2f0b4f43fc
Fix #112593 by adding support in lowering to concatenation with an absent optional _assumed length_ dummy argument because: 1. Most compilers seem to support it (most likely by accident). 2. This actually makes the compiler codegen simpler. Codegen was going out of its way to poke the LLVM optimizer bear by producing an undef argument for the length. I insist on the fact that no compiler support this with _explicit length_ optional arguments and the executable will segfault and I would discourage users from using that "feature" because runtime checks for bad optional dereference will kick when used (For instance, "nagfor -C=present" will produce an executable that abort with an error message . Flang does not have such runtime check option so far). Hence, I am not updating the Extensions.md document because this is not something I think we should advertise.
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.