[Fuchsia][cmake] Allow using FatLTO when building runtimes (#112277)

We'd like to build runtimes using FatLTO (see
https://llvm.org/docs/FatLTO.html for details). This gives us more
control over how libc++ can be consumed by users of our toolchain, like
the Fuchsia SDK.

Author: ilovepi

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -190,6 +190,10 @@ foreach(target aarch64-unknown-linux-gnu;armv7-unknown-linux-gnueabihf;i386-unkn
     set(RUNTIMES_${target}_LLVM_TOOLS_DIR "${CMAKE_BINARY_DIR}/bin" CACHE BOOL "")
     set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
 
+    # Enable FatLTO for Linux and baremetal runtimes
+    set(RUNTIMES_${target}_LLVM_ENABLE_LTO ON CACHE BOOL "")
+    set(RUNTIMES_${target}_LLVM_ENABLE_FATLTO ON CACHE BOOL "")
+
     # Use .build-id link.
     list(APPEND RUNTIME_BUILD_ID_LINK "${target}")
   endif()
@@ -272,6 +276,10 @@ if(FUCHSIA_SDK)
     set(RUNTIMES_${target}+asan+noexcept_LIBCXXABI_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
     set(RUNTIMES_${target}+asan+noexcept_LIBCXX_ENABLE_EXCEPTIONS OFF CACHE BOOL "")
 
+    # Enable FatLTO for Fuchsia runtimes
+    set(RUNTIMES_${target}_LLVM_ENABLE_LTO ON CACHE BOOL "")
+    set(RUNTIMES_${target}_LLVM_ENABLE_FATLTO ON CACHE BOOL "")
+
     # Use .build-id link.
     list(APPEND RUNTIME_BUILD_ID_LINK "${target}")
   endforeach()
@@ -363,6 +371,10 @@ foreach(target armv6m-none-eabi;armv7m-none-eabi;armv8m.main-none-eabi;armv8.1m.
   set(RUNTIMES_${target}_LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_ENABLE_ASSERTIONS OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "libc;libcxx" CACHE STRING "")
+
+  # Enable FatLTO for baremetal runtimes
+  set(RUNTIMES_${target}_LLVM_ENABLE_LTO ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LLVM_ENABLE_FATLTO ON CACHE BOOL "")
 endforeach()
 
 foreach(target riscv32-unknown-elf)
@@ -414,6 +426,10 @@ foreach(target riscv32-unknown-elf)
   set(RUNTIMES_${target}_LLVM_INCLUDE_TESTS OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_ENABLE_ASSERTIONS OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "libc;libcxx" CACHE STRING "")
+
+  # Enable FatLTO for baremetal runtimes
+  set(RUNTIMES_${target}_LLVM_ENABLE_LTO ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LLVM_ENABLE_FATLTO ON CACHE BOOL "")
 endforeach()
 
 set(LLVM_BUILTIN_TARGETS "${BUILTIN_TARGETS}" CACHE STRING "")

llvm/cmake/modules/HandleLLVMOptions.cmake

@@ -1285,7 +1285,7 @@ elseif(LLVM_ENABLE_LTO)
   endif()
 endif()
 
-if(LLVM_ENABLE_FATLTO AND UNIX AND NOT APPLE)
+if(LLVM_ENABLE_FATLTO AND ((UNIX AND NOT APPLE) OR FUCHSIA))
   append("-ffat-lto-objects" CMAKE_C_FLAGS CMAKE_CXX_FLAGS)
   if(NOT LINKER_IS_LLD_LINK)
     append("-ffat-lto-objects" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS CMAKE_MODULE_LINKER_FLAGS)