Index: cmake/modules/HandleLLVMOptions.cmake
===================================================================
--- cmake/modules/HandleLLVMOptions.cmake
+++ cmake/modules/HandleLLVMOptions.cmake
@@ -17,6 +17,42 @@
   set(LINKER_IS_LLD_LINK FALSE)
 endif()
 
+set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
+string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
+if(LLVM_ENABLE_LTO AND LLVM_ON_WIN32 AND NOT LINKER_IS_LLD_LINK)
+  message(FATAL_ERROR "When compiling for Windows, LLVM_ENABLE_LTO requires using lld as the linker (point CMAKE_LINKER at lld-link.exe)")
+endif()
+if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
+  append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
+  if(NOT LINKER_IS_LLD_LINK)
+    append("-flto=thin" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  endif()
+  # If the linker supports it, enable the lto cache. This improves initial build
+  # time a little since we re-link a lot of the same objects, and significantly
+  # improves incremental build time.
+  # FIXME: We should move all this logic into the clang driver.
+  if(APPLE)
+    append("-Wl,-cache_path_lto,${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=${PROJECT_BINARY_DIR}/lto.cache"
+           CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  elseif(LLVM_USE_LINKER STREQUAL "gold")
+    append("-Wl,--plugin-opt,cache-dir=${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)
+    append("-flto" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+  endif()
+endif()
+
 # Ninja Job Pool support
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
@@ -32,6 +68,11 @@
 
 set(LLVM_PARALLEL_LINK_JOBS "" CACHE STRING
   "Define the maximum number of concurrent link jobs.")
+if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN" AND NOT LLVM_PARALLEL_LINK_JOBS)
+  # ThinLTO does its own parallelism. Limit concurrent link jobs to 2
+  # by default to avoid spawning too many threads.
+  set(LLVM_PARALLEL_LINK_JOBS "2")
+endif()
 if(LLVM_PARALLEL_LINK_JOBS)
   if(NOT CMAKE_MAKE_PROGRAM MATCHES "ninja")
     message(WARNING "Job pooling is only available with Ninja generators.")
@@ -713,42 +754,6 @@
   CMAKE_EXE_LINKER_FLAGS
   CMAKE_SHARED_LINKER_FLAGS)
 
-set(LLVM_ENABLE_LTO OFF CACHE STRING "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO")
-string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
-if(LLVM_ENABLE_LTO AND LLVM_ON_WIN32 AND NOT LINKER_IS_LLD_LINK)
-  message(FATAL_ERROR "When compiling for Windows, LLVM_ENABLE_LTO requires using lld as the linker (point CMAKE_LINKER at lld-link.exe)")
-endif()
-if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
-  append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS)
-  if(NOT LINKER_IS_LLD_LINK)
-    append("-flto=thin" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
-  endif()
-  # If the linker supports it, enable the lto cache. This improves initial build
-  # time a little since we re-link a lot of the same objects, and significantly
-  # improves incremental build time.
-  # FIXME: We should move all this logic into the clang driver.
-  if(APPLE)
-    append("-Wl,-cache_path_lto,${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=${PROJECT_BINARY_DIR}/lto.cache"
-           CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
-  elseif(LLVM_USE_LINKER STREQUAL "gold")
-    append("-Wl,--plugin-opt,cache-dir=${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)
-    append("-flto" CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
-  endif()
-endif()
-
 # This option makes utils/extract_symbols.py be used to determine the list of
 # symbols to export from LLVM tools. This is necessary when using MSVC if you
 # want to allow plugins, though note that the plugin has to explicitly link