cmake: Add LTO option

CMakeLists.txt

@@ -54,6 +54,7 @@ cmake_dependent_option(BUILD_WALLET_TOOL "Build bitcoin-wallet tool." ON "ENABLE
 cmake_dependent_option(CXX20 "Enable compilation in C++20 mode." OFF "NOT MSVC" ON)
 option(THREADLOCAL "Enable features that depend on the C++ thread_local keyword (currently just thread names in debug logs)." ON)
 option(HARDENING "Attempt to harden the resulting executables." ON)
+option(LTO "Build using LTO. To override the default CMake LTO flags, please use the LTO_FLAGS variable." OFF)
 option(REDUCE_EXPORTS "Attempt to reduce exported symbols in the resulting executables." OFF)
 option(WERROR "Treat compiler warnings as errors." OFF)
 
@@ -81,6 +82,19 @@ set(CMAKE_CXX_EXTENSIONS OFF)
 
 set(configure_warnings)
 
+if(LTO)
+  if(DEFINED LTO_FLAGS)
+    set(CMAKE_CXX_COMPILE_OPTIONS_IPO ${LTO_FLAGS})
+  endif()
+  include(CheckIPOSupported)
+  check_ipo_supported(RESULT result OUTPUT output)
+  if(result)
+    set(CMAKE_INTERPROCEDURAL_OPTIMIZATION TRUE)
+  else()
+    message(FATAL_ERROR "LTO is not supported: ${output}")
+  endif()
+endif()
+
 include(CheckPIESupported)
 check_pie_supported(OUTPUT_VARIABLE check_pie_output LANGUAGES CXX)
 if(CMAKE_CXX_LINK_PIE_SUPPORTED)
@@ -369,6 +383,14 @@ message("Linker flags for executables .......... ${CMAKE_EXE_LINKER_FLAGS}")
 message("Linker flags for shared libraries ..... ${CMAKE_SHARED_LINKER_FLAGS}")
 print_config_flags()
 message("Use assembly routines ................. ${ASM}")
+if(LTO)
+  list(JOIN CMAKE_CXX_COMPILE_OPTIONS_IPO " " lto_flags)
+  string(PREPEND lto_flags ", with flags: ")
+else()
+  set(lto_flags)
+endif()
+message("Build using LTO ....................... ${LTO}${lto_flags}")
+unset(lto_flags)
 message("Attempt to harden executables ......... ${HARDENING}")
 message("Treat compiler warnings as errors ..... ${WERROR}")
 message("Use ccache for compiling .............. ${CCACHE}")