[Flang][Docs] Add a GettingStarted.md for build instructions

This patch fix first point of llvm#60730.
https://flang.llvm.org/docs/ has no build instructions. This doc page is just
a copy of README page so that it can be accessible from website.

Differential Revision: https://reviews.llvm.org/D148070

Author: xgupta

flang/README.md

@@ -38,338 +38,5 @@ also review [how flang uses modern C++ features](docs/C++17.md).
 If you are interested in writing new documentation, follow
 [LLVM's Markdown style guide](https://github.com/llvm/llvm-project/blob/main/llvm/docs/MarkdownQuickstartTemplate.md).
 
-## Building flang
-There are two ways to build flang. The first method is to build it at the same
-time that you build all of the projects on which it depends. This is called
-building in tree. The second method is to first do an in tree build to create
-all of the projects on which flang depends.  Then, after creating this base
-build, only build the flang code itself. This is called building standalone.
-Building standalone has the advantage of being smaller and faster. Once you
-create the base build and base install areas, you can create multiple
-standalone builds using them.
-
-Note that instructions for building LLVM can be found at
-https://llvm.org/docs/GettingStarted.html.
-
-All of the examples below use GCC as the C/C++ compilers and ninja as the build
-tool.
-
-### Building flang in tree
-Building flang in tree means building flang along with all of the projects on
-which it depends.  These projects include mlir, clang, flang, openmp, and
-compiler-rt.  Note that compiler-rt is only needed to access libraries that
-support 16 bit floating point numbers.  It's not needed to run the automated
-tests.  You can use several different C++ compilers for most of the build,
-includig GNU and clang.  But building compiler-rt requres using the clang
-compiler built in the initial part of the build.
-
-Here's a directory structure that works.  Create a root directory for the
-cloned and built files.  Under that root directory, clone the source code
-into a directory called llvm-project.  The build will also
-create subdirectories under the root directory called build (holds most of
-the built files), install (holds the installed files, and compiler-rt (holds
-the result of building compiler-rt).
-
-Here's a complete set of commands to clone all of the necessary source and do
-the build.
-
-First, create the root directory and `cd` into it.
-```bash
-mkdir root
-cd root
-
-Now clone the source:
-```bash
-git clone https://github.com/llvm/llvm-project.git
-```
-Once the clone is complete, execute the following commands:
-```bash
-rm -rf build
-mkdir build
-rm -rf install
-mkdir install
-ROOTDIR=`pwd`
-INSTALLDIR=$ROOTDIR/install
-
-cd build
-
-cmake \
-  -G Ninja \
-  -DCMAKE_BUILD_TYPE=Release \
-  -DCMAKE_INSTALL_PREFIX=$INSTALLDIR \
-  -DCMAKE_CXX_STANDARD=17 \
-  -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
-  -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,$LD_LIBRARY_PATH" \
-  -DFLANG_ENABLE_WERROR=ON \
-  -DLLVM_ENABLE_ASSERTIONS=ON \
-  -DLLVM_TARGETS_TO_BUILD=host \
-  -DLLVM_LIT_ARGS=-v \
-  -DLLVM_ENABLE_PROJECTS="clang;mlir;flang;openmp" \
-  -DLLVM_ENABLE_RUNTIMES="compiler-rt" \
-  ../llvm-project/llvm
-
-ninja
-```
-
-By default flang tests that do not specify an explicit `--target` flag use
-LLVM's default target triple. For these tests, if there is a need to test on a
-different triple by overriding the default, the following needs to be added to
-the cmake command above:
-`-DLLVM_TARGET_TRIPLE_ENV="<some string>" -DFLANG_TEST_TARGET_TRIPLE="<your triple>"`.
-
-To run the flang tests on this build, execute the command in the "build"
-directory:
-```bash
-ninja check-flang
-```
-
-To create the installed files:
-```bash
-ninja install
-
-echo "latest" > $INSTALLDIR/bin/versionrc
-```
-
-To build compiler-rt:
-```bash
-cd $ROOTDIR
-rm -rf compiler-rt
-mkdir compiler-rt
-cd compiler-rt
-CC=$INSTALLDIR/bin/clang \
-CXX=$INSTALLDIR/bin/clang++ \
-cmake \
-  -G Ninja \
-  ../llvm-project/compiler-rt \
-  -DCMAKE_BUILD_TYPE=Release \
-  -DCMAKE_INSTALL_PREFIX=$INSTALLDIR \
-  -DCMAKE_CXX_STANDARD=11 \
-  -DCMAKE_C_CFLAGS=-mlong-double-128 \
-  -DCMAKE_CXX_CFLAGS=-mlong-double-128 \
-  -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
-  -DCOMPILER_RT_BUILD_ORC=OFF \
-  -DCOMPILER_RT_BUILD_XRAY=OFF \
-  -DCOMPILER_RT_BUILD_MEMPROF=OFF \
-  -DCOMPILER_RT_BUILD_LIBFUZZER=OFF \
-  -DCOMPILER_RT_BUILD_SANITIZERS=OFF \
-  -DLLVM_CONFIG_PATH=$INSTALLDIR/bin/llvm-config
-
-ninja
-ninja install
-```
-
-Note that these instructions specify flang as one of the projects to build in
-the in tree build.  This is not strictly necessary for subsequent standalone
-builds, but doing so lets you run the flang tests to verify that the source
-code is in good shape.
-### Building flang standalone
-To do the standalone build, start by building flang in tree as described above.
-This build is base build for subsequent standalone builds.  Start each
-standalone build the same way by cloning the source for llvm-project:
-```bash
-mkdir standalone
-cd standalone
-git clone https://github.com/llvm/llvm-project.git
-```
-Once the clone is complete, execute the following commands:
-```bash
-cd llvm-project/flang
-rm -rf build
-mkdir build
-cd build
-
-cmake \
-  -G Ninja \
-  -DCMAKE_BUILD_TYPE=Release \
-  -DCMAKE_CXX_STANDARD=17 \
-  -DCMAKE_CXX_LINK_FLAGS="-Wl,-rpath,$LD_LIBRARY_PATH" \
-  -DCMAKE_EXPORT_COMPILE_COMMANDS=ON \
-  -DFLANG_ENABLE_WERROR=ON \
-  -DLLVM_TARGETS_TO_BUILD=host \
-  -DLLVM_ENABLE_ASSERTIONS=ON \
-  -DLLVM_BUILD_MAIN_SRC_DIR=$ROOTDIR/build/lib/cmake/llvm \
-  -DLLVM_EXTERNAL_LIT=$ROOTDIR/build/bin/llvm-lit \
-  -DLLVM_LIT_ARGS=-v \
-  -DLLVM_DIR=$ROOTDIR/build/lib/cmake/llvm \
-  -DCLANG_DIR=$ROOTDIR/build/lib/cmake/clang \
-  -DMLIR_DIR=$ROOTDIR/build/lib/cmake/mlir \
-  ..
-
-ninja
-```
-
-To run the flang tests on this build, execute the command in the "flang/build"
-directory:
-```bash
-ninja check-flang
-```
-
-## 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.
-
-Note that flang is not supported on 32 bit CPUs.
-
-### 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.
-
-```bash
-export CXX=g++-8.3
-```
-or
-```bash
-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.
-```bash
-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.
-
-# 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 standalone builds
-To run all tests:
-```bash
-cd ~/flang/build
-cmake -DLLVM_DIR=$LLVM -DMLIR_DIR=$MLIR ~/flang/src
-ninja 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:
-```bash
-<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 \
-  <path-to-fortran-test>
-
-```
-
-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 standalone build as it does not include googletest related headers and libraries.
-
-There are various ways to run unit-tests.
-
-```
-
-1. ninja check-flang-unit
-2. ninja check-all or ninja check-flang
-3. <path-to-llvm-lit>/llvm-lit \
-        test/Unit
-4. Invoking tests from <standalone 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:
-```bash
-ninja check-flang-unit
-```
-To run all of the flang regression tests use the `check-flang` target:
-```bash
-ninja 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 `ninja flang-doc`. This will
-create `<build-dir>/tools/flang/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.
-
-```bash
-cd ~/llvm-project/build
-cmake -DLLVM_ENABLE_DOXYGEN=ON -DFLANG_INCLUDE_DOCS=ON ../llvm
-ninja doxygen-flang
-```
-
-It will generate html in
-
-```bash
-    <build-dir>/tools/flang/docs/doxygen/html # for flang docs
-```
-## Generate Sphinx-based Documentation
-<!TODO: Add webpage once we have a website.
-!>
-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.
-
-```bash
-cd ~/llvm-project/build
-cmake -DLLVM_ENABLE_SPHINX=ON -DSPHINX_WARNINGS_AS_ERRORS=OFF ../llvm
-ninja docs-flang-html
-```
-
-It will generate html in
-
-```bash
-   $BROWSER <build-dir>/tools/flang/docs/html/
-```
+Consult the [Getting Started with Flang](docs/GettingStarted.md)
+for information on building and running flang.