# Flang Flang is a ground-up implementation of a Fortran front end written in modern C++. It started off as the f18 project (https://github.com/flang-compiler/f18) with an aim to replace the previous flang project (https://github.com/flang-compiler/flang) 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](docs). Start with the [compiler overview](docs/Overview.md). To better understand Fortran as a language and the specific grammar accepted by flang, read [Fortran For C Programmers](docs/FortranForCProgrammers.md) and flang's specifications of the [Fortran grammar](docs/f2018-grammar.txt) and the [OpenMP grammar](docs/OpenMP-4.5-grammar.txt). Treatment of language extensions is covered in [this document](docs/Extensions.md). To understand the compilers handling of intrinsics, see the [discussion of intrinsics](docs/Intrinsics.md). To understand how a flang program communicates with libraries at runtime, see the discussion of [runtime descriptors](docs/RuntimeDescriptor.md). If you're interested in contributing to the compiler, read the [style guide](docs/C++style.md) and also review [how flang uses modern C++ features](docs/C++17.md). If you are interested in writing new documentation, follow [markdown style guide from LLVM](https://github.com/llvm/llvm-project/blob/master/llvm/docs/MarkdownQuickstartTemplate.md). ## 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 https://llvm.org/docs/GettingStarted.html. 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 the variable `LLVM_DIR` to find the installed LLVM components and the variable `MLIR_DIR` to find the installed MLIR components. To get the correct LLVM and MLIR libraries included in your flang build, define LLVM_DIR and MLIR_DIR on the cmake command line. ``` LLVM=/lib/cmake/llvm \ MLIR=/lib/cmake/mlir \ cmake -DLLVM_DIR=$LLVM -DMLIR_DIR=$MLIR ... ``` 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 ``` or ``` 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=` to the cmake command where `` 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 cmake -DLLVM_DIR=$LLVM -DMLIR_DIR=$MLIR ~/flang/src make ``` ### Build The New Flang Driver The new Flang driver, `flang-new`, is currently under active development and should be considered as an experimental feature. For this reason it is disabled by default. This will change once the new driver replaces the _throwaway_ driver, `flang`. In order to build the new driver, add `-DFLANG_BUILD_NEW_DRIVER=ON` to your CMake invocation line. Additionally, when building out-of-tree, use `CLANG_DIR` (similarly to `LLVM_DIR` and `MLIR_DIR`) to find the installed Clang components. **Note:** `CLANG_DIR` is only required when building the new Flang driver, which currently depends on Clang. # How to Run Tests Flang supports 2 different categories of tests 1. Regression tests (https://www.llvm.org/docs/TestingGuide.html#regression-tests) 2. Unit tests (https://www.llvm.org/docs/TestingGuide.html#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: ``` /llvm-lit \ --param flang_site_config=/test-lit/lit.site.cfg.py \ --param flang_config=/test-lit/lit.cfg.py \ ``` 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. /llvm-lit \ test/Unit 4. Invoking tests from /unittests/ ``` ## 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/FIRLangRef.md` in flang build directory. ## Generate Doxygen-based Documentation To generate doxygen-style documentation from source code - Pass `-DLLVM_ENABLE_DOXYGEN=ON -DFLANG_INCLUDE_DOCS=ON` to the cmake command. ``` cd ~/llvm-project/build cmake -DLLVM_ENABLE_DOXYGEN=ON -DFLANG_INCLUDE_DOCS=ON ../llvm make doxygen-flang ``` It will generate html in ``` /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](http://sphinx-doc.org/), including the [sphinx-markdown-tables](https://pypi.org/project/sphinx-markdown-tables/) extension. - Pass `-DLLVM_ENABLE_SPHINX=ON -DSPHINX_WARNINGS_AS_ERRORS=OFF` to the cmake command. ``` cd ~/llvm-project/build cmake -DLLVM_ENABLE_SPHINX=ON -DSPHINX_WARNINGS_AS_ERRORS=OFF ../llvm make docs-flang-html ``` It will generate html in ``` $BROWSER /tools/flang/docs/html/ ```