CMake: IPO/LTO (#1890)

Add optional interprocedural optimization (IPO) aka
link-time optimization (LTO) to AMReX.

In my WarpX tests recently on x86_64 and ppc64le, using those options
incurred a 2% performance hit (although one would expect a slight
increase). Thus, they are not enabled by default. Also, enabling
them with some toolchains needs users to set the linker properly.

Nontheless, enabling IPO shrinks the binary size a lot. This is
very valuable and worth the runtime penalty for our binary
deployments, e.g. when shipping generic x86_64, aarch64 and ppc64le
binaries on conda-forge to users.

WarpX binary size (default build):
* no IPO: 217MiB
* WarpX IPO, AMReX no IPO: 155MiB
* both WarpX and AMReX with IPO: 127M

Author: ax3l

Docs/sphinx_documentation/source/BuildingAMReX.rst

@@ -433,6 +433,8 @@ The list of available options is reported in the :ref:`table <tab:cmakevar>` bel
    +------------------------------+-------------------------------------------------+-------------------------+-----------------------+
    | AMReX_PIC                    |  Build Position Independent Code                | NO                      | YES, NO               |
    +------------------------------+-------------------------------------------------+-------------------------+-----------------------+
+   | AMReX_IPO                    |  Interprocedural optimization (IPO/LTO)         | NO                      | YES, NO               |
+   +------------------------------+-------------------------------------------------+-------------------------+-----------------------+
    | AMReX_MPI                    |  Build with MPI support                         | YES                     | YES, NO               |
    +------------------------------+-------------------------------------------------+-------------------------+-----------------------+
    | AMReX_OMP                    |  Build with OpenMP support                      | NO                      | YES, NO               |

Tools/CMake/AMReXBuildInfo.cmake

@@ -226,4 +226,16 @@ function (generate_buildinfo _target _git_dir)
       POSITION_INDEPENDENT_CODE ${_pic}
       WINDOWS_EXPORT_ALL_SYMBOLS ${_sym} )
 
+   # IPO/LTO
+   if (AMReX_IPO)
+      include(CheckIPOSupported)
+      check_ipo_supported(RESULT is_IPO_available)
+      if(is_IPO_available)
+          set_target_properties(buildInfo${_target} PROPERTIES
+              INTERPROCEDURAL_OPTIMIZATION TRUE)
+      else()
+          message(FATAL_ERROR "Interprocedural optimization is not available, set AMReX_IPO=OFF")
+      endif()
+   endif()
+
 endfunction ()

Tools/CMake/AMReXOptions.cmake

@@ -269,6 +269,9 @@ endif ()
 option( AMReX_PIC "Build position-independent code" OFF)
 print_option( AMReX_PIC )
 
+option( AMReX_IPO "Enable interprocedural optimization (IPO/LTO)" OFF)
+print_option( AMReX_IPO )
+
 option(AMReX_FPE "Enable Floating Point Exceptions checks" OFF)
 print_option( AMReX_FPE )
 

Tools/CMake/AMReXTypecheck.cmake

@@ -182,6 +182,18 @@ function( add_typecheck_target _target)
       WINDOWS_EXPORT_ALL_SYMBOLS ON
       )
 
+   # IPO/LTO
+   if (AMReX_IPO)
+      include(CheckIPOSupported)
+      check_ipo_supported(RESULT is_IPO_available)
+      if(is_IPO_available)
+          set_target_properties(${_typecheckobjs} PROPERTIES
+              INTERPROCEDURAL_OPTIMIZATION TRUE)
+      else()
+          message(FATAL_ERROR "Interprocedural optimization is not available, set AMReX_IPO=OFF")
+      endif()
+   endif()
+
    # >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>
    # STEP 2: create CPPD files from C++ headers
    # <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<

Tools/CMake/AMReX_Config.cmake

@@ -148,6 +148,17 @@ function (configure_amrex)
         WINDOWS_EXPORT_ALL_SYMBOLS ON )
    endif ()
 
+   # IPO/LTO
+   if (AMReX_IPO)
+      include(CheckIPOSupported)
+      check_ipo_supported(RESULT is_IPO_available)
+      if(is_IPO_available)
+          set_target_properties(amrex PROPERTIES INTERPROCEDURAL_OPTIMIZATION TRUE)
+      else()
+          message(FATAL_ERROR "Interprocedural optimization is not available, set AMReX_IPO=OFF")
+      endif()
+   endif()
+
    #
    # Setup third-party profilers
    #