cmake: Configure the ThinLTO cache directory when using ELF lld or gold.

[mehdi_amini]

CC Bob, I suspect we should add the support for this option for lld-link on windows as well.

llvm/cmake/modules/HandleLLVMOptions.cmake

724

Is LLVM_USE_LINKER already normalized into lower case? It seems that STREQUAL is case sensitive.

730

I'm fine with this for now, but I feel we should provide a clang driver option to abstract away all this logic.

[pcc]

llvm/cmake/modules/HandleLLVMOptions.cmake

724

I don't think so, but I wouldn't normally expect something like -DLLVM_USE_LINKER=LLD to work anyway because this will translate into -fuse-ld=LLD which probably wouldn't work on case-sensitive file systems (which ELF targets normally are).

730

Agreed, I've added a FIXME.

llvm/cmake/modules/HandleLLVMOptions.cmake

 if(LLVM_ENABLE_LTO AND LLVM_ON_WIN32 AND NOT LINKER_IS_LLD_LINK)
   message(FATAL_ERROR "When compiling for Windows, LLVM_ENABLE_LTO requires using lld as the linker (point CMAKE_LINKER at lld-link.exe)")
 endif()
 if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
   append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
   if(NOT LINKER_IS_LLD_LINK)
     append("-flto=thin" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   endif()
-  # On darwin, enable the lto cache. This improves initial build time a little
-  # since we re-link a lot of the same objects, and significantly improves
-  # incremental build time.
-  append_if(APPLE "-Wl,-cache_path_lto,${PROJECT_BINARY_DIR}/lto.cache"
-            CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  # If the linker supports it, enable the lto cache. This improves initial build
+  # time a little since we re-link a lot of the same objects, and significantly
+  # improves incremental build time.
+  if(APPLE)
+    append("-Wl,-cache_path_lto,${PROJECT_BINARY_DIR}/lto.cache"
+           CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  elseif(UNIX AND LLVM_USE_LINKER STREQUAL "lld")
+    append("-Wl,--thinlto-cache-dir=${PROJECT_BINARY_DIR}/lto.cache"
+  elseif(LLVM_USE_LINKER STREQUAL "gold")
+    append("-Wl,--plugin-opt,cache-dir=${PROJECT_BINARY_DIR}/lto.cache"
+           CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  endif()
 elseif(uppercase_LLVM_ENABLE_LTO STREQUAL "FULL")
   append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
   if(NOT LINKER_IS_LLD_LINK)
     append("-flto=full" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
   endif()
 elseif(LLVM_ENABLE_LTO)
   append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
   if(NOT LINKER_IS_LLD_LINK)