[cmake] Build executables with -no_exported_symbols when building App…

…le toolchain (#87684)

Building the Apple way turns off plugin support, meaning we don't need
to export unloadable symbols from all executables. While deadstripping
effects aren't expected to change, enabling this across all tools
prevents the creation of export tries. This saves us ~3.5 MB in just the
universal build of `clang`.

clang/cmake/caches/Apple-stage2.cmake

@@ -15,6 +15,7 @@ set(LLVM_ENABLE_ZLIB ON CACHE BOOL "")
 set(LLVM_ENABLE_BACKTRACES OFF CACHE BOOL "")
 set(LLVM_ENABLE_MODULES ON CACHE BOOL "")
 set(LLVM_EXTERNALIZE_DEBUGINFO ON CACHE BOOL "")
+set(LLVM_ENABLE_EXPORTED_SYMBOLS_IN_EXECUTABLES OFF CACHE BOOL "")
 set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")
 set(CLANG_SPAWN_CC1 ON CACHE BOOL "")
 set(BUG_REPORT_URL "http://developer.apple.com/bugreporter/" CACHE STRING "")

lldb/cmake/caches/Apple-lldb-base.cmake

@@ -3,6 +3,7 @@ set(CMAKE_EXPORT_COMPILE_COMMANDS ON CACHE BOOL "")
 
 set(LLVM_TARGETS_TO_BUILD X86;ARM;AArch64 CACHE STRING "")
 set(LLVM_ENABLE_ASSERTIONS ON CACHE BOOL "")
+set(LLVM_ENABLE_EXPORTED_SYMBOLS_IN_EXECUTABLES OFF CACHE BOOL "")
 
 set(LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
 set(LIBCXX_ENABLE_STATIC OFF CACHE BOOL "")

llvm/CMakeLists.txt

@@ -673,6 +673,9 @@ option(LLVM_USE_OPROFILE
 option(LLVM_EXTERNALIZE_DEBUGINFO
   "Generate dSYM files and strip executables and libraries (Darwin Only)" OFF)
 
+option(LLVM_ENABLE_EXPORTED_SYMBOLS_IN_EXECUTABLES
+  "Preserve exported symbols in executables" ON)
+
 set(LLVM_CODESIGNING_IDENTITY "" CACHE STRING
   "Sign executables and dylibs with the given identity or skip if empty (Darwin Only)")
 

llvm/cmake/modules/AddLLVM.cmake

@@ -258,15 +258,24 @@ if (NOT DEFINED LLVM_LINKER_DETECTED AND NOT WIN32)
     endif()
   endif()
 
-  # Apple's linker complains about duplicate libraries, which CMake likes to do
-  # to support ELF platforms. To silence that warning, we can use
-  # -no_warn_duplicate_libraries, but only in versions of the linker that
-  # support that flag.
-  if(NOT LLVM_USE_LINKER AND ${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
+  if(${CMAKE_SYSTEM_NAME} MATCHES "Darwin")
     include(CheckLinkerFlag)
-    check_linker_flag(C "-Wl,-no_warn_duplicate_libraries" LLVM_LINKER_SUPPORTS_NO_WARN_DUPLICATE_LIBRARIES)
-  else()
-    set(LLVM_LINKER_SUPPORTS_NO_WARN_DUPLICATE_LIBRARIES OFF CACHE INTERNAL "")
+    # Linkers that support Darwin allow a setting to internalize all symbol exports, 
+    # aiding in reducing binary size and often is applicable for executables.
+    check_linker_flag(C "-Wl,-no_exported_symbols" LLVM_LINKER_SUPPORTS_NO_EXPORTED_SYMBOLS)
+
+    if (NOT LLVM_USE_LINKER) 
+      # Apple's linker complains about duplicate libraries, which CMake likes to do
+      # to support ELF platforms. To silence that warning, we can use
+      # -no_warn_duplicate_libraries, but only in versions of the linker that
+      # support that flag.
+      check_linker_flag(C "-Wl,-no_warn_duplicate_libraries" LLVM_LINKER_SUPPORTS_NO_WARN_DUPLICATE_LIBRARIES)
+    else()
+      set(LLVM_LINKER_SUPPORTS_NO_WARN_DUPLICATE_LIBRARIES OFF CACHE INTERNAL "")
+    endif()
+
+  else() 
+    set(LLVM_LINKER_SUPPORTS_NO_EXPORTED_SYMBOLS OFF CACHE INTERNAL "")
   endif()
 endif()
 
@@ -1029,6 +1038,16 @@ macro(add_llvm_executable name)
     add_llvm_symbol_exports( ${name} ${LLVM_EXPORTED_SYMBOL_FILE} )
   endif(LLVM_EXPORTED_SYMBOL_FILE)
 
+  if (NOT LLVM_ENABLE_EXPORTED_SYMBOLS_IN_EXECUTABLES) 
+    if(LLVM_LINKER_SUPPORTS_NO_EXPORTED_SYMBOLS)
+      set_property(TARGET ${name} APPEND_STRING PROPERTY
+        LINK_FLAGS " -Wl,-no_exported_symbols")
+    else()
+      message(FATAL_ERROR 
+        "LLVM_ENABLE_EXPORTED_SYMBOLS_IN_EXECUTABLES cannot be disabled when linker does not support \"-no_exported_symbols\"")
+    endif()
+  endif()
+
   if (LLVM_LINK_LLVM_DYLIB AND NOT ARG_DISABLE_LLVM_LINK_LLVM_DYLIB)
     set(USE_SHARED USE_SHARED)
   endif()

llvm/docs/CMake.rst

@@ -654,6 +654,11 @@ enabled sub-projects. Nearly all of these variable names begin with
   Generate dSYM files and strip executables and libraries (Darwin Only).
   Defaults to OFF.
 
+**LLVM_ENABLE_EXPORTED_SYMBOLS_IN_EXECUTABLES**:BOOL
+  When building executables, preserve symbol exports. Defaults to ON. 
+  You can use this option to disable exported symbols from all 
+  executables (Darwin Only).
+
 **LLVM_FORCE_USE_OLD_TOOLCHAIN**:BOOL
   If enabled, the compiler and standard library versions won't be checked. LLVM
   may not compile at all, or might fail at runtime due to known bugs in these