Document LLVM_ENABLE_RUNTIMES

[MaskRay]

llvm/docs/CMake.rst

480

484

Worth mentioning that the list should not duplicate with LLVM_ENABLE_PROJECTS.

[beanz]

Probably also worth noting it builds the builtins separately from the other runtimes to preserve correct dependency ordering.

[sylvestre.ledru]

In D108890#2971290, @beanz wrote:

Probably also worth noting it builds the builtins separately from the other runtimes to preserve correct dependency ordering.

Not sure what you mean here? Do you have an example?

llvm/docs/CMake.rst

480

oups

[sylvestre.ledru]

Take in account the comments

[beanz]

In D108890#2971295, @sylvestre.ledru wrote:

Not sure what you mean here? Do you have an example?

The goal of the runtimes build initially was to preserve correct dependency ordering when building. You need the just-built clang to build the builtins libraries, and you need the builtins libraries before you can build any fully linked dynamic libraries or executables. So when you use the runtimes build the build ordering is clang -> compiler-rt builtins only -> everything else.

[jdoerfert]

In D108890#2971330, @beanz wrote:

In D108890#2971295, @sylvestre.ledru wrote:

Not sure what you mean here? Do you have an example?

The goal of the runtimes build initially was to preserve correct dependency ordering when building. You need the just-built clang to build the builtins libraries, and you need the builtins libraries before you can build any fully linked dynamic libraries or executables. So when you use the runtimes build the build ordering is clang -> compiler-rt builtins only -> everything else.

OpenMP also relies on the just build clang to create the (GPU) device runtimes. That is why our build with offloading support is based on the runtimes and not project.

[phosek]

llvm/docs/CMake.rst

487

LLVM libc is also supported, see https://github.com/llvm/llvm-project/blob/9f7873784dcce4177a759fc657b267d2f189bcf0/runtimes/CMakeLists.txt#L5

You can also use LLVM_ENABLE_RUNTIMES=all to build all supported runtimes.

[sylvestre.ledru]

Add libc to the list + document =all

[phosek]

LGTM

[MaskRay]

Looks great!

[tambre]

llvm/docs/CMake.rst

481

This sentence doesn't make much sense.

485

Probably worth wording more strongly and without it being a sidenote.

486

The full list of what? Not the defaults, so probably worth explicitly clarifying that these are possible values.

llvm/docs/CMake.rst

**LLVM_ENABLE_PROJECTS**:STRING
   (clang, libcxx, libcxxabi, lldb, compiler-rt, lld, polly, etc) projects.
   This flag assumes that projects are checked out side-by-side and not nested,
   i.e. clang needs to be in parallel of llvm instead of nested in `llvm/tools`.
   This feature allows to have one build for only LLVM and another for clang+llvm
   using the same source checkout.
   The full list is:
   ``clang;clang-tools-extra;compiler-rt;cross-project-tests;libc;libclc;libcxx;libcxxabi;libunwind;lld;lldb;openmp;parallel-libs;polly;pstl``
 
+**LLVM_ENABLE_RUNTIMES**:STRING
+  Build libc++, libc++abi or other projects using that a just-built compiler.
+  This is the correct way to build libc++ when putting together a toolchain.
+  It will build the builtins separately from the other runtimes to preserve
+  correct dependency ordering.
+  Note: the list should not have duplicates with `LLVM_ENABLE_PROJECTS`.
+  The full list is:
+  ``compiler-rt;libc;libcxx;libcxxabi;libunwind;openmp``
+  To enable all of them, use:
+  ``LLVM_ENABLE_RUNTIMES=all``
+
+
 **LLVM_ENABLE_RTTI**:BOOL
   Build LLVM with run-time type information. Defaults to OFF.
 
 **LLVM_ENABLE_SPHINX**:BOOL
   If specified, CMake will search for the ``sphinx-build`` executable and will make
   the ``SPHINX_OUTPUT_HTML`` and ``SPHINX_OUTPUT_MAN`` CMake options available.
   Defaults to OFF.