[CMake] Allow custom extensions for externalized debug info

[sgraenitz]

Summary

Extra flexibility for emitting debug info to external files (remains Darwin only for now).
LLDB needs this functionality to emit a LLDB.framework.dSYM instead of LLDB.dSYM when building the framework, because the latter could conflict with the driver's lldb.dSYM when emitted in the same directory on case-insensitive file systems.

[sgraenitz]

Enables https://reviews.llvm.org/D60863

[friss]

LGTM

[friss]

Actually, thinking more about this, how do you use it? LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION is a global but this would be different per target.

[sgraenitz]

In D60862#1471755, @friss wrote:

Actually, thinking more about this, how do you use it? LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION is a global but this would be different per target.

It can be set in the scope of the add_lldb_library call as in the matching LLDB patch https://reviews.llvm.org/D60863

[friss]

Interesting. Cmake scoping rules are still a mystery to me. So when you `set(FOO ...), it's set only for the current scope and in any functions/macros you call in that scope? Then this seems like an ok solution.

[sgraenitz]

Yes, scopes are per directory; macros operate on the caller's scope and functions add one.

llvm/trunk/cmake/modules/AddLLVM.cmake

if(APPLE)
         set(CMAKE_STRIP xcrun strip)
       endif()
       set(strip_command COMMAND ${CMAKE_STRIP} -Sxl $<TARGET_FILE:${name}>)
     else()
       set(strip_command COMMAND ${CMAKE_STRIP} -g -x $<TARGET_FILE:${name}>)
     endif()
   endif()
 
-  if(LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR)
-    if(APPLE)
-      set(output_name "$<TARGET_FILE_NAME:${name}>.dSYM")
-      set(output_path "-o=${LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR}/${output_name}")
+    if(LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION)
+      set(file_ext ${LLVM_EXTERNALIZE_DEBUGINFO_EXTENSION})
+    else()
+      set(file_ext dSYM)
     endif()
+
+    set(output_name "$<TARGET_FILE_NAME:${name}>.${file_ext}")
+
+    if(LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR)
+      set(output_path "-o=${LLVM_EXTERNALIZE_DEBUGINFO_OUTPUT_DIR}/${output_name}")
+    else()
+      set(output_path "-o=${output_name}")
     endif()
 
-  if(APPLE)
     if(CMAKE_CXX_FLAGS MATCHES "-flto"
       OR CMAKE_CXX_FLAGS_${uppercase_CMAKE_BUILD_TYPE} MATCHES "-flto")
 
       set(lto_object ${CMAKE_CURRENT_BINARY_DIR}/${CMAKE_CFG_INTDIR}/${name}-lto.o)
       set_property(TARGET ${name} APPEND_STRING PROPERTY
         LINK_FLAGS " -Wl,-object_path_lto,${lto_object}")
     endif()
     if(NOT CMAKE_DSYMUTIL)