CMake: Use LLD if available

CMakeLists.txt

@@ -50,12 +50,16 @@ option(DEBUG_EXTRA "Expensive debug options" OFF)
 option(SET_WARNING_FLAGS "Adjust compiler warning flags" ON)
 option(SET_NOISY_WARNING_FLAGS "Enable noisy compiler warnings" OFF)
 option(SET_OPTIMIZATION_FLAGS "Adjust compiler optimization flags" ON)
-suboption(USE_LDGOLD "Use the Gold linker" BOOL ${SET_OPTIMIZATION_FLAGS})
 set(default_FASTLINK OFF)
 if(DEVELOPER OR CONTINUOUS_INTEGRATION)
 	set(default_FASTLINK ON)
 endif()
 suboption(FASTLINK "Optimize (incremental) linking speed" BOOL ${default_FASTLINK})
+set(default_USE_LD "default")
+if(SET_OPTIMIZATION_FLAGS OR FASTLINK)
+	set(default_USE_LD "best")
+endif()
+suboption(USE_LD "The linker to use (mold, lld, gold, bfd, best or default)" STRING "${default_USE_LD}")
 set(default_USE_LTO OFF)
 if(SET_OPTIMIZATION_FLAGS AND NOT FASTLINK)
 	set(default_USE_LTO ON)

README.md

@@ -68,6 +68,7 @@ The default build settings are tuned for users - if you plan to make changes to
 | `DEVELOPER`               | `OFF`     | Enable build options suitable for developers⁵.
 | `FASTLINK`                | `OFF`⁶    | Optimize for link speed.
 | `USE_LTO`                 | `ON`²     | Use link-time code generation.
+| `USE_LD`                  | `best`⁸   | Linker to use - `default`, `mold`, `lld`, `gold`, `bfd` or `best`
 | `BUILD_TESTS`             | `OFF`⁶    | Build unit tests that can be run using `make check`
 | `RUN_TESTS`               | `OFF`⁷    | Automatically run tests
 | `RUN_TARGET`              | (none)    | Wrapper to run binaries produced in the build process
@@ -78,6 +79,7 @@ The default build settings are tuned for users - if you plan to make changes to
 5. Currently this and enables `DEBUG`, `BUILD_TESTS`, `RUN_TESTS` and `FASTLINK` for faster incremental builds and improved debug output, unless those options have been explicitly specified by the user.
 6. Enabled automatically if `DEVELOPER` is enabled.
 7. Enabled automatically if `DEVELOPER` is enabled unless cross-compiling without `RUN_TARGET` set
+8. Disabled automatically (set to `default`) if both `SET_OPTIMIZATION_FLAGS` and `FASTLINK` are disabled. `best` will select the most suited linker based on availability and other settings such as `USE_LTO`.
 
 Install options:
 

cmake/BuildType.cmake

@@ -130,8 +130,52 @@ if(MSVC)
 	
 else(MSVC)
 	
-	if(USE_LDGOLD)
+	set(linker_used)
+	if(NOT linker_used AND USE_LD STREQUAL "mold")
+		# Does not really support LTO yet
+		add_ldflag("-fuse-ld=mold")
+		if(FLAG_FOUND)
+			set(linker_used "mold")
+		elseif(STRICT_USE AND NOT USE_LD STREQUAL "best")
+			message(FATAL_ERROR "Requested linker is not available")
+		endif()
+	endif()
+	if(NOT linker_used AND (USE_LD STREQUAL "lld" OR
+	                        (USE_LD STREQUAL "best" AND (NOT USE_LTO OR CMAKE_CXX_COMPILER_ID MATCHES "Clang"))))
+		# Only supports LTO with LLVM-based compilers and old versions are unstable
+		if(USE_LD STREQUAL "best")
+			execute_process(COMMAND ${CMAKE_CXX_COMPILER} "-fuse-ld=lld" "-Wl,-version"
+			                OUTPUT_VARIABLE _LLD_Version ERROR_QUIET)
+		endif()
+		if(USE_LD STREQUAL "best" AND _LLD_Version MATCHES "LLD [0-8]\\.[0-9\\.]*")
+			message(STATUS "Not using ancient ${CMAKE_MATCH_0}")
+		elseif(USE_LD STREQUAL "best" AND CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND
+		       CMAKE_CXX_COMPILER_VERSION VERSION_LESS 9)
+			message(STATUS "Not using ancient Clang ${CMAKE_CXX_COMPILER_VERSION}")
+		else()
+			add_ldflag("-fuse-ld=lld")
+			if(FLAG_FOUND)
+				set(linker_used "lld")
+			elseif(STRICT_USE AND NOT USE_LD STREQUAL "best")
+				message(FATAL_ERROR "Requested linker is not available")
+			endif()
+		endif()
+	endif()
+	if(NOT linker_used AND (USE_LD STREQUAL "gold" OR USE_LD STREQUAL "best"))
 		add_ldflag("-fuse-ld=gold")
+		if(FLAG_FOUND)
+			set(linker_used "gold")
+		elseif(STRICT_USE AND NOT USE_LD STREQUAL "best")
+			message(FATAL_ERROR "Requested linker is not available")
+		endif()
+	endif()
+	if(NOT linker_used AND (USE_LD STREQUAL "bfd"))
+		add_ldflag("-fuse-ld=bfd")
+		if(FLAG_FOUND)
+			set(linker_used "bfd")
+		elseif(STRICT_USE AND NOT USE_LD STREQUAL "best")
+			message(FATAL_ERROR "Requested linker is not available")
+		endif()
 	endif()
 	
 	if(USE_LTO)
@@ -146,7 +190,12 @@ else(MSVC)
 	if(FASTLINK)
 		
 		# Optimize for link speed in developer builds
-		add_cxxflag("-gsplit-dwarf")
+		if(linker_used STREQUAL "mold" OR linker_used STREQUAL "lld")
+			# mold and lld are fast enough without -gsplit-dwarf that we don't need to deal with its issues
+		else()
+			add_cxxflag("-gsplit-dwarf")
+			add_cxxflag("-gdwarf-4") # -gsplit-dwarf is broken with DWARF 5
+		endif()
 		
 	elseif(SET_OPTIMIZATION_FLAGS)