79a25e11fe
Refine handling of NULL(...) in semantics to properly distinguish NULL(), NULL(objectPointer), NULL(procPointer), and NULL(allocatable) from each other in relevant contexts. Add IsNullAllocatable() and IsNullPointerOrAllocatable() utility functions. IsNullAllocatable() is true only for NULL(allocatable); it is false for a bare NULL(), which can be detected independently with IsBareNullPointer(). IsNullPointer() now returns false for NULL(allocatable). ALLOCATED(NULL(allocatable)) now works, and folds to .FALSE. These utilities were modified to accept const pointer arguments rather than const references; I usually prefer this style when the result should clearly be false for a null argument (in the C sense), and it helped me find all of their use sites in the code.
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.