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Recursion, both direct and indirect, prevents accurate stack size calculation at link time for GPU device code. Restructure these recursive (often mutually so) routines in the Fortran runtime with new implementations based on an iterative work queue with suspendable/resumable work tickets: Assign, Initialize, initializeClone, Finalize, and Destroy. Default derived type I/O is also recursive, but already disabled. It can be added to this new framework later if the overall approach succeeds. Note that derived type FINAL subroutine calls, defined assignments, and defined I/O procedures all perform callbacks into user code, which may well reenter the runtime library. This kind of recursion is not handled by this change, although it may be possible to do so in the future using thread-local work queues. (Relanding this patch after reverting initial attempt due to some test failures that needed some time to analyze and fix.) Fixes https://github.com/llvm/llvm-project/issues/142481.
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.