llvm-project/flang

F18

F18 is a ground-up implementation of a Fortran front end written in modern C++. F18, when combined with LLVM, is intended to replace the Flang compiler.

Flang is a Fortran compiler targeting LLVM. Visit the Flang wiki for more information about Flang.

Getting Started

Read more about f18 in the documentation directory. Start with the compiler overview.

To better understand Fortran as a language and the specific grammar accepted by f18, read Fortran For C Programmers and f18'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 an f18 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 f18 uses modern C++ features.

Building F18

Get the Source Code

cd where/you/want/the/source
git clone https://github.com/flang-compiler/f18.git

Supported C++ compilers

F18 is written in C++17.

The code has been compiled and tested with GCC versions 7.2.0, 7.3.0, 8.1.0, and 8.2.0.

The code has been compiled and tested with clang version 7.0 and 8.0 using either GNU's libstdc++ or LLVM's libc++.

LLVM dependency

F18 uses components from LLVM.

The instructions to build LLVM can be found at https://llvm.org/docs/GettingStarted.html.

We highly recommend using the same compiler to compile both llvm and f18.

The f18 CMakeList.txt file uses the variable LLVM_DIR to find the installed components.

To get the correct LLVM libraries included in your f18 build, define LLVM_DIR on the cmake command line.

LLVM=<LLVM_BUILD_DIR>/lib/cmake/llvm cmake -DLLVM_DIR=$LLVM ...

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

LLVM dependency when building f18 with Fortran IR

If you do not want to build Fortran IR, add -DLINK_WITH_FIR=Off to f18 cmake command and ignore the rest of this section.

If you intend to build f18 with Fortran IR (-DLINK_WITH_FIR On by default), you must:

• build LLVM with the same compiler and options as the one you are using to build F18.
• pass -DCMAKE_CXX_STANDARD=17 -DLLVM_ENABLE_PROJECTS="mlir" to LLVM cmake command.
• install LLVM somewhere with make install in order to get the required AddMLIR cmake file (it is not generated in LLVM build directory).

Installing LLVM from packages is most likely not an option as it will not include MLIR and not be built following C++17 standard.

MLIR is under active development and the most recent development version may be incompatible. A branch named f18 is available inside LLVM fork in https://github.com/flang-compiler/f18-llvm-project. It contains a version of LLVM that is known be compatible to build f18 with FIR.

The fastest way to get set up is to do:

cd where/you/want/to/build/llvm
git clone --depth=1 -b f18 https://github.com/flang-compiler/f18-llvm-project.git
mkdir build
mkdir install
cd build
cmake ../f18-llvm-project/llvm -DCMAKE_BUILD_TYPE=Release \
    -DLLVM_ENABLE_PROJECTS=mlir -DCMAKE_CXX_STANDARD=17 \
    -DLLVM_INSTALL_UTILS=On \
    -DCMAKE_INSTALL_PREFIX=../install
make
make install

Then, -DLLVM_DIR would have to be set to <where/you/want/to/build/llvm>/install/lib/cmake/llvm in f18 cmake command.

To run lit tests, -DLLVM_EXTERNAL_LIT=<where/you/want/to/build/llvm>/build/bin/llvm-lit must be added to f18 cmake command. This is because llvm-lit is not part of LLVM installation.

Note that when using some advanced options from f18 cmake file it may be necessary to reproduce their effects in LLVM cmake command.

Building f18 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 f18.

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++-7.2, assuming g++-7.2 is on your PATH.

export CXX=g++-7.2

or

CXX=/opt/gcc-7.2/bin/g++-7.2 cmake ...

There's a third option! The CMakeList.txt file uses the variable GCC as the path to the bin directory containing the C++ compiler.

GCC can be defined on the cmake command line where <GCC_DIRECTORY> is the path to a GCC installation with bin, lib, etc:

cmake -DGCC=<GCC_DIRECTORY> ...

Building f18 with clang

To build f18 with clang, cmake needs to know how to find clang++ and the GCC library and tools that were used to build clang++.

The CMakeList.txt file expects either CXX or BUILD_WITH_CLANG to be set.

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

export CXX=clang++

BUILD_WITH_CLANG can be defined on the cmake command line where <CLANG_DIRECTORY> is the path to a clang installation with bin, lib, etc:

cmake -DBUILD_WITH_CLANG=<CLANG_DIRECTORY>

Or GCC can be defined on the f18 cmake command line where <GCC_DIRECTORY> is the path to a GCC installation with bin, lib, etc:

cmake -DGCC=<GCC_DIRECTORY> ...

To use f18 after it is built, the environment variables PATH and LD_LIBRARY_PATH must be set to use GCC and its associated libraries.

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 f18 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 F18

cd ~/f18/build
cmake -DLLVM_DIR=$LLVM ~/f18/src
make

How to Run the Regression Tests

To run all tests:

cd ~/f18/build
cmake -DLLVM_DIR=$LLVM ~/f18/src
make test check-all

To run individual regression tests llvm-lit needs to know the lit configuration for f18. The parameters in charge of this are: flang_site_config and flang_config. And they can be set as shown bellow:

<path-to-llvm-lit>/llvm-lit \
 --param flang_site_config=<path-to-f18-build>/test-lit/lit.site.cfg.py \
 --param flang_config=<path-to-f18-build>/test-lit/lit.cfg.py \
  <path-to-fortran-test>

How to Generate FIR Documentation

If f18 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/FIRLangRef.md in f18 build directory.