To: J3                                                     J3/21-156
From: Gary Klimowicz
Subject: Flang Liaison Report
Date: 2021-June-21

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.
    - Flang biweekly sync call ("umbrella call")
    - Flang biweekly technical call
    - "Classic Flang" biweekly call
    - Flang/Clang driver refactoring call
    - OpenMP and OpenMP IR weekly call (refactor for clang and flang)
    - OpenACC biweekly call for upstreaming work to LLVM

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. As of LLVM 12, it will be a
full-fledged member of the LLVM projects.

LLVM Flang is being developed under the Apache 2.0 license with LLVM
exceptions (the same as LLVM, Clang, OpenMP, etc.).

Recent efforts have concentrated on
    - upstreaming the lowering code from the 'fir-dev' branch to LLVM
      'main'
    - adding Fortran 95 semantics
    - adding intrinsics, including folding and runtime support
    - creating a new driver program to replace the original temporary
      driver
    - OpenMP semantics and runtime
    - fixing issues identified from test cases

The current state of LLVM Flang compiler is
    - written in modern C++ following LLVM conventions
    - over 200,000 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" (FIR) based on
      LLVM's MLIR
    - can lower most OpenMP and OpenACC constructs
    - can compile and correctly run Fortran 77 programs (passes FCVS
      test suite)
    - has a new driver, refactored to share libraries with the clang
      driver

Upcoming work includes
    - completing constraint checking and semantics for modern Fortran
      (344 checked of 606 constraints in F2018)
    - pull request of code required to support feature set (up through
      current F95 support) lowering to FIR
    - completing implementation of the runtime environment
    - completing implementation of intrinsics
    - continued work on the new driver
    - adding Fortran tests to LLVM suite (separate from Flang)


Current ("Classic") Flang Compiler
----------------------------------

The initial version of Flang is derived from the PGI Fortran compiler,
with some proprietary features removed (OpenACC support, inter-procedure
analysis). It is the basis of the Arm and AMD commercial Fortran
compilers.

Recent improvements to Flang include
    - support for LLVM 10.0, LLVM 11.0 using the LLVM monorepo
    - support for LLVM 12.0 being added
    - performance improvements (function specialization, etc.)
    - enhanced Fortran debug metadata support
    - continuing to add support for Windows; investigating Arm

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.

Detailed status can be found in the biweekly call Google doc
//docs.google.com/document/d/1-OuiKx4d7O6eLEJDBDKSRnSiUO2rgRR-c2Ga4AkrzOI