From c36cff8937ae2a370cc2ea76d48c11be616dbb6a Mon Sep 17 00:00:00 2001
From: Harmenszoon <25753539+Harmenszoon@users.noreply.github.com>
Date: Mon, 11 May 2026 14:41:30 -0400
Subject: [PATCH] [Build] enable IPO/LTO for release builds (opt-out via
 -DSC_ENABLE_LTO=OFF)

Adds SC_ENABLE_LTO, default ON, and uses check_ipo_supported() to enable target-level IPO/LTO only for non-Debug configurations. Unsupported toolchains emit a status message and continue without IPO/LTO.

Initial no-code A/B on MSVC 19.44.35217.0, CMake 3.31.6, Ninja, Release BUILD_GUI=OFF compared baseline /O2 /Ob2 /DNDEBUG against IPO /GL + /LTCG with threads=1 target_error=0 deterministic=1 fixed seed. DPS and TotalEvents matched exactly. Mean/median wins: CI_ST_10 5.48/6.23%; Windwalker DungeonSlice 9.26/10.65%; Windwalker ST 12.48/12.21%; Shadow Priest ST 10.40/10.35%; Havoc DH 5T 8.91/9.47%; Dev Evoker 5T 9.05/10.32%.

Build time on that toolchain was 36.9s baseline vs 59.8s IPO. The full CI DungeonSlice profile was invalid because Demon Hunter disables that fight style by default (engine/class_modules/sc_demon_hunter.cpp:11273), so Windwalker DungeonSlice was used as the substituted control.

Patched CMake validation using SC_ENABLE_LTO=OFF vs default ON remained output-equivalent: CI_ST_10 mean/median wins 3.74/4.63%; Windwalker ST 11.45/10.85%. The existing MSVC C4244 warning at engine/class_modules/sc_druid.cpp:8909 was unchanged.
---
 CMakeLists.txt        | 22 ++++++++++++++++++++++
 engine/CMakeLists.txt |  1 +
 qt/CMakeLists.txt     |  3 ++-
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8662ba4b15e..43c4daa7b06 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -17,6 +17,7 @@ option(SC_NO_NETWORKING "Disable all networking related stuff." OFF)
 option(SC_USE_FLAT_INSTALL "Install files into a flat folder structure" ${WIN32})
 
 option(SC_EVENT_QUEUE_DEBUG "Enable Event Queue Debug Info" OFF)
+option(SC_ENABLE_LTO "Enable IPO/LTO for non-Debug builds" ON)
 
 set(CMAKE_CXX_STANDARD 17 CACHE STRING "C++ standard to conform to")
 set(CMAKE_CXX_STANDARD_REQUIRED YES)
@@ -53,6 +54,26 @@ function(sc_common_compiler_options target)
   )
 endfunction()
 
+if(SC_ENABLE_LTO)
+  include(CheckIPOSupported)
+  check_ipo_supported(RESULT SC_IPO_SUPPORTED OUTPUT SC_IPO_OUTPUT LANGUAGES CXX)
+  if(SC_IPO_SUPPORTED)
+    message(STATUS "IPO/LTO enabled for non-Debug builds")
+  else()
+    message(STATUS "IPO/LTO requested but not supported by this toolchain: ${SC_IPO_OUTPUT}")
+  endif()
+endif()
+
+function(sc_enable_lto target)
+  if(SC_ENABLE_LTO AND SC_IPO_SUPPORTED)
+    set_target_properties(${target} PROPERTIES
+      INTERPROCEDURAL_OPTIMIZATION_RELEASE ON
+      INTERPROCEDURAL_OPTIMIZATION_RELWITHDEBINFO ON
+      INTERPROCEDURAL_OPTIMIZATION_MINSIZEREL ON
+    )
+  endif()
+endfunction()
+
 
 ### Git Hash ###
 # Get the current working branch
@@ -80,6 +101,7 @@ endif()
 add_executable(simc engine/sc_main.cpp)
 target_link_libraries(simc engine)
 sc_common_compiler_options(simc)
+sc_enable_lto(simc)
 
 install(TARGETS simc DESTINATION ${SIMC_INSTALL_BIN})
 
diff --git a/engine/CMakeLists.txt b/engine/CMakeLists.txt
index f680840d5c9..a457aba3587 100644
--- a/engine/CMakeLists.txt
+++ b/engine/CMakeLists.txt
@@ -9,6 +9,7 @@ include(../source_files/cmake_engine.txt)
 add_library(engine ${source_files})
 target_include_directories(engine PUBLIC . ./include ./lib)
 sc_common_compiler_options(engine)
+sc_enable_lto(engine)
 
 # Make cmake selections visible to C++ code
 if(SC_NO_THREADING)
diff --git a/qt/CMakeLists.txt b/qt/CMakeLists.txt
index d2b11b18dd4..df068722b40 100644
--- a/qt/CMakeLists.txt
+++ b/qt/CMakeLists.txt
@@ -36,6 +36,7 @@ if (${QT_VERSION_MAJOR} EQUAL 6)
 endif()
 
 sc_common_compiler_options(SimulationCraft)
+sc_enable_lto(SimulationCraft)
 
 target_include_directories(SimulationCraft PUBLIC ../engine/)
 target_link_libraries(SimulationCraft 
@@ -74,4 +75,4 @@ DESTINATION ${SIMC_INSTALL_SHARED}/locale)
 if(WIN32)
   include(${PROJECT_SOURCE_DIR}/cmake/windeployqt.cmake)
   windeployqt(SimulationCraft)
-endif()
\ No newline at end of file
+endif()