CMake: implement LTO for clang. Fixes #478 (#487)

* CMake: implement LTO for clang. Fixes #478

* LTO: add basic docs about required executables.

CMakeLists.txt

@@ -19,6 +19,7 @@ option(BENCHMARK_BUILD_32_BITS "Build a 32 bit version of the library." OFF)
 option(BENCHMARK_ENABLE_INSTALL "Enable installation of benchmark. (Projects embedding benchmark may want to turn this OFF.)" ON)
 
 # Make sure we can import out CMake functions
+list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake/Modules")
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
 
 # Read the git tags to determine the project version
@@ -132,6 +133,8 @@ else()
       if (GCC_RANLIB)
         set(CMAKE_RANLIB ${GCC_RANLIB})
       endif()
+    elseif("${CMAKE_C_COMPILER_ID}" STREQUAL "Clang")
+      include(llvm-toolchain)
     endif()
   endif()
 

README.md

@@ -848,6 +848,9 @@ To enable link-time optimisation, use
 cmake -DCMAKE_BUILD_TYPE=Release -DBENCHMARK_ENABLE_LTO=true
 ```
 
+If you are using gcc, you might need to set `GCC_AR` and `GCC_RANLIB` cmake cache variables, if autodetection fails.
+If you are using clang, you may need to set `LLVMAR_EXECUTABLE`, `LLVMNM_EXECUTABLE` and `LLVMRANLIB_EXECUTABLE` cmake cache variables.
+
 ## Linking against the library
 When using gcc, it is necessary to link against pthread to avoid runtime exceptions.
 This is due to how gcc implements std::thread.

cmake/Modules/FindLLVMAr.cmake

@@ -0,0 +1,16 @@
+include(FeatureSummary)
+
+find_program(LLVMAR_EXECUTABLE
+  NAMES llvm-ar
+  DOC "The llvm-ar executable"
+  )
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(LLVMAr
+  DEFAULT_MSG
+  LLVMAR_EXECUTABLE)
+
+SET_PACKAGE_PROPERTIES(LLVMAr PROPERTIES
+  URL https://llvm.org/docs/CommandGuide/llvm-ar.html
+  DESCRIPTION "create, modify, and extract from archives"
+)

cmake/Modules/FindLLVMNm.cmake

@@ -0,0 +1,16 @@
+include(FeatureSummary)
+
+find_program(LLVMNM_EXECUTABLE
+  NAMES llvm-nm
+  DOC "The llvm-nm executable"
+  )
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(LLVMNm
+  DEFAULT_MSG
+  LLVMNM_EXECUTABLE)
+
+SET_PACKAGE_PROPERTIES(LLVMNm PROPERTIES
+  URL https://llvm.org/docs/CommandGuide/llvm-nm.html
+  DESCRIPTION "list LLVM bitcode and object file’s symbol table"
+)

cmake/Modules/FindLLVMRanLib.cmake

@@ -0,0 +1,15 @@
+include(FeatureSummary)
+
+find_program(LLVMRANLIB_EXECUTABLE
+  NAMES llvm-ranlib
+  DOC "The llvm-ranlib executable"
+  )
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(LLVMRanLib
+  DEFAULT_MSG
+  LLVMRANLIB_EXECUTABLE)
+
+SET_PACKAGE_PROPERTIES(LLVMRanLib PROPERTIES
+  DESCRIPTION "generate index for LLVM archive"
+)

cmake/llvm-toolchain.cmake

@@ -0,0 +1,8 @@
+find_package(LLVMAr REQUIRED)
+set(CMAKE_AR "${LLVMAR_EXECUTABLE}" CACHE FILEPATH "" FORCE)
+
+find_package(LLVMNm REQUIRED)
+set(CMAKE_NM "${LLVMNM_EXECUTABLE}" CACHE FILEPATH "" FORCE)
+
+find_package(LLVMRanLib REQUIRED)
+set(CMAKE_RANLIB "${LLVMRANLIB_EXECUTABLE}" CACHE FILEPATH "" FORCE)