Align LLVM_EXTERNALIZE_DEBUGINFO behaviours on Linux w/ Darwin

[directhex]

Summary

Despite what it says in the documentation[1], LLVM_EXTERNALIZE_DEBUGINFO is not only for Darwin, and has been functional on Linux since 2017[2].

However, It is hardcoded to emit ${name}.debug in the same directory, whereas macOS allows overriding both the extension and the output dir.

This commit adds support for LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION and LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR on Linux.

[1] https://www.llvm.org/docs/CMake.html#llvm-related-variables
[2] https://reviews.llvm.org/D28575

[directhex]

This fails when cross-compiling, and needs changes.

[directhex]

Hm, the cross compiler case can be handled by passing a path to an arch-appropriate objcopy/strip binary with CMAKE_OBJCOPY/CMAKE_STRIP, i'm not sure it's worth trying to do some kind of magical auto-handling in lieu of letting the user provide the correct binaries.

llvm/cmake/modules/AddLLVM.cmake

       ${strip_command}
       )
   else()
+    if(LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION)
+      set(file_ext ${LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION})
+    else()
+      set(file_ext debug)
+    endif()
+
+    set(output_name "$<TARGET_FILE_NAME:${name}>.${file_ext}")
+
+    if(LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR)
+      set(output_path "${LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR}/${output_name}")
+      # If an output dir is specified, it must be manually mkdir'd on Linux,
+      # as that directory needs to exist before we can pipe to a file in it.
+      add_custom_command(TARGET ${name} POST_BUILD
+        COMMAND ${CMAKE_COMMAND} -E make_directory ${LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR}
+        )
+    else()
+      set(output_path "${output_name}")
+    endif()
+
     add_custom_command(TARGET ${name} POST_BUILD
-      COMMAND ${CMAKE_OBJCOPY} --only-keep-debug $<TARGET_FILE:${name}> $<TARGET_FILE:${name}>.debug
+      COMMAND ${CMAKE_OBJCOPY} --only-keep-debug $<TARGET_FILE:${name}> ${output_path}
       ${strip_command} -R .gnu_debuglink
-      COMMAND ${CMAKE_OBJCOPY} --add-gnu-debuglink=$<TARGET_FILE:${name}>.debug $<TARGET_FILE:${name}>
+      COMMAND ${CMAKE_OBJCOPY} --add-gnu-debuglink=${output_path} $<TARGET_FILE:${name}>
       )
   endif()
 endfunction()