[llvm-branch-commits] [clang] [flang] [lld] [llvm] [Flang] LLVM_ENABLE_RUNTIMES=flang-rt (PR #110217)

[Daniel Chen via llvm-branch-commits]

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+<!--===- README.md
+
+   Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+   See https://llvm.org/LICENSE.txt for license information.
+   SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+-->
+
+# Fortran Runtime (Flang-RT)
+
+Flang-RT is the runtime library for code emitted by the Flang compiler
+(https://flang.llvm.org).
+
+
+## Getting Started
+
+There are two build modes for the Flang-RT. The bootstrap build, also
+called the in-tree build, and the runtime-only build, also called the
+out-of-tree build.
+Not to be confused with the terms
+[in-source and out-of-source](https://cmake.org/cmake/help/latest/manual/cmake.1.html#introduction-to-cmake-buildsystems)
+builds as defined by CMake. In an in-source build, the source directory and the
+build directory are identical, whereas with an out-of-source build the
+build artifacts are stored somewhere else, possibly in a subdirectory of the
+source directory. LLVM does not support in-source builds.
+
+
+### Requirements
+
+Requirements:
+  * [Same as LLVM](https://llvm.org/docs/GettingStarted.html#requirements).
+
+
+### Bootstrap/In-Tree Build
+
+The bootstrap build will first build Clang and Flang, then use these compilers
+to compile Flang-RT. CMake will create a secondary build tree
+configured to use these just-built compilers. The secondary build will reuse
+the same build options (Flags, Debug/Release, ...) as the primary build.
+It will also ensure that once built, Flang-RT is found by Flang from either
+the build- or install-prefix. To enable, add `flang-rt` to
+`LLVM_ENABLE_RUNTIMES`:
+
+```bash
+cmake -S <path-to-llvm-project-source>/llvm \
+  -GNinja                                   \
+  -DLLVM_ENABLE_PROJECTS="clang;flang"      \
+  -DLLVM_ENABLE_RUNTIMES=flang-rt           \
+  ...
+```
+
+It is recommended to enable building OpenMP alongside Flang and Flang-RT
+as well. This will build `omp_lib.mod` required to use OpenMP from Fortran.
+Building Compiler-RT may also be required, particularly on platforms that do
+not provide all C-ABI functionality (such as Windows).
+
+```bash
+cmake -S <path-to-llvm-project-source>/llvm     \
+  -GNinja                                       \
+  -DCMAKE_BUILD_TYPE=Release                    \
+  -DLLVM_ENABLE_PROJECTS="clang;flang;openmp"   \
----------------
DanielCChen wrote:

Building `openmp` as `LLVM_ENABLE_PROJECTS` is not supported on AIX as we believe it is more appropriate to build `openmp` as `LLVM_ENABLE_RUNTIMES`.
May be it is better to use a command that works on all platforms.

https://github.com/llvm/llvm-project/pull/110217