[compiler-rt] Add option to disable libc++ build

[smeenai]

Summary

Having libc++ checked out doesn't necessarily mean it should be built;
for example, the same source tree might be used for multiple build
configurations, and libc++ might not build in some of those
configurations. Add an option to compiler-rt's build to disable building
libc++. This defaults to ON, so it shouldn't change any existing build
configurations.

[mgorny]

Could you also add a local switch to control using it independently of main build? The sanitized builds are quite time-consuming.

[smeenai]

In D56479#1350922, @mgorny wrote:

Could you also add a local switch to control using it independently of main build? The sanitized builds are quite time-consuming.

As in, add a compiler-rt specific option to disable using libc++? Maybe something like COMPILER_RT_DISABLE_LIBCXX (or its positive version)? I have no opinions on that (I don't really work on compiler-rt), although I'm wondering if that flag should be in addition to the LLVM_TOOL_LIBCXX_BUILD check or just replace it.

[mgorny]

I think it'd be better to have a positive version, e.g. something like COMPILER_RT_USE_LIBCXX_SOURCES. Maybe it could default to ${LLVM_TOOL_LIBCXX_BUILD}.

[smeenai]

Sounds good ... I'll update this patch.

[smeenai]

@mgorny suggestion

[phosek]

This also needs to support the runtimes build which defines HAVE_LIBCXX variable when libc++ is being built.

Where's LLVM_TOOL_LIBCXX_BUILD handled? Searching through the tree, I haven't found any references to LLVM_TOOL_LIBCXX_BUILD, I have found a few references to other LLVM_TOOL_${TOOL} variables in llvm/tools but I haven't found any logic that's consuming them aside from llvm/test/CMakeList.txt and polly/test/CMakeList.txt.

[smeenai]

In D56479#1352333, @phosek wrote:

This also needs to support the runtimes build which defines HAVE_LIBCXX variable when libc++ is being built.

Where's LLVM_TOOL_LIBCXX_BUILD handled? Searching through the tree, I haven't found any references to LLVM_TOOL_LIBCXX_BUILD, I have found a few references to other LLVM_TOOL_${TOOL} variables in llvm/tools but I haven't found any logic that's consuming them aside from llvm/test/CMakeList.txt and polly/test/CMakeList.txt.

The LLVM_TOOL_*_BUILD variables are checked by add_llvm_subdirectory (which is called by add_llvm_external_project with project being LLVM and type being TOOL, which is where the LLVM_TOOL_ prefix comes from). They'll default to on for anything that lives inside llvm/tools or llvm/projects, and anything that's enabled via LLVM_ENABLE_PROJECTS or LLVM_EXTERNAL_PROJECTS.

[smeenai]

Take HAVE_LIBCXX into account

[smeenai]

I updated to use HAVE_LIBCXX as well, and confirmed that the following scenarios work:

• Having both compiler-rt and libc++ checked out under llvm/projects
• Using -DLLVM_ENABLE_PROJECTS=compiler-rt;libcxx (the order of projects doesn't matter)
• Using -DLLVM_ENABLE_RUNTIMES=compiler-rt;libcxx (the order of runtimes doesn't matter)

What doesn't work is having libc++ under llvm/projects and compiler-rt under llvm/runtimes (or correspondingly, having libc++ in LLVM_ENABLE_PROJECTS and compiler-rt in LLVM_ENABLE_RUNTIMES). I can fix that by passing through the LLVM_TOOL_*_BUILD variables to the sub-configures, but I'm not sure that's a build setup anyone would use?

[smeenai]

The other option would be to just have COMPILER_RT_USE_LIBCXX always default to ON and require users to set it to OFF explicitly if they disable the build. That would require setting an additional cache variable during configuration instead of it being inferred, but it removes the possibility of breaking any existing setups. What do you think?

[smeenai]

Default to ON

[smeenai]

Okay, I decided to make it default ON. I think this should be a pretty safe and uncontroversial change at this point?

[phosek]

In D56479#1356671, @smeenai wrote:

Okay, I decided to make it default ON. I think this should be a pretty safe and uncontroversial change at this point?

This is fine with me, although I wouldn't be concerned about mixing llvm/projects and compiler-rt under llvm/runtimes either since that setup is untested and most likely broken anyway.

compiler-rt/CMakeLists.txt

if (CMAKE_LINKER MATCHES "link.exe$")
   # it, but CMake doesn't seem to have a way to set linker flags for
   # individual static libraries, so we enable the suppression flag for
   # the whole compiler-rt project.
   set(CMAKE_STATIC_LINKER_FLAGS "${CMAKE_STATIC_LINKER_FLAGS} /IGNORE:4221")
 endif()
 
 add_subdirectory(include)
 
+option(COMPILER_RT_USE_LIBCXX
+  "Enable compiler-rt to use libc++ from the source tree" ${LLVM_TOOL_LIBCXX_BUILD})
+if(COMPILER_RT_USE_LIBCXX)
   foreach(path IN ITEMS ${LLVM_MAIN_SRC_DIR}/projects/libcxx
                         ${LLVM_MAIN_SRC_DIR}/runtimes/libcxx
                         ${LLVM_MAIN_SRC_DIR}/../libcxx
                         ${LLVM_EXTERNAL_LIBCXX_SOURCE_DIR})
     if(IS_DIRECTORY ${path})
       set(COMPILER_RT_LIBCXX_PATH ${path})
       break()
     endif()
   endforeach()
+endif()
 
 set(COMPILER_RT_LLD_PATH ${LLVM_MAIN_SRC_DIR}/tools/lld)
 if(EXISTS ${COMPILER_RT_LLD_PATH}/ AND LLVM_TOOL_LLD_BUILD)
   set(COMPILER_RT_HAS_LLD TRUE)
 else()
   set(COMPILER_RT_LLD_PATH ${LLVM_MAIN_SRC_DIR}/../lld)
   if(EXISTS ${COMPILER_RT_LLD_PATH}/ AND LLVM_TOOL_LLD_BUILD)
     set(COMPILER_RT_HAS_LLD TRUE)