tree: b777b8de3736109c678085715813f459a9ccd67c [path history] [tgz]
  1. cmake/
  2. docs/
  3. examples/
  4. include/
  5. lib/
  6. module/
  7. runtime/
  8. test/
  9. tools/
  10. unittests/
  11. .clang-format
  12. .clang-tidy
  14. .gitignore
  15. CMakeLists.txt


Flang is a ground-up implementation of a Fortran front end written in modern C++. It started off as the f18 project ( with an aim to replace the previous flang project ( 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.

Building flang

There are two ways to build flang. The first method is to build it at the same time that you build all of the projects on which it depends. This is called building in tree. The second method is to first do an in tree build to create all of the projects on which flang depends. Then, after creating this base build, only build the flang code itself. This is called building standalone. Building standalone has the advantage of being smaller and faster. Once you create the base build and base install areas, you can create multiple standalone builds using them.

Note that instructions for building LLVM can be found at

All of the examples below use GCC as the C/C++ compilers and ninja as the build tool.

Building flang in tree

Building flang in tree means building flang along with all of the projects on which it depends. These projects include mlir, clang, flang, openmp, and compiler-rt. Note that compiler-rt is only needed to access libraries that support 16 bit floating point numbers. It's not needed to run the automated tests. You can use several different C++ compilers for most of the build, includig GNU and clang. But building compiler-rt requres using the clang compiler built in the initial part of the build.

Here's a directory structure that works. Create a root directory for the cloned and built files. Under that root directory, clone the source code into a directory called llvm-project. The build will also create subdirectories under the root directory called build (holds most of the built files), install (holds the installed files, and compiler-rt (holds the result of building compiler-rt).

Here's a complete set of commands to clone all of the necessary source and do the build.

First, create the root directory and cd into it.

mkdir root
cd root

Now clone the source:
git clone

Once the clone is complete, execute the following commands:

rm -rf build
mkdir build
rm -rf install
mkdir install

cd build

cmake \
  -G Ninja \
  -DLLVM_ENABLE_PROJECTS="clang;mlir;flang;openmp" \
  -DLLVM_ENABLE_RUNTIMES="compiler-rt" \


By default flang tests that do not specify an explicit --target flag use LLVM's default target triple. For these tests, if there is a need to test on a different triple by overriding the default, the following needs to be added to the cmake command above: -DLLVM_TARGET_TRIPLE_ENV="<some string>" -DFLANG_TEST_TARGET_TRIPLE="<your triple>".

To run the flang tests on this build, execute the command in the “build” directory:

ninja check-flang

To create the installed files:

ninja install

echo "latest" > $INSTALLDIR/bin/versionrc

To build compiler-rt:

rm -rf compiler-rt
mkdir compiler-rt
cd compiler-rt
CC=$INSTALLDIR/bin/clang \
CXX=$INSTALLDIR/bin/clang++ \
cmake \
  -G Ninja \
  ../llvm-project/compiler-rt \
  -DCMAKE_C_CFLAGS=-mlong-double-128 \
  -DCMAKE_CXX_CFLAGS=-mlong-double-128 \

ninja install

Note that these instructions specify flang as one of the projects to build in the in tree build. This is not strictly necessary for subsequent standalone builds, but doing so lets you run the flang tests to verify that the source code is in good shape.

Building flang standalone

To do the standalone build, start by building flang in tree as described above. This build is base build for subsequent standalone builds. Start each standalone build the same way by cloning the source for llvm-project:

mkdir standalone
cd standalone
git clone

Once the clone is complete, execute the following commands:

cd llvm-project/flang
rm -rf build
mkdir build
cd build

cmake \
  -G Ninja \
  -DLLVM_BUILD_MAIN_SRC_DIR=$ROOTDIR/build/lib/cmake/llvm \
  -DLLVM_EXTERNAL_LIT=$ROOTDIR/build/bin/llvm-lit \
  -DLLVM_DIR=$ROOTDIR/build/lib/cmake/llvm \
  -DCLANG_DIR=$ROOTDIR/build/lib/cmake/clang \
  -DMLIR_DIR=$ROOTDIR/build/lib/cmake/mlir \


To run the flang tests on this build, execute the command in the “flang/build” directory:

ninja check-flang

Supported C++ compilers

Flang is written in C++17.

The code has been compiled and tested with GCC versions from 7.2.0 to 9.3.0.

The code has been compiled and tested with clang version 7.0, 8.0, 9.0 and 10.0 using either GNU‘s libstdc++ or LLVM’s libc++.

The code has been compiled on AArch64, x86_64 and ppc64le servers with CentOS7, Ubuntu18.04, Rhel, MacOs, Mojave, XCode and Apple Clang version 10.0.1.

Note that flang is not supported on 32 bit CPUs.

Building flang with GCC

By default, cmake will search for g++ on your PATH. The g++ version must be one of the supported versions in order to build flang.

Or, cmake will use the variable CXX to find the C++ compiler. CXX should include the full path to the compiler or a name that will be found on your PATH, e.g. g++-8.3, assuming g++-8.3 is on your PATH.

export CXX=g++-8.3


CXX=/opt/gcc-8.3/bin/g++-8.3 cmake ...

Building flang with clang

To build flang with clang, cmake needs to know how to find clang++ and the GCC library and tools that were used to build clang++.

CXX should include the full path to clang++ or clang++ should be found on your PATH.

export CXX=clang++

Installation Directory

To specify a custom install location, add -DCMAKE_INSTALL_PREFIX=<INSTALL_PREFIX> to the cmake command where <INSTALL_PREFIX> is the path where flang should be installed.

Build Types

To create a debug build, add -DCMAKE_BUILD_TYPE=Debug to the cmake command. Debug builds execute slowly.

To create a release build, add -DCMAKE_BUILD_TYPE=Release to the cmake command. Release builds execute quickly.

How to Run Tests

Flang supports 2 different categories of tests

  1. Regression tests (
  2. Unit tests (

For standalone builds

To run all tests:

cd ~/flang/build
cmake -DLLVM_DIR=$LLVM -DMLIR_DIR=$MLIR ~/flang/src
ninja check-all

To run individual regression tests llvm-lit needs to know the lit configuration for flang. The parameters in charge of this are: flang_site_config and flang_config. And they can be set as shown below:

<path-to-llvm-lit>/llvm-lit \
 --param flang_site_config=<path-to-flang-build>/test-lit/ \
 --param flang_config=<path-to-flang-build>/test-lit/ \

Unit tests:

If flang was built with -DFLANG_INCLUDE_TESTS=ON (ON by default), it is possible to generate unittests. Note: Unit-tests will be skipped for LLVM install for an standalone build as it does not include googletest related headers and libraries.

There are various ways to run unit-tests.

1. ninja check-flang-unit 2. ninja check-all or ninja check-flang 3. <path-to-llvm-lit>/llvm-lit \ test/Unit 4. Invoking tests from <standalone flang build>/unittests/<respective unit test folder>

For in tree builds

If flang was built with -DFLANG_INCLUDE_TESTS=ON (ON by default), it is possible to generate unittests.

To run all of the flang unit tests use the check-flang-unit target:

ninja check-flang-unit

To run all of the flang regression tests use the check-flang target:

ninja check-flang

How to Generate Documentation

Generate FIR Documentation

If flang was built with -DLINK_WITH_FIR=ON (ON by default), it is possible to generate FIR language documentation by running ninja flang-doc. This will create <build-dir>/tools/flang/docs/Dialect/ in flang build directory.

Generate Doxygen-based Documentation

To generate doxygen-style documentation from source code

cd ~/llvm-project/build
ninja doxygen-flang

It will generate html in

    <build-dir>/tools/flang/docs/doxygen/html # for flang docs

Generate Sphinx-based Documentation

Flang documentation should preferably be written in markdown(.md) syntax (they can be in reStructuredText(.rst) format as well but markdown is recommended in first place), it is mostly meant to be processed by the Sphinx documentation generation system to create HTML pages which would be hosted on the webpage of flang and updated periodically.

If you would like to generate and view the HTML locally:

  • Install Sphinx, including the sphinx-markdown-tables extension.
cd ~/llvm-project/build
ninja docs-flang-html

It will generate html in

   $BROWSER <build-dir>/tools/flang/docs/html/