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The MappableType OpenACC type interface is a richer interface that allows OpenACC dialect to be capable to better interact with a source dialect, FIR in this case. fir.box and fir.class types already implemented this interface. Now the same is being done with the other FIR types that represent variables. One additional notable change is that fir.array no longer implements this interface. This is because MappableType is primarily intended for variables - and FIR variables of this type have storage associated and thus there's a pointer-like type (fir.ref/heap/pointer) that holds the array type. The end goal of promoting these FIR types to MappableType is that we will soon implement ability to generate recipes outside of the frontend via this interface.
Flang
Flang is a ground-up implementation of a Fortran front end written in modern C++. It started off as the f18 project (https://github.com/flang-compiler/f18) with an aim to replace the previous flang project (https://github.com/flang-compiler/flang) and address its various deficiencies. F18 was subsequently accepted into the LLVM project and rechristened as Flang.
Please note that flang is not ready yet for production usage.
Getting Started
Read more about flang in the docs directory. Start with the compiler overview.
To better understand Fortran as a language and the specific grammar accepted by flang, read Fortran For C Programmers and flang's specifications of the Fortran grammar and the OpenMP grammar.
Treatment of language extensions is covered in this document.
To understand the compilers handling of intrinsics, see the discussion of intrinsics.
To understand how a flang program communicates with libraries at runtime, see the discussion of runtime descriptors.
If you're interested in contributing to the compiler, read the style guide and also review how flang uses modern C++ features.
If you are interested in writing new documentation, follow LLVM's Markdown style guide.
Consult the Getting Started with Flang for information on building and running flang.