Merge pull request 'Add LTO support' (#616) from mueller/add-lto-support into development

Reviewed-on: fsfw/fsfw#616

CHANGELOG.md

@@ -46,6 +46,20 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 
 ## Additions
 
+- LTO support: Allow using LTO/IPO by setting `FSFW_ENABLE_LTO=1`. CMake is able to detect whether
+  the user compiler supports IPO/LPO. LTO is on by default now. Most modern compilers support it,
+  can make good use of it and it usually makes the code faster and/or smaller.
+  After some more research:
+  Enabling LTO will actually cause the compiler to only produce thin LTO by adding 
+  `-flto -fno-fat-lto-objects` to the compiler options. I am not sure this is an ideal choice
+  because if an application linking against the FSFW does not use LTO, there can be compile
+  issues (e.g. observed when compiling the FSFW tests without LTO). This is a known issue as
+  can be seen in the multiple CMake issues for it: 
+    - https://gitlab.kitware.com/cmake/cmake/-/issues/22913, 
+    - https://gitlab.kitware.com/cmake/cmake/-/issues/16808, 
+    - https://gitlab.kitware.com/cmake/cmake/-/issues/21696
+  Easiest solution for now: Keep this option OFF by default.
+  PR: https://egit.irs.uni-stuttgart.de/fsfw/fsfw/pulls/616
 - Linux HAL: Add wiretapping option for I2C. Enabled with `FSFW_HAL_I2C_WIRETAPPING` defined to 1
 - Dedicated Version class and constant `fsfw::FSFW_VERSION` containing version information
   inside `fsfw/version.h`

CMakeLists.txt

@@ -17,6 +17,7 @@ set(FSFW_REVISION 0)
 # Add the cmake folder so the FindSphinx module is found
 set(CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake" ${CMAKE_MODULE_PATH})
 
+set(FSFW_ETL_LIB_NAME etl)
 set(FSFW_ETL_LIB_MAJOR_VERSION 20 CACHE STRING
   "ETL library major version requirement"
 )
@@ -32,7 +33,16 @@ set(FSFW_CATCH2_LIB_VERSION v${FSFW_CATCH2_LIB_MAJOR_VERSION}.0.0-preview5 CACHE
     "Catch2 library exact version requirement"
 )
 
-set(FSFW_ETL_LIB_NAME etl)
+# Keep this off by default for now. See PR: https://egit.irs.uni-stuttgart.de/fsfw/fsfw/pulls/616
+# for information which keeping this on by default is problematic
+option(FSFW_ENABLE_IPO "Enable interprocedural optimization or link-time optimization if available" OFF)
+if(FSFW_ENABLE_IPO)
+  include(CheckIPOSupported)
+  check_ipo_supported(RESULT IPO_SUPPORTED OUTPUT IPO_ERROR)
+  if(NOT IPO_SUPPORTED)
+    message(STATUS "FSFW | IPO/LTO not supported: ${IPO_ERROR}")
+  endif()
+endif()
 
 option(FSFW_GENERATE_SECTIONS
   "Generate function and data sections. Required to remove unused code" ON
@@ -71,6 +81,10 @@ set(FSFW_DUMMY_TGT fsfw-dummy)
 
 add_library(${LIB_FSFW_NAME})
 
+if(IPO_SUPPORTED AND FSFW_ENABLE_IPO)
+  set_property(TARGET ${LIB_FSFW_NAME} PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+endif()
+
 if(FSFW_BUILD_UNITTESTS)
     message(STATUS "Building the FSFW unittests in addition to the static library")
     # Check whether the user has already installed Catch2 first
@@ -95,6 +109,9 @@ if(FSFW_BUILD_UNITTESTS)
 
     project(${FSFW_TEST_TGT} CXX C)
     add_executable(${FSFW_TEST_TGT})
+    if(IPO_SUPPORTED AND FSFW_ENABLE_IPO)
+      set_property(TARGET ${FSFW_TEST_TGT} PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+    endif()
 
     if(FSFW_TESTS_GEN_COV)
         message(STATUS "Generating coverage data for the library")