add LLVM_INCLUDE_LIB CMake option

[ilelann]

Summary

Including lib does not seem to be required when one want, example given, to build libcxx.
Same rationale as the others LLLM_INCLUDE options: it reduces the number of targets, and makes life inside IDE easier.

[ldionne]

Sorry, I'm not sufficiently familiar with LLVM's build system to review this. I work on the runtimes almost exclusively. @smeenai might be a better fit here.

[ldionne]

In D90025#2350097, @ldionne wrote:

Sorry, I'm not sufficiently familiar with LLVM's build system to review this. I work on the runtimes almost exclusively. @smeenai might be a better fit here.

Oh, I looked a bit fast. I think I understand the intent here -- you're trying to make it easier to build libcxx as part of the monorepo without the rest of LLVM?

I don't think that's the right way forward, I think we need to setup a separate build root for the runtimes.

[ilelann]

In D90025#2350101, @ldionne wrote:

Oh, I looked a bit fast. I think I understand the intent here -- you're trying to make it easier to build libcxx as part of the monorepo without the rest of LLVM?

That is the intent indeed.
Starting from the "official" build instructions found here https://libcxx.llvm.org/docs/BuildingLibcxx.html, I ultimately got this CMake incantation to get a minimal libunwind+libcxxabi+libcxx static build:
https://github.com/ilelann/IncludeOS/blob/master/CMakeLists.txt#L77-L130

And I am totally happy with it, despite the arguably long list of flags.
The additionnal LLVM_INCLUDE_LIB proposed here is only to avoid configuring and building a large set of irrelevant targets.
With my patch and LLVM_INCLUDE_LIB set to OFF, the "pollution" brought by including the main LLVM CMake file is really minimal: when I "make" my super-project I don't notice any irrelevant target.

[smeenai]

Yeah, in general, we've tried to move away from configure-time options for parts of the build like this (CC @beanz and @tstellar). The separate build root for the runtimes is the way to go here.

llvm/CMakeLists.txt

 option(LLVM_INCLUDE_GO_TESTS "Include the Go bindings tests in test build targets." ON)
 
 option(LLVM_BUILD_BENCHMARKS "Add LLVM benchmark targets to the list of default
 targets. If OFF, benchmarks still could be built using Benchmarks target." OFF)
 option(LLVM_INCLUDE_BENCHMARKS "Generate benchmark targets. If OFF, benchmarks can't be built." ON)
 
 option (LLVM_BUILD_DOCS "Build the llvm documentation." OFF)
 option (LLVM_INCLUDE_DOCS "Generate build targets for llvm documentation." ON)
+option (LLVM_INCLUDE_LIB "Generate build targets for llvm libraries." ON)
 option (LLVM_ENABLE_DOXYGEN "Use doxygen to generate llvm API documentation." OFF)
 option (LLVM_ENABLE_SPHINX "Use Sphinx to generate llvm documentation." OFF)
 option (LLVM_ENABLE_OCAMLDOC "Build OCaml bindings documentation." ON)
 option (LLVM_ENABLE_BINDINGS "Build bindings." ON)
 
 set(LLVM_INSTALL_DOXYGEN_HTML_DIR "share/doc/llvm/doxygen-html"
     CACHE STRING "Doxygen-generated HTML documentation install directory")
 set(LLVM_INSTALL_OCAMLDOC_HTML_DIR "share/doc/llvm/ocaml-html"

 include(TableGen)
 
 if( MINGW AND NOT "${CMAKE_CXX_COMPILER_ID}" MATCHES "Clang" )
   # People report that -O3 is unreliable on MinGW. The traditional
   # build also uses -O2 for that reason:
   llvm_replace_compiler_option(CMAKE_CXX_FLAGS_RELEASE "-O3" "-O2")
 endif()
 
+if( LLVM_INCLUDE_UTILS AND NOT LLVM_INCLUDE_LIB )
+    message(FATAL_ERROR "Including utils when not building lib will not work.
+    Either set LLVM_INCLUDE_LIB to On, or set LLVM_INCLUDE_UTILS to Off.")
+endif()
+
+if( LLVM_INCLUDE_LIB )
   # Put this before tblgen. Else we have a circular dependence.
   add_subdirectory(lib/Demangle)
   add_subdirectory(lib/Support)
   add_subdirectory(lib/TableGen)
 
+  if( LLVM_INCLUDE_UTILS )
     add_subdirectory(utils/TableGen)
+  endif()
 
   add_subdirectory(include/llvm)
 
   add_subdirectory(lib)
+endif()
 
 if( LLVM_INCLUDE_UTILS )
   add_subdirectory(utils/FileCheck)
   add_subdirectory(utils/PerfectShuffle)
   add_subdirectory(utils/count)
   add_subdirectory(utils/not)
   add_subdirectory(utils/yaml-bench)
 else()