llvm-project/flang/README.md

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# 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](https://github.com/flang-compiler/flang/wiki)
for more information about Flang.

## Getting Started

Read more about f18 in the [documentation directory](documentation).
Start with the [compiler overview](documentation/Overview.md).

To better understand Fortran as a language
and the specific grammar accepted by f18,
read [Fortran For C Programmers](documentation/FortranForCProgrammers.md)
and
f18's specifications of the [Fortran grammar](documentation/f2018-grammar.txt)
and
the [OpenMP grammar](documentation/OpenMP-4.5-grammar.txt).

Treatment of language extensions is covered
in [this document](documentation/Extensions.md).

To understand the compilers handling of intrinsics,
see the [discussion of intrinsics](documentation/Intrinsics.md).

To understand how an f18 program communicates with libraries at runtime,
see the discussion of [runtime descriptors](documentation/RuntimeDescriptor.md).

If you're interested in contributing to the compiler,
read the [style guide](documentation/C++style.md)
and
also review [how f18 uses modern C++ features](documentation/C++17.md).

## 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_INSTALLATION_DIR>/lib/cmake/llvm cmake -DLLVM_DIR=$LLVM ...
```
where `LLVM_INSTALLATION_DIR` is
the top-level directory
where llvm is installed.

### 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
```