doerfert actually argued the exact opposite - extending LLVM IR to do some/many of the things that MLIR does).

It depends what you mean by "new subsystem" and "transitioning to": what seems like a given is that the notion of "one size fits all" of LLVM IR is behind us and the need to multi-level IR is embraced. LLVM IR is evolving to accommodate this better, within reason (that is: it stay organized around a pretty well defined core instruction set and type system), and MLIR is just the fully extensible framework beyond this. It is to be seen if anyone would have the appetite to port LLVM IR (and the LLVM framework) to be a dialect, I think there are challenges for this.

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