[CMake] Add custom target to create build directory

[Hahnfeld]

Summary

CMake does a poor job in tracking dependencies on files and directories
directly. Create custom target similar to the configuration step.
On my system, this avoids the reconfiguration on each build.

[Hahnfeld]

Gentle ping

[beanz]

LGTM. I keep meaning to spend time rewriting all this to use the LLVMExternalProjectUtils module which avoids issues like this, but I never really have the time to do it.

[Hahnfeld]

In D39298#918632, @beanz wrote:

LGTM. I keep meaning to spend time rewriting all this to use the LLVMExternalProjectUtils module which avoids issues like this, but I never really have the time to do it.

Sorry, by far no CMake expert. I'd volunteer otherwise to get that fixed correctly...

llvm/trunk/cmake/modules/CrossCompile.cmake

set(CROSS_TOOLCHAIN_FLAGS_${target_name}
         -DCMAKE_TOOLCHAIN_FILE=\"${LLVM_MAIN_SRC_DIR}/cmake/platforms/${toolchain}.cmake\"
         CACHE STRING "Toolchain file for ${target_name}")
   endif()
 
   add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}
     COMMAND ${CMAKE_COMMAND} -E make_directory ${LLVM_${target_name}_BUILD}
     COMMENT "Creating ${LLVM_${target_name}_BUILD}...")
 
+  add_custom_target(CREATE_LLVM_${target_name}
+                    DEPENDS ${LLVM_${target_name}_BUILD})
+
   add_custom_command(OUTPUT ${LLVM_${target_name}_BUILD}/CMakeCache.txt
     COMMAND ${CMAKE_COMMAND} -G "${CMAKE_GENERATOR}"
         ${CROSS_TOOLCHAIN_FLAGS_${target_name}} ${CMAKE_SOURCE_DIR}
         -DLLVM_TARGET_IS_CROSSCOMPILE_HOST=TRUE
     WORKING_DIRECTORY ${LLVM_${target_name}_BUILD}
-    DEPENDS ${LLVM_${target_name}_BUILD}
+    DEPENDS CREATE_LLVM_${target_name}
     COMMENT "Configuring ${target_name} LLVM...")
 
   add_custom_target(CONFIGURE_LLVM_${target_name}
                     DEPENDS ${LLVM_${target_name}_BUILD}/CMakeCache.txt)
 
   set_directory_properties(PROPERTIES ADDITIONAL_MAKE_CLEAN_FILES
                                       ${LLVM_${target_name}_BUILD})