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  1. 29c3e8a [flang] Complain about more cases of calls to insufficiently defined procedures by peter klausler · 9 days ago main master
  2. 3e5ec6a [flang] Fix ARM/POWER test failure (folding20.f90) by peter klausler · 8 days ago
  3. 8f73695 [flang] Implement runtime for IALL & IANY by peter klausler · 9 days ago
  4. f1add8b [flang] Use a "double-double" accumulator in SUM by peter klausler · 9 days ago
  5. 5cab9bb [flang] Fold MAXVAL & MINVAL by peter klausler · 9 days ago


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.

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 markdown style guide from LLVM.

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.

The code does not compile with Windows and a compiler that does not have support for C++17.

Building Flang out of tree

These instructions are for building Flang separately from LLVM; if you are building Flang alongside LLVM then follow the standard LLVM build instructions and add flang to LLVM_ENABLE_PROJECTS instead, as detailed there.

LLVM dependency

The instructions to build LLVM can be found at If you are building flang as part of LLVM, follow those instructions and add flang to LLVM_ENABLE_PROJECTS.

We highly recommend using the same compiler to compile both llvm and flang.

The flang CMakeList.txt file uses

  • LLVM_DIR to find the installed LLVM components
  • MLIR_DIR to find the installed MLIR components
  • CLANG_DIR to find the installed Clang components

To get the correct LLVM, MLIR and Clang libraries included in your flang build, define LLVM_DIR, MLIR_DIR and CLANG_DIR on the cmake command line.

LLVM=<LLVM_BUILD_DIR>/lib/cmake/llvm \
MLIR=<LLVM_BUILD_DIR>/lib/cmake/mlir \
CLANG=<LLVM_BUILD_DIR>/lib/cmake/clang \

where LLVM_BUILD_DIR is the top-level directory where LLVM was built.

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.

Build Flang out of tree

cd ~/flang/build

Disable The New Flang Driver

The new Flang compiler driver, flang-new, is implemented in terms of clangDriver and hence it introduces a dependency on Clang. This dependency is otherwise not required. If you do not require the new driver, you can disable it by adding -DFLANG_BUILD_NEW_DRIVER=OFF to your CMake invocation. With the new driver disabled, you no longer need to add clang to LLVM_ENABLE_PROJECTS (or to specify CLANG_DIR when building out-of-tree).

How to Run Tests

Flang supports 2 different categories of tests

  1. Regression tests (
  2. Unit tests (

For out of tree builds

To run all tests:

cd ~/flang/build
cmake -DLLVM_DIR=$LLVM -DMLIR_DIR=$MLIR ~/flang/src
make test 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 out-of-tree build as it does not include googletest related headers and libraries.

There are various ways to run unit-tests.

1. make check-flang-unit 2. make check-all or make check-flang 3. <path-to-llvm-lit>/llvm-lit \ test/Unit 4. Invoking tests from <out-of-tree 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:

make check-flang-unit

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

make 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 make flang-doc. This will create docs/Dialect/ in flang build directory.

Generate Doxygen-based Documentation

To generate doxygen-style documentation from source code

cd ~/llvm-project/build
make 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
make docs-flang-html

It will generate html in

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