[llvm-dev] [RFC] [CMake] Removing support for LLVM_TOOL_ CMake cache variables (original) (raw)

Dan Liew via llvm-dev llvm-dev at lists.llvm.org
Tue Feb 5 01:59:13 PST 2019

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Hi,

In our CMake build system there are currently two ways of specifying which LLVM sub projects to build by setting CMake cache variables.

• Setting LLVM_ENABLE_PROJECTS to the list of projects to enable (e.g. -DLLVM_ENABLE_PROJECTS=clang;compiler-rt)
• Setting LLVM_TOOL_<PROJECT>_BUILD boolean CMake cache variables (e.g. -DLLVM_TOOL_CLANG_BUILD=ON -DLLVM_TOOL_COMPILER_RT_BUILD=ON)

Having two different ways of specifying the same thing is problematic because from the CMake perspective because we can't detect which way the user actually wants to use.

Since r353148 if LLVM_ENABLE_PROJECTS is set by the user then that is used to determine which projects are built and any user specified value for the LLVM_TOOL_<PROJECT>_BUILD variables get overridden. LLVM_ENABLE_PROJECTS currently only works with the new mono-repo layout (projects outside of the LLVM source tree) which basically means that:

• LLVM_ENABLE_PROJECTS is used for the mono repo project layout
• LLVM_TOOL_<PROJECT>_BUILD is used for the traditional in-tree project layout (e.g. projects located at tools/clang, tools/lldb, projects/compiler-rt).

This is a bit of a mess and I'd like to propose we switch to only using LLVM_ENABLE_PROJECTS and remove support for LLVM_TOOL_<PROJECT>_BUILD variables.

I see two ways of doing this:

• Graceful. We'll eventually move everyone over to the mono repo layout anyway so just drop support for LLVM_TOOL_<PROJECT>_BUILD variables as part of the process of removing support for the in-tree subprojects inside the LLVM source tree. We just need to document this change clearly.

• Aggressive. Remove support for setting LLVM_TOOL_<PROJECT>_BUILD variables and only use LLVM_ENABLE_PROJECTS. The logic for LLVM_ENABLE_PROJECTS would need to be changed to work with in-tree subprojects.

I'd prefer Graceful because it's less work and I actually have old scripts that rely on setting LLVM_TOOL_<PROJECT>_BUILD variables. Others might too.

Thoughts?

Thanks, Dan.

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