[PATCH] D141050: [standalone-build] outsource, simplify and unify repetitive CMake code

[Konrad Wilhelm Kleine via Phabricator via cfe-commits]

kwk created this revision.
kwk added reviewers: tstellar, phosek.
Herald added subscribers: bzcheeseman, rriddle.
Herald added a project: All.
kwk requested review of this revision.
Herald added subscribers: cfe-commits, stephenneuendorffer.
Herald added a project: clang.

Rationale
=========

This is an opinionated change for the standalone build mode CMake code.

The rationale behind this change is to unify the parts of standalone
builds that historically had to be kept in each and every project. With
the advent of the top-level `cmake` directory inside the LLVM source
tree, it is now possible to bundle the CMake instructions into a file,
aka `cmake/Modules/StandaloneBuildHelpers.cmake`, and include that file
in each project that wants to build in standalone-mode.

Historically the standalone build mode is used mostly by Linux
distributions. Certainly not every LLVM contributor cares about Linux
distributions. To reduce the frictions it makes even more sense to have
a unified place where to keep the specialities of building in standalone
mode.

Affected projects (so far)
--------------------------

This change brings the unified standalone build mode to the clang and
lld project.

Assumptions
===========

One radical assumption for this change is that in order to build clang
or ldd in standalone mode, you have to first build the `llvm` subproject
and *install* it into any location. You can assist the build process to
find LLVM using `find_package(LLVM)` by specifying
`-DCMAKE_PREFIX_PATH=${LLVM_INSTALL_DIR}/lib/cmake/llvm` in the cmake
configuration process.
You have to build the `llvm subproject with utilies included and
installed
(`-DLLVM_INCLUDE_UTILS:BOOL=ON` and `-DLLVM_INSTALL_UTILS:BOOL=ON`. But
I'm sure that this is done most of the time anyways, no?

Don't build as you go: No more cross-project dependencies on LLVM utilties
--------------------------------------------------------------------------

Another assumption is that in standalone build mode it makes no sense to
build clang and try to build an LLVM utility binary like `FileCheck` if
that is missing. This only adds noise to the cmake files and creates an
indirect dependency on the LLVM utilities directory which doesn't exist
in the the clang source tarball. Therefore we go and search for

Don't silently turn off tests
-----------------------------

Before this change, we would silently turn off tests if a binary like
`FileCheck`, `count` or `not` was missing. This is not only dangerous
but IMHO not helpful. If someone asks for tests by passing
`-DLLVM_INCLUDE_TESTS=On` we should error out at configure time because
we cannot fulfil this request when a binary is missing. This is exactly
what this tests does. If you want to check if an LLVM utility binary
exists and what the path to it is, you can call
`get_llvm_utility_binary_path("FileCheck" "FileCheck_EXE")` and it will
get the import location for the target, aka the path to the binary that
was found when we did `find_package(LLVM)`.

NOTE: You can take a look at this small example project which shows you
how importing of an installed project works:
https://github.com/kwk/cmake-export-binary-example/blob/2b429fccef97eb93c1717ceddb27bbe0022339d2/project-b/CMakeLists.txt#L7-L10



Require external LIT in standalone mode
---------------------------------------

We also think that in standalone mode you always want to use an external
lit and not build it as you go. That's why the `find_external_lit` macro
checks if `LLVM_EXTERNAL_LIT` is set and the path exists. If one of
these conditions doesn't hold true, we error out.

TODO:

( ) make sure the correct binaries of `FileCheck` and `count` and `not`
get substituted in lit test files.
( ) get feedback on this change or just opinions
( ) extend usage to other projects like `mlir`, `libomp` and so on.
( ) more encapsulation in `cmake/Modules/StandaloneBuildHelpers.cmake`


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D141050

Files:
  clang/CMakeLists.txt
  cmake/Modules/StandaloneBuildHelpers.cmake
  lld/CMakeLists.txt

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