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Currently, all procedures from intrinsic modules that are not BIND(C) are expected to be intercepted by the compiler in lowering and to have a handler in IntrinsicCall.cpp. As more "intrinsic" modules are being added (OpenMP, OpenACC, CUF, ...), this requirement is preventing seamless implementation of intrinsic modules in Fortran. Procedures from intrinsic modules are different from generic intrinsics defined in section 16 of the standard. They are declared in Fortran file seating in the intrinsic module directory and inside the compiler they look like regular user call except for the INTRINSIC attribute set on their module. So an easy implementation is just to have the implementation done in Fortran and linked to the runtime without any need for the compiler to necessarily understand and handle these calls in special ways. This patch splits the lookup and generation part of IntrinsicCall.cpp so that it can be allowed to only intercept calls to procedure from intrinsic module if they have a handler. Otherwise, the assumption is that they should be implemented in Fortran. Add explicit TODOs handler for the IEEE procedure that are known to not yet been implemented and won't be implemented via Fortran code so that this patch is an NFC for what is currently supported. This patch also prevents doing two lookups in the intrinsic table (There was one to get argument lowering rules, and another one to generate 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.