[CMake] Provide an option to use relative paths in debug info

[phosek]

Summary

CMake always uses absolute file paths in the generated compiler
invocation which results in absolute file paths being embedded in debug
info. This is undesirable when building a toolchain e.g. on bots as the
debug info may embed the bot source checkout path which is meaningless
anywhere else.

This change introduces the LLVM_USE_RELATIVE_PATHS which uses
-fdebug-prefix-map (where supported) options to rewrite paths embedded
into debug info with relative ones. Additionally, LLVM_SOURCE_PREFIX can
be used to override the path to source directory with a different one.

[aprantl]

-DLLVM_USE_RELATIVE_PATHS is very generic sounding and might give a false impression. Is there a name that emphasizes that this affects the debug info of the built compiler?

[phosek]

In D62622#1522247, @aprantl wrote:

-DLLVM_USE_RELATIVE_PATHS is very generic sounding and might give a false impression. Is there a name that emphasizes that this affects the debug info of the built compiler?

How about LLVM_USE_RELATIVE_PATHS_IN_DEBUG_INFO or LLVM_ENABLE_RELOCATABLE_DEBUG_INFO?

[aprantl]

I like LLVM_USE_RELATIVE_PATHS_IN_DEBUG_INFO.

[phosek]

In D62622#1523269, @aprantl wrote:

I like LLVM_USE_RELATIVE_PATHS_IN_DEBUG_INFO.

Done

clang/cmake/caches/Fuchsia-stage2.cmake

 endif()
 set(LLVM_ENABLE_LTO ON CACHE BOOL "")
 set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ON CACHE BOOL "")
 set(LLVM_ENABLE_TERMINFO OFF CACHE BOOL "")
 set(LLVM_ENABLE_ZLIB ON CACHE BOOL "")
 set(LLVM_EXTERNALIZE_DEBUGINFO ON CACHE BOOL "")
 set(LLVM_INCLUDE_EXAMPLES OFF CACHE BOOL "")
 set(LLVM_INCLUDE_DOCS OFF CACHE BOOL "")
+set(LLVM_USE_RELATIVE_PATHS ON CACHE BOOL "")
 
 set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
 if(NOT APPLE)
   set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
   set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
 endif()
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
 set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")

llvm/cmake/modules/HandleLLVMOptions.cmake

elseif( NOT DEFINED LLVM_ENABLE_SUPPORT_XCODE_SIGNPOSTS )
       # Treat LLVM_ENABLE_SUPPORT_XCODE_SIGNPOSTS like "FORCE_OFF" when it has not been
       # defined.
     else()
       message(FATAL_ERROR "Unknown value for LLVM_ENABLE_SUPPORT_XCODE_SIGNPOSTS:"
                           " \"${LLVM_ENABLE_SUPPORT_XCODE_SIGNPOSTS}\"!")
     endif()
   endif()
 endif()
+
+option(LLVM_USE_RELATIVE_PATHS "Make builds independent of absolute file paths." OFF)
+set(LLVM_SOURCE_PREFIX "" CACHE STRING "LLVM source directory prefix")
+
+if(LLVM_USE_RELATIVE_PATHS)
+  check_c_compiler_flag("-fdebug-prefix-map=foo=bar" SUPPORTS_FDEBUG_PREFIX_MAP)
+  if(LLVM_ENABLE_PROJECTS_USED)
+    get_filename_component(source_root "${LLVM_MAIN_SRC_DIR}/.." ABSOLUTE)
+  else()
+    set(source_root "${LLVM_MAIN_SRC_DIR}")
+  endif()
+  file(RELATIVE_PATH relative_root "${source_root}" "${CMAKE_BINARY_DIR}")
+  append_if(SUPPORTS_FDEBUG_PREFIX_MAP "-fdebug-prefix-map=${CMAKE_BINARY_DIR}=${relative_root}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  append_if(SUPPORTS_FDEBUG_PREFIX_MAP "-fdebug-prefix-map=${source_root}/=${LLVM_SOURCE_PREFIX}" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
+  add_flag_if_supported("-no-canonical-prefixes" NO_CANONICAL_PREFIXES)
+endif()

llvm/cmake/modules/LLVMExternalProjectUtils.cmake

CMAKE_ARGS ${${nameCanon}_CMAKE_ARGS}
                ${compiler_args}
                -DCMAKE_INSTALL_PREFIX=${CMAKE_INSTALL_PREFIX}
                ${sysroot_arg}
                -DLLVM_BINARY_DIR=${PROJECT_BINARY_DIR}
                -DLLVM_CONFIG_PATH=${llvm_config_path}
                -DLLVM_ENABLE_WERROR=${LLVM_ENABLE_WERROR}
                -DLLVM_HOST_TRIPLE=${LLVM_HOST_TRIPLE}
                -DLLVM_HAVE_LINK_VERSION_SCRIPT=${LLVM_HAVE_LINK_VERSION_SCRIPT}
+               -DLLVM_USE_RELATIVE_PATHS=${LLVM_USE_RELATIVE_PATHS}
+               -DLLVM_SOURCE_PREFIX=${LLVM_SOURCE_PREFIX}
                -DPACKAGE_VERSION=${PACKAGE_VERSION}
                -DCMAKE_BUILD_TYPE=${CMAKE_BUILD_TYPE}
                -DCMAKE_MAKE_PROGRAM=${CMAKE_MAKE_PROGRAM}
                -DCMAKE_EXPORT_COMPILE_COMMANDS=1
                ${cmake_args}
                ${PASSTHROUGH_VARIABLES}
     INSTALL_COMMAND ""
     STEP_TARGETS configure build

llvm/runtimes/CMakeLists.txt

function(runtime_default_target)
     llvm_ExternalProject_Add(runtimes
                              ${CMAKE_CURRENT_SOURCE_DIR}
                              DEPENDS ${ARG_DEPENDS} ${CXX_HEADER_TARGET}
                              # Builtins were built separately above
                              CMAKE_ARGS -DCOMPILER_RT_BUILD_BUILTINS=Off
                                         -DLLVM_INCLUDE_TESTS=${LLVM_INCLUDE_TESTS}
                                         -DLLVM_BINARY_DIR=${LLVM_BINARY_DIR}
                                         -DLLVM_LIBRARY_DIR=${LLVM_LIBRARY_DIR}
+                                        -DLLVM_MAIN_SRC_DIR=${LLVM_MAIN_SRC_DIR}
                                         -DLLVM_DEFAULT_TARGET_TRIPLE=${TARGET_TRIPLE}
+                                        -DLLVM_ENABLE_PROJECTS_USED=${LLVM_ENABLE_PROJECTS_USED}
                                         -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
                                         -DCMAKE_C_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_CXX_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_ASM_COMPILER_TARGET=${TARGET_TRIPLE}
                                         -DCMAKE_C_COMPILER_WORKS=ON
                                         -DCMAKE_CXX_COMPILER_WORKS=ON
                                         -DCMAKE_ASM_COMPILER_WORKS=ON
                              PASSTHROUGH_PREFIXES LLVM_ENABLE_RUNTIMES
function(runtime_register_target name target)
     llvm_ExternalProject_Add(runtimes-${name}
                              ${CMAKE_CURRENT_SOURCE_DIR}
                              DEPENDS ${${name}_deps} ${CXX_HEADER_TARGET}
                              # Builtins were built separately above
                              CMAKE_ARGS -DCOMPILER_RT_BUILD_BUILTINS=Off
                                         -DLLVM_INCLUDE_TESTS=${LLVM_INCLUDE_TESTS}
                                         -DLLVM_BINARY_DIR=${LLVM_BINARY_DIR}
                                         -DLLVM_LIBRARY_DIR=${LLVM_LIBRARY_DIR}
+                                        -DLLVM_MAIN_SRC_DIR=${LLVM_MAIN_SRC_DIR}
                                         -DLLVM_DEFAULT_TARGET_TRIPLE=${target}
+                                        -DLLVM_ENABLE_PROJECTS_USED=${LLVM_ENABLE_PROJECTS_USED}
                                         -DLLVM_ENABLE_PER_TARGET_RUNTIME_DIR=ON
                                         -DCMAKE_C_COMPILER_TARGET=${target}
                                         -DCMAKE_CXX_COMPILER_TARGET=${target}
                                         -DCMAKE_ASM_COMPILER_TARGET=${target}
                                         -DCMAKE_C_COMPILER_WORKS=ON
                                         -DCMAKE_CXX_COMPILER_WORKS=ON
                                         -DCMAKE_ASM_COMPILER_WORKS=ON
                                         -DCOMPILER_RT_DEFAULT_TARGET_ONLY=ON