To: J3                                                     J3/21-133
From: Gary Klimowicz
Subject: Flang Open Source Liaison Report
Date: 2021-March-03

Flang Open Source Fortran Report
================================

Flang is an open source compiler for Fortran, sponsored by the US
Department of Energy Exascale Computing Project (particularly, LLNL,
Sandia and LANL).

The goals of the project are to
    - Create a new, open source Fortran 2018 compiler with
      Apache 2.0 licensing,
    - that can be used for language and parallelization
      experimentation,
    - that exists as a peer in the LLVM community of languages,
      like Clang,
    - that can rely on LLVM code generation and parallelism
      support for CPUs and GPUs.

There is a Slack workspace for Flang and a number of conference calls
related to Flang development. Details can be found on the Getting
Involved page http://flang.llvm.org/docs/GettingInvolved.html.

The New LLVM Flang Compiler
---------------------------

The new front-end is available at https://github.com/llvm/llvm-project
(the LLVM monorepo) in the flang/ directory.

No contributor license agreement is needed to contribute to LLVM Flang,
which is being developed under the Apache 2.0 license with LLVM
exceptions (the same as LLVM and Clang).

The current state of LLVM Flang compiler is
    - written in modern C++ following LLVM conventions
    - over 183,000 lines of code, tests and scripts
    - parses all of Fortran 2018 to abstract syntax trees (AST)
    - parses  OpenMP 4.5 and some OpenMP 5.0
    - parses OpenACC 3.0
    - defines a "Fortran Intermediate Representation" based on
      LLVM's MLIR; this work is being upstreamed into LLVM
    - can lower most OpenMP and OpenACC constructs
    - can compile and correctly run Fortran 77 programs (passes FCVS
      test suite)
    - new driver being developed for Flang

Upcoming work includes
    - completing constraint checking and semantics for modern Fortran
    - continuing merge of FIR dialect definitions; starting on operations
    - completing definition of the runtime environment
    - implementation of runtime
    - continued work on the new driver
    - adding Fortran tests to LLVM suite (separate from Flang)


Current Flang Compiler
----------------------

The initial version of Flang is derived from the PGI Fortran compiler,
with some proprietary features removed (OpenACC support, inter-procedure
analysis).

Recent improvements to Flang include
    - many bug fixes and enhancements from the PGI compiler
    - support for  LLVM 9.0 being phased out
    - support for LLVM 10.0, LLVM 11.0 using the LLVM monorepo
    - support for  LLVM 12.0 being added
    - bug fixes (array initialization, etc.)
    - enhanced Fortran debug metadata support (adjustable, allocatable,
      assumed size, assumed rank arrays)
    - continuing to add support for Windows

Current Flang is available for Linux on x86-64, OpenPOWER and Arm
processors, and is the basis of the Arm and AMD commercial Fortran
compilers.  Huawei has made significant contributions as well.