[cfe-dev] Separating clang-tools-extra from clang in LLVM_ENABLE_PROJECTS

[Mehdi AMINI via cfe-dev]

On Wed, Feb 13, 2019 at 3:15 PM Reid Kleckner <rnk at google.com> wrote:

> The context is https://reviews.llvm.org/D58157
>
> Back when Mehdi originally introduced the LLVM_ENABLE_PROJECTS cmake
> variable, people were discussing the idea of folding clang-tools-extra into
> clang, if we moved to multiple git repos away from a monorepo. To support
> that goal, this code was added:
> https://github.com/llvm/llvm-project/blob/master/llvm/CMakeLists.txt#L144
>       # There is a widely spread opinion that clang-tools-extra should be
> merged
>       # into clang. The following simulates it by always enabling
> clang-tools-extra
>       # when enabling clang.
>       if (proj STREQUAL "clang")
>         set(LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR
> "${CMAKE_CURRENT_SOURCE_DIR}/../clang-tools-extra")
>       endif()
>
> This makes it so that if you're building clang, you're building
> clang-tidy, clangd, etc. However, we have use cases where we want to check
> out the monorepo and just build clang, so we wanted to remove that block.
>

Just to mention for completeness that the use case you mention can be
supported also by:
1) building the clang target specifically: `ninja clang`
2) the same way we support building without llc/opt/... by setting
-DLLVM_BUILD_TOOLS=OFF : there could be a -DCLANG_BUILD_TOOLS=OFF ; having
clang-tools-extra be "part of" clang does not *forces* to build them. There
are good question to ask about what the *default* should be when
configuring and building clang/llvm.

Having clang-tools-extra considered part of clang is something that can be
evaluated separately (and I have no idea what makes sense today on this
topic) than the build targets (the same way that people were afraid at the
time that "monorepo" meant that we would always build everything, organize
the source and the build components are somehow orthogonal concerns).

Best,

-- 
Mehdi



> The upshot is that if you use LLVM_ENABLE_PROJECTS today and you want to
> build clang tools, you will have to add "clang-tools-extra" to your CMake
> invocation after D58157 lands. We've identified the one upstream buildbot
> that uses this variable and plan to update it, but if you have downstream
> bots using this variable, you may also need to add "clang-tools-extra" to
> build clang-tidy & co.
>
> We have some consensus that this is the direction we want to go in the
> code review from Sam Mccall, Shoaib, myself, and Nico, but please let us
> know if you think this is the wrong direction.
>
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