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What's the difference between Legion and Regent, by the way?

I noticed the Regent code is inside the Legion repo. Is Legion the system, and Regent the language?

Can Legion be used without Regent, or vice versa?

Legion is a C++ runtime system. It exposes APIs in C++ and C. You can write code directly to it with C++ (and CUDA/HIP/SyCL if you want to use GPUs). But the only requirement is a C++ compiler and standard build system (Make/CMake).

Regent is a programming language. The compiler for Regent generates Legion code. Semantically, Regent is mostly a simplification of Legion. There are fewer moving pieces, so fewer things you need to worry about. Many of the "gotchas" that exist in Legion are taken care of by the language/compiler so idiomatic code usually "just works". It also does GPU code generation for you so you don't need to hand-write CUDA/HIP/etc. The tradeoff is that you're using a new programming language, so you have to be willing to take that risk.

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