[CMake] Simplify two binary variables into one tri-state variable

Also remove some ancient logic to detect and ignore requests to use LLD.
If people want to explicitly use LLD, they probably have a reason and we
shouldn't second guess them.

Author: davezarzycki

CMakeLists.txt

@@ -147,24 +147,20 @@ set(SWIFT_COMPILER_VERSION "" CACHE STRING
 set(CLANG_COMPILER_VERSION "" CACHE STRING
     "The internal version of the Clang compiler")
 
-# Indicate whether Swift should attempt to use the lld linker.
-if(CMAKE_SYSTEM_NAME STREQUAL Windows AND NOT CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
-  set(SWIFT_ENABLE_LLD_LINKER_default TRUE)
+# Which default linker to use. Prefer LLVM_USE_LINKER if it set, otherwise use
+# our own defaults. This should only be possible in a unified (not stand alone)
+# build environment.
+if(LLVM_USE_LINKER)
+  set(SWIFT_USE_LINKER_default "${LLVM_USE_LINKER}")
+elseif(CMAKE_SYSTEM_NAME STREQUAL Windows AND NOT CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
+  set(SWIFT_USE_LINKER_default "lld")
+elseif(CMAKE_SYSTEM_NAME STREQUAL Darwin)
+  set(SWIFT_USE_LINKER_default "")
 else()
-  set(SWIFT_ENABLE_LLD_LINKER_default FALSE)
+  set(SWIFT_USE_LINKER_default "gold")
 endif()
-set(SWIFT_ENABLE_LLD_LINKER ${SWIFT_ENABLE_LLD_LINKER_default} CACHE BOOL
-    "Enable using the lld linker when available")
-
-# Indicate whether Swift should attempt to use the gold linker.
-# This is not used on Darwin.
-if(CMAKE_SYSTEM_NAME STREQUAL Darwin OR CMAKE_SYSTEM_NAME STREQUAL Windows)
-  set(SWIFT_ENABLE_GOLD_LINKER_default FALSE)
-else()
-  set(SWIFT_ENABLE_GOLD_LINKER_default TRUE)
-endif()
-set(SWIFT_ENABLE_GOLD_LINKER ${SWIFT_ENABLE_GOLD_LINKER_default} CACHE BOOL
-    "Enable using the gold linker when available")
+set(SWIFT_USE_LINKER ${SWIFT_USE_LINKER_default} CACHE STRING
+    "Build Swift with a non-default linker")
 
 set(SWIFT_TOOLS_ENABLE_LTO OFF CACHE STRING "Build Swift tools with LTO. One
     must specify the form of LTO by setting this to one of: 'full', 'thin'. This

cmake/modules/AddSwift.cmake

@@ -116,7 +116,7 @@ function(_add_host_variant_c_compile_link_flags name)
   if(SWIFT_HOST_VARIANT_SDK STREQUAL ANDROID)
     # lld can handle targeting the android build.  However, if lld is not
     # enabled, then fallback to the linker included in the android NDK.
-    if(NOT SWIFT_ENABLE_LLD_LINKER)
+    if(NOT SWIFT_USE_LINKER STREQUAL "lld")
       swift_android_tools_path(${SWIFT_HOST_VARIANT_ARCH} tools_path)
       target_compile_options(${name} PRIVATE -B${tools_path})
     endif()
@@ -368,12 +368,9 @@ function(_add_host_variant_link_flags target)
   endif()
 
   if(NOT SWIFT_COMPILER_IS_MSVC_LIKE)
-    if(SWIFT_ENABLE_LLD_LINKER)
+    if(SWIFT_USE_LINKER)
       target_link_options(${target} PRIVATE
-        -fuse-ld=lld$<$<STREQUAL:${CMAKE_HOST_SYSTEM_NAME},Windows>:.exe>)
-    elseif(SWIFT_ENABLE_GOLD_LINKER)
-      target_link_options(${target} PRIVATE
-        -fuse-ld=gold$<$<STREQUAL:${CMAKE_HOST_SYSTEM_NAME},Windows>:.exe>)
+        -fuse-ld=${SWIFT_USE_LINKER}$<$<STREQUAL:${CMAKE_HOST_SYSTEM_NAME},Windows>:.exe>)
     endif()
   endif()
 

cmake/modules/AddSwiftUnittests.cmake

@@ -57,15 +57,9 @@ function(add_swift_unittest test_dirname)
       _ENABLE_EXTENDED_ALIGNED_STORAGE)
   endif()
 
-  find_program(LDLLD_PATH "ld.lld")
-  # Strangely, macOS finds lld and then can't find it when using -fuse-ld=
-  if(SWIFT_ENABLE_LLD_LINKER AND LDLLD_PATH AND NOT APPLE)
+  if(SWIFT_USE_LINKER)
     set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY
-      LINK_FLAGS " -fuse-ld=lld")
-  elseif(SWIFT_ENABLE_GOLD_LINKER AND
-     "${SWIFT_SDK_${SWIFT_HOST_VARIANT_SDK}_OBJECT_FORMAT}" STREQUAL "ELF")
-    set_property(TARGET "${test_dirname}" APPEND_STRING PROPERTY
-      LINK_FLAGS " -fuse-ld=gold")
+      LINK_FLAGS " -fuse-ld=${SWIFT_USE_LINKER}")
   endif()
 
   if(SWIFT_ANALYZE_CODE_COVERAGE)

stdlib/cmake/modules/AddSwiftStdlib.cmake

@@ -93,7 +93,7 @@ function(_add_target_variant_c_compile_link_flags)
   if("${CFLAGS_SDK}" STREQUAL "ANDROID")
     # lld can handle targeting the android build.  However, if lld is not
     # enabled, then fallback to the linker included in the android NDK.
-    if(NOT SWIFT_ENABLE_LLD_LINKER)
+    if(NOT SWIFT_USE_LINKER STREQUAL "lld")
       swift_android_tools_path(${CFLAGS_ARCH} tools_path)
       list(APPEND result "-B" "${tools_path}")
     endif()
@@ -414,20 +414,11 @@ function(_add_target_variant_link_flags)
     list(APPEND library_search_directories "${SWIFT_${LFLAGS_SDK}_${LFLAGS_ARCH}_ICU_I18N_LIBDIR}")
   endif()
 
-  if(NOT SWIFT_COMPILER_IS_MSVC_LIKE)
-    # FIXME: On Apple platforms, find_program needs to look for "ld64.lld"
-    find_program(LDLLD_PATH "ld.lld")
-    if((SWIFT_ENABLE_LLD_LINKER AND LDLLD_PATH AND NOT APPLE) OR
-       ("${LFLAGS_SDK}" STREQUAL "WINDOWS" AND
-        NOT "${CMAKE_SYSTEM_NAME}" STREQUAL "WINDOWS"))
-      list(APPEND result "-fuse-ld=lld")
-    elseif(SWIFT_ENABLE_GOLD_LINKER AND
-        "${SWIFT_SDK_${LFLAGS_SDK}_OBJECT_FORMAT}" STREQUAL "ELF")
-      if(CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
-        list(APPEND result "-fuse-ld=gold.exe")
-      else()
-        list(APPEND result "-fuse-ld=gold")
-      endif()
+  if(SWIFT_USE_LINKER AND NOT SWIFT_COMPILER_IS_MSVC_LIKE)
+    if(CMAKE_HOST_SYSTEM_NAME STREQUAL Windows)
+      list(APPEND result "-fuse-ld=${SWIFT_USE_LINKER}.exe")
+    else()
+      list(APPEND result "-fuse-ld=${SWIFT_USE_LINKER}")
     endif()
   endif()
 

test/lit.site.cfg.in

@@ -116,10 +116,10 @@ if "@SWIFT_ENABLE_SOURCEKIT_TESTS@" == "TRUE":
 if "@SWIFT_HAVE_LIBXML2@" == "TRUE":
     config.available_features.add('libxml2')
 
-if "@SWIFT_ENABLE_LLD_LINKER@" == "TRUE":
+if "@SWIFT_USE_LINKER@" == "lld":
     config.android_linker_name = "lld"
 else:
-    # even if SWIFT_ENABLE_GOLD_LINKER isn't set, we cannot use BFD for Android
+    # even if SWIFT_USE_LINKER isn't set, we cannot use BFD for Android
     config.android_linker_name = "gold"
 
 if '@SWIFT_INCLUDE_TOOLS@' == 'TRUE':

validation-test/lit.site.cfg.in

@@ -98,10 +98,10 @@ if "@CMAKE_GENERATOR@" == "Xcode":
 
 config.available_features.add("CMAKE_GENERATOR=@CMAKE_GENERATOR@")
 
-if "@SWIFT_ENABLE_LLD_LINKER@" == "TRUE":
+if "@SWIFT_USE_LINKER@" == "lld":
     config.android_linker_name = "lld"
 else:
-    # even if SWIFT_ENABLE_GOLD_LINKER isn't set, we cannot use BFD for Android
+    # even if SWIFT_USE_LINKER isn't set, we cannot use BFD for Android
     config.android_linker_name = "gold"
 
 # Let the main config do the real work.