[certik]

There is old Flang, which motivated me to start LFortran. The new Flang, which presumably you are referring to, started possibly in the same month as LFortran, but we didn't know about each other.

It's best if you ask Flang developers what they see as the advantages of Flang over LFortran. From my biased perspective, LFortran can run interactively, it is fast to compile the compiler (30s on my laptop) and LFortran compiles your code very quickly (especially with our direct x64 or C backends). It runs in a browser: https://dev.lfortran.org/. We have many backends (LLVM, C, C++, Julia, WASM, x64). We plan to add Python and Fortran (the latter could be used to modernize your old Fortran code). It is easy to add new backends, and it is also easy to add new frontends, so we have LPython and LFortran as two thin frontends, to our intermediate representation that we call ASR (Abstract Semantic Representation). The internal design is simple, so a small team can develop LCompilers at a fast pace. New contributors without any prior compiler experience get up to speed very quickly (typically a few weeks or even less).

We are still in alpha, which means it is expected to break for your code (and when it does, please report all bugs!). To choose between them, I recommend to test them out and pick the one that you like the most, based on your criteria. Note that the most mature and widespread open source Fortran compiler is GFortran.