cmake: Accept "thin" or "full" as arguments to -DLLVM_ENABLE_LTO

Mehdi suggested in a review of r259766 that it's also useful to easily
set the type of LTO. Augment the cmake variable to support that.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@260143 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 1d15d57)

Author: bogner

cmake/modules/HandleLLVMOptions.cmake

@@ -635,12 +635,18 @@ append_if(LLVM_BUILD_INSTRUMENTED "-fprofile-instr-generate"
   CMAKE_EXE_LINKER_FLAGS
   CMAKE_SHARED_LINKER_FLAGS)
 
-option(LLVM_ENABLE_LTO "Enable link-time optimization" OFF)
-append_if(LLVM_ENABLE_LTO "-flto"
-  CMAKE_CXX_FLAGS
-  CMAKE_C_FLAGS
-  CMAKE_EXE_LINKER_FLAGS
-  CMAKE_SHARED_LINKER_FLAGS)
+option(LLVM_ENABLE_LTO "Build LLVM with LTO. May be specified as Thin or Full to use a particular kind of LTO" OFF)
+string(TOUPPER "${LLVM_ENABLE_LTO}" uppercase_LLVM_ENABLE_LTO)
+if(uppercase_LLVM_ENABLE_LTO STREQUAL "THIN")
+  append("-flto=thin" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
+                      CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+elseif(uppercase_LLVM_ENABLE_LTO STREQUAL "FULL")
+  append("-flto=full" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
+                 CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+elseif(LLVM_ENABLE_LTO)
+  append("-flto" CMAKE_CXX_FLAGS CMAKE_C_FLAGS
+                 CMAKE_EXE_LINKER_FLAGS CMAKE_SHARED_LINKER_FLAGS)
+endif()
 
 # Plugin support
 # FIXME: Make this configurable.

docs/CMake.rst

@@ -347,9 +347,10 @@ LLVM-specific variables
   are ``Address``, ``Memory``, ``MemoryWithOrigins``, ``Undefined``, ``Thread``,
   and ``Address;Undefined``. Defaults to empty string.
 
-**LLVM_ENABLE_LTO**:BOOL
-  Add the ``-flto`` flag to the compile and link command lines,
-  enabling link-time optimization. Defaults to OFF.
+**LLVM_ENABLE_LTO**:STRING
+  Add ``-flto`` or ``-flto=`` flags to the compile and link command
+  lines, enabling link-time optimization. Possible values are ``Off``,
+  ``On``, ``Thin`` and ``Full``. Defaults to OFF.
 
 **LLVM_PARALLEL_COMPILE_JOBS**:STRING
   Define the maximum number of concurrent compilation jobs.