Optionally install clang-tblgen to aid cross-compiling

[chewi]

Summary

clang-tblgen is required to cross-compile clang itself. Unlike before, most of the infrastructure is in place to do this now, and the only thing preventing it is LLVM_BUILD_UTILS, which doesn't apply to Clang.

I thought about changing this to ${project}_BUILD_UTILS and adding a CLANG_BUILD_UTILS option, but there seems little point for just one tool.

Instead, it checks whether clang-tblgen was explicitly requested in LLVM_DISTRIBUTION_COMPONENTS, which is good enough for Gentoo and other distributions.

Closes: https://github.com/llvm/llvm-project/issues/20282

[chewi]

Apologies. I conceived this against 15.0.6, but have now noticed that the latest TableGen.cmake may have changed things. I'll retest tomorrow and adjust as necessary.

[chewi]

Reworked and retested against the main branch. Thankfully, it is simpler now.

[chewi]

I forgot to add: I notice you no longer need llvm-tblgen already present when cross-compiling LLVM, but sadly, that still isn't the case for clang-tblgen. I tried leveraging CrossCompile.cmake to perform a "NATIVE" Clang build, but I think a lot more work would be needed to do it this way.

[nikic]

LGTM

[chewi]

I don't have commit access. Please merge this for me.

[nikic]

Can you please share the Name <email> to use for the commit?

[chewi]

James Le Cuirot <[e-mail]>

I'm new to this process, so thanks for bearing with me.

llvm/cmake/modules/TableGen.cmake

if( ${${project}_TABLEGEN} STREQUAL "${target}" )
       # If we're using the host tablegen, and utils were not requested, we have no
       # need to build this tablegen.
       if ( NOT LLVM_BUILD_UTILS )
         set_target_properties(${target} PROPERTIES EXCLUDE_FROM_ALL ON)
       endif()
     endif()
   endif()
 
-  if (ADD_TABLEGEN_DESTINATION AND NOT LLVM_INSTALL_TOOLCHAIN_ONLY AND LLVM_BUILD_UTILS)
+  if (ADD_TABLEGEN_DESTINATION AND NOT LLVM_INSTALL_TOOLCHAIN_ONLY AND
+      (LLVM_BUILD_UTILS OR ${target} IN_LIST LLVM_DISTRIBUTION_COMPONENTS))
     set(export_arg)
     if(ADD_TABLEGEN_EXPORT)
       get_target_export_arg(${target} ${ADD_TABLEGEN_EXPORT} export_arg)
     endif()
     install(TARGETS ${target}
             ${export_arg}
             COMPONENT ${target}
             RUNTIME DESTINATION "${ADD_TABLEGEN_DESTINATION}")