From: Wolfgang Bangerth <bangerth@colostate.edu>
Date: Fri, 18 Oct 2024 23:31:26 +0000 (-0600)
Subject: Enable the use of interprocedural/link-time optimization.
X-Git-Url: https://gitweb.dealii.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=898f51baec42e2cdcfb9b10629262c1cd1b106b4;p=dealii.git

Enable the use of interprocedural/link-time optimization.
---

diff --git a/cmake/checks/check_02_compiler_features.cmake b/cmake/checks/check_02_compiler_features.cmake
index 59251dca98..cab88ab5bb 100644
--- a/cmake/checks/check_02_compiler_features.cmake
+++ b/cmake/checks/check_02_compiler_features.cmake
@@ -407,3 +407,25 @@ if(NOT CMAKE_CXX_COMPILER_ID MATCHES "MSVC")
 
   reset_cmake_required()
 endif()
+
+
+#
+# Check whether the compiler supports interprocedural and link-time
+# optimization. If so, we can set the corresponding property on the
+# compile and link targets later on.
+#
+if(DEAL_II_USE_LTO)
+  include(CheckIPOSupported)
+  check_ipo_supported(RESULT _res OUTPUT _out LANGUAGES CXX)
+  if (_res)
+    message(STATUS "Compiler supports interprocedural/link-time optimizations")
+    set(DEAL_II_USE_LTO "YES")
+  else()
+    message(FATAL_ERROR "You asked for interprocedural/link-time optimizations, but the compiler does not support these optimizations: ${_out}")
+    set(DEAL_II_USE_LTO "NO")
+  endif()
+else()
+  # User did not set DEAL_II_USE_LTO on the command line. Set the
+  # variable to a specific (negative) value.
+  set(DEAL_II_USE_LTO "NO")
+endif()
diff --git a/cmake/macros/macro_populate_target_properties.cmake b/cmake/macros/macro_populate_target_properties.cmake
index b85a803ccd..7223089c5b 100644
--- a/cmake/macros/macro_populate_target_properties.cmake
+++ b/cmake/macros/macro_populate_target_properties.cmake
@@ -15,7 +15,7 @@
 #
 # populate_target_properties(<target> <build>)
 #
-# This function populate target properties according to (globally) defined
+# This function populates target properties according to (globally) defined
 # DEAL_II_* variables. Specifically:
 #
 #   DEAL_II_LIBRARIES DEAL_II_LIBRARIES_<build>
@@ -126,4 +126,13 @@ function(populate_target_properties _target _build)
     ${DEAL_II_LIBRARIES} ${DEAL_II_LIBRARIES_${_build}}
     ${DEAL_II_TARGETS} ${DEAL_II_TARGETS_${_build}}
     )
+
+
+   # For release builds (and their corresponding object files),
+   # use interprocedural optimizations if possible
+   if (DEAL_II_USE_LTO AND ("${_build}" STREQUAL "RELEASE"))
+     set_property(TARGET ${_target}
+                  PROPERTY INTERPROCEDURAL_OPTIMIZATION TRUE)
+   endif()
+
 endfunction()
diff --git a/cmake/setup_cached_variables.cmake b/cmake/setup_cached_variables.cmake
index b3fc8b9e5d..eb2d5957bf 100644
--- a/cmake/setup_cached_variables.cmake
+++ b/cmake/setup_cached_variables.cmake
@@ -31,6 +31,7 @@
 #
 #     CMAKE_BUILD_TYPE
 #     DEAL_II_ALLOW_PLATFORM_INTROSPECTION
+#     DEAL_II_USE_LTO
 #     DEAL_II_SETUP_COVERAGE
 #     DEAL_II_UNITY_BUILD
 #     DEAL_II_EARLY_DEPRECATIONS
@@ -159,6 +160,12 @@ option(DEAL_II_ALLOW_PLATFORM_INTROSPECTION
   )
 mark_as_advanced(DEAL_II_ALLOW_PLATFORM_INTROSPECTION)
 
+option(DEAL_II_USE_LTO
+  "Allow the compiler to use interprocedural and link-time optimization (LTO)."
+  OFF
+  )
+mark_as_advanced(DEAL_II_USE_LTO)
+
 option(DEAL_II_SETUP_COVERAGE
   "Setup debug compiler flags to provide additional test coverage information. Currently only gprof is supported."
   OFF
diff --git a/doc/users/cmake_dealii.html b/doc/users/cmake_dealii.html
index ddc2f12b8e..26305f34dd 100644
--- a/doc/users/cmake_dealii.html
+++ b/doc/users/cmake_dealii.html
@@ -781,6 +781,14 @@ cmake -DLAPACK_FOUND=true \
           (default), <acronym>deal.II</acronym> will perform platform
           introspection for the given CPU.
 
+        <li>
+          <code>DEAL_II_USE_LTO</code>: If set to "ON" (the default is
+          "OFF"), allow the compiler to use interprocedural and
+          link-time optimization for release builds. These
+          optimizations often substantially improve performance, but
+          take both a substantial amount of memory and CPU time when
+          compiling the library.
+
         <li>
           <code>BUILD_SHARED_LIBS</code>: If set (default),
           <acronym>deal.II</acronym> will be linked as a shared library
diff --git a/source/CMakeLists.txt b/source/CMakeLists.txt
index 5d4f55ba06..502e7e921d 100644
--- a/source/CMakeLists.txt
+++ b/source/CMakeLists.txt
@@ -94,6 +94,7 @@ foreach(build ${DEAL_II_BUILD_TYPES})
     set(build_camelcase "Release")
   endif()
 
+
   #
   # Combine all ${build} OBJECT targets to a ${build} library:
   #