[flang] Detect circularly defined interfaces of procedures

It's possible to define a procedure whose interface depends on a procedure
which has an interface that depends on the original procedure.  Such a circular
definition was causing the compiler to fall into an infinite loop when
resolving the name of the second procedure.  It's also possible to create
circular dependency chains of more than two procedures.

I fixed this by adding the function HasCycle() to the class DeclarationVisitor
and calling it from DeclareProcEntity() to detect procedures with such
circularly defined interfaces.  I marked the associated symbols of such
procedures by calling SetError() on them.  When processing subsequent
procedures, I called HasError() before attempting to analyze their interfaces.
Unfortunately, this did not work.

With help from Tim, we determined that the SymbolSet used to track the
erroneous symbols was instantiated using a "<" operator which was
defined using the name of the procedure.  But the procedure name was
being changed by a call to ReplaceName() between the times that the
calls to SetError() and HasError() were made.  This caused HasError() to
incorrectly report that a symbol was not in the set of erroneous
symbols.  I fixed this by making SymbolSet be an ordered set, which does
not use the "<" operator.

I also added tests that will crash the compiler without this change.
And I fixed the formatting on an error message from a previous update.

Differential Revision: https://reviews.llvm.org/D97201

GitOrigin-RevId: 07de0846a5055015b55dc2b8faa2143f9902e549
4 files changed
tree: f7aabffd185331a2614067535ebf106d2c8e9b51
  1. .clang-format
  2. .clang-tidy
  3. .drone.star
  4. .gitignore
  5. CMakeLists.txt
  7. LICENSE.txt
  8. README.md
  9. cmake/
  10. docs/
  11. include/
  12. lib/
  13. module/
  14. runtime/
  15. test/
  16. tools/
  17. unittests/


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. 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 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=<LLVM_BUILD_DIR>/lib/cmake/llvm \
MLIR=<LLVM_BUILD_DIR>/lib/cmake/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


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
cmake -DLLVM_DIR=$LLVM -DMLIR_DIR=$MLIR ~/flang/src

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:

<path-to-llvm-lit>/llvm-lit \
 --param flang_site_config=<path-to-flang-build>/test-lit/lit.site.cfg.py \
 --param flang_config=<path-to-flang-build>/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. <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/FIRLangRef.md 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/