From ade72310b8dcb6ab50f6ea8904ad0bd9026b6b1c Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@google.com>
Date: Tue, 20 Jan 2026 04:32:00 +0000
Subject: [PATCH 1/5] Set LTO option only if not using debug config

Previously, the CMakeLists.txt files added `-flto` to some of the
compilation targets unconditionally. The addition should only be done if
the compiler supports LTO, and it shouldn't be done if the cmake
configuration is the debug configuration.
---
 CMakeLists.txt                         | 4 ++++
 pybind_interface/avx2/CMakeLists.txt   | 8 +++++++-
 pybind_interface/avx512/CMakeLists.txt | 8 +++++++-
 pybind_interface/basic/CMakeLists.txt  | 8 +++++++-
 pybind_interface/sse/CMakeLists.txt    | 8 +++++++-
 5 files changed, 32 insertions(+), 4 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 8b824c81d..604e39dc3 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -54,6 +54,10 @@ endif()
 
 find_package(OpenMP REQUIRED)
 
+# Check for LTO support (which in CMake is lumped in with IPO).
+include(CheckIPOSupported)
+check_ipo_supported(RESULT HAVE_IPO)
+
 # Always build the basic part.
 add_subdirectory(pybind_interface/basic)
 add_subdirectory(pybind_interface/decide)
diff --git a/pybind_interface/avx2/CMakeLists.txt b/pybind_interface/avx2/CMakeLists.txt
index b8a989d2b..eaee747ab 100644
--- a/pybind_interface/avx2/CMakeLists.txt
+++ b/pybind_interface/avx2/CMakeLists.txt
@@ -21,7 +21,7 @@ if(WIN32)
     # This prevents a conflict with /RTC1 in DEBUG builds.
     add_compile_options($<$<NOT:$<CONFIG:Debug>>:/O2>)
 elseif(LINUX)
-    add_compile_options(-mavx2 -mfma -O3 -flto=auto)
+    add_compile_options(-mavx2 -mfma -O3)
     execute_process(
         COMMAND bash --noprofile -c "grep -qs bmi2 /proc/cpuinfo"
         RESULT_VARIABLE _EXIT_CODE
@@ -50,3 +50,9 @@ include(../GetPybind11.cmake)
 pybind11_add_module(qsim_avx2 pybind_main_avx2.cpp)
 
 target_link_libraries(qsim_avx2 PUBLIC OpenMP::OpenMP_CXX)
+
+if(HAVE_IPO)
+    set_property(TARGET qsim_avx2 PROPERTY
+        INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
+    )
+endif()
diff --git a/pybind_interface/avx512/CMakeLists.txt b/pybind_interface/avx512/CMakeLists.txt
index a718a99f3..46321e11a 100644
--- a/pybind_interface/avx512/CMakeLists.txt
+++ b/pybind_interface/avx512/CMakeLists.txt
@@ -21,7 +21,7 @@ if(WIN32)
     # This prevents a conflict with /RTC1 in DEBUG builds.
     add_compile_options($<$<NOT:$<CONFIG:Debug>>:/O2>)
 else()
-    add_compile_options(-mavx512f -mbmi2 -O3 -flto=auto)
+    add_compile_options(-mavx512f -mbmi2 -O3)
 endif()
 
 if(APPLE)
@@ -43,3 +43,9 @@ include(../GetPybind11.cmake)
 pybind11_add_module(qsim_avx512 pybind_main_avx512.cpp)
 
 target_link_libraries(qsim_avx512 PUBLIC OpenMP::OpenMP_CXX)
+
+if(HAVE_IPO)
+    set_property(TARGET qsim_avx512 PROPERTY
+        INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
+    )
+endif()
diff --git a/pybind_interface/basic/CMakeLists.txt b/pybind_interface/basic/CMakeLists.txt
index c8731ec8a..380cf7cc8 100644
--- a/pybind_interface/basic/CMakeLists.txt
+++ b/pybind_interface/basic/CMakeLists.txt
@@ -21,7 +21,7 @@ if(WIN32)
     # This prevents a conflict with /RTC1 in DEBUG builds.
     add_compile_options($<$<NOT:$<CONFIG:Debug>>:/O2>)
 else()
-    add_compile_options(-O3 -flto=auto)
+    add_compile_options(-O3)
 endif()
 
 if(APPLE)
@@ -43,3 +43,9 @@ include(../GetPybind11.cmake)
 pybind11_add_module(qsim_basic pybind_main_basic.cpp)
 
 target_link_libraries(qsim_basic PUBLIC OpenMP::OpenMP_CXX)
+
+if(HAVE_IPO)
+    set_property(TARGET qsim_basic PROPERTY
+        INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
+    )
+endif()
diff --git a/pybind_interface/sse/CMakeLists.txt b/pybind_interface/sse/CMakeLists.txt
index 195af28c3..3562ca2b9 100644
--- a/pybind_interface/sse/CMakeLists.txt
+++ b/pybind_interface/sse/CMakeLists.txt
@@ -21,7 +21,7 @@ if(WIN32)
     # This prevents a conflict with /RTC1 in DEBUG builds.
     add_compile_options($<$<NOT:$<CONFIG:Debug>>:/O2>)
 else()
-    add_compile_options(-msse4 -O3 -flto=auto)
+    add_compile_options(-msse4 -O3)
 endif()
 
 if(APPLE)
@@ -43,3 +43,9 @@ include(../GetPybind11.cmake)
 pybind11_add_module(qsim_sse pybind_main_sse.cpp)
 
 target_link_libraries(qsim_sse PUBLIC OpenMP::OpenMP_CXX)
+
+if(HAVE_IPO)
+    set_property(TARGET qsim_sse PROPERTY
+        INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
+    )
+endif()

From c42bd3f52cc4e29d9345b768b4ae2426572ac960 Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@google.com>
Date: Tue, 20 Jan 2026 04:49:35 +0000
Subject: [PATCH 2/5] Encapsulate LTO option setting in a function

---
 CMakeLists.txt                         | 9 +++++++++
 pybind_interface/avx2/CMakeLists.txt   | 6 +-----
 pybind_interface/avx512/CMakeLists.txt | 6 +-----
 pybind_interface/basic/CMakeLists.txt  | 6 +-----
 pybind_interface/sse/CMakeLists.txt    | 6 +-----
 5 files changed, 13 insertions(+), 20 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 604e39dc3..e10984ee6 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -58,6 +58,15 @@ find_package(OpenMP REQUIRED)
 include(CheckIPOSupported)
 check_ipo_supported(RESULT HAVE_IPO)
 
+# Helper function for setting LTO flag (but only if the config is not debug).
+function(enable_lto target_name)
+    if(HAVE_IPO)
+        set_property(TARGET ${target_name} PROPERTY
+            INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
+        )
+    endif()
+endfunction()
+
 # Always build the basic part.
 add_subdirectory(pybind_interface/basic)
 add_subdirectory(pybind_interface/decide)
diff --git a/pybind_interface/avx2/CMakeLists.txt b/pybind_interface/avx2/CMakeLists.txt
index eaee747ab..411cada99 100644
--- a/pybind_interface/avx2/CMakeLists.txt
+++ b/pybind_interface/avx2/CMakeLists.txt
@@ -51,8 +51,4 @@ pybind11_add_module(qsim_avx2 pybind_main_avx2.cpp)
 
 target_link_libraries(qsim_avx2 PUBLIC OpenMP::OpenMP_CXX)
 
-if(HAVE_IPO)
-    set_property(TARGET qsim_avx2 PROPERTY
-        INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
-    )
-endif()
+enable_lto(qsim_avx2)
diff --git a/pybind_interface/avx512/CMakeLists.txt b/pybind_interface/avx512/CMakeLists.txt
index 46321e11a..7932b3d7b 100644
--- a/pybind_interface/avx512/CMakeLists.txt
+++ b/pybind_interface/avx512/CMakeLists.txt
@@ -44,8 +44,4 @@ pybind11_add_module(qsim_avx512 pybind_main_avx512.cpp)
 
 target_link_libraries(qsim_avx512 PUBLIC OpenMP::OpenMP_CXX)
 
-if(HAVE_IPO)
-    set_property(TARGET qsim_avx512 PROPERTY
-        INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
-    )
-endif()
+enable_lto(qsim_avx512)
diff --git a/pybind_interface/basic/CMakeLists.txt b/pybind_interface/basic/CMakeLists.txt
index 380cf7cc8..6b8a04b87 100644
--- a/pybind_interface/basic/CMakeLists.txt
+++ b/pybind_interface/basic/CMakeLists.txt
@@ -44,8 +44,4 @@ pybind11_add_module(qsim_basic pybind_main_basic.cpp)
 
 target_link_libraries(qsim_basic PUBLIC OpenMP::OpenMP_CXX)
 
-if(HAVE_IPO)
-    set_property(TARGET qsim_basic PROPERTY
-        INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
-    )
-endif()
+enable_lto(qsim_basic)
diff --git a/pybind_interface/sse/CMakeLists.txt b/pybind_interface/sse/CMakeLists.txt
index 3562ca2b9..4c5ee2b57 100644
--- a/pybind_interface/sse/CMakeLists.txt
+++ b/pybind_interface/sse/CMakeLists.txt
@@ -44,8 +44,4 @@ pybind11_add_module(qsim_sse pybind_main_sse.cpp)
 
 target_link_libraries(qsim_sse PUBLIC OpenMP::OpenMP_CXX)
 
-if(HAVE_IPO)
-    set_property(TARGET qsim_sse PROPERTY
-        INTERPROCEDURAL_OPTIMIZATION $<NOT:$<CONFIG:Debug>>
-    )
-endif()
+enable_lto(qsim_sse)

From d1545c12288269c80a6de051669e0d87af890d57 Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@google.com>
Date: Tue, 20 Jan 2026 20:01:26 +0000
Subject: [PATCH 3/5] The Makefile should also test if can use `-fopenmp`

The CMake files now check if OpenMP can be used. The Makefiles should too.
---
 Makefile | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/Makefile b/Makefile
index 0f7e4ed25..cf5c60e94 100644
--- a/Makefile
+++ b/Makefile
@@ -38,7 +38,7 @@ CXX ?= g++
 NVCC ?= nvcc
 HIPCC ?= hipcc
 
-BASE_CXXFLAGS := -std=c++17 -fopenmp
+BASE_CXXFLAGS := -std=c++17
 BASE_NVCCFLAGS := -std c++17 -Wno-deprecated-gpu-targets
 BASE_HIPCCFLAGS :=
 
@@ -48,8 +48,16 @@ HIPCCFLAGS := $(BASE_HIPCCFLAGS) $(HIPCCFLAGS)
 
 LTO_FLAGS := -flto=auto
 USING_CLANG := $(shell $(CXX) --version | grep -isq clang && echo "true")
-ifeq ($(USING_CLANG),"true")
-	LTO_FLAGS := -flto
+ifeq ($(USING_CLANG),true)
+    LTO_FLAGS := -flto
+endif
+
+# Test if OpenMP header files are available and we can link with the library.
+OMP_CHECK_CMD := echo "int main() { return 0; }" | \
+    $(CXX) -fopenmp -x c++ - -o /dev/null 2>/dev/null
+HAVE_OPENMP := $(shell $(OMP_CHECK_CMD) && echo "true")
+ifeq ($(HAVE_OPENMP),true)
+    OPENMP_FLAGS := -fopenmp
 endif
 
 ifdef DEBUG
@@ -58,7 +66,7 @@ ifdef DEBUG
     NVCCFLAGS += $(DEBUG_FLAGS)
     HIPCCFLAGS += $(DEBUG_FLAGS)
 else
-    CXXFLAGS += -O3 $(LTO_FLAGS)
+    CXXFLAGS += -O3 $(OPENMP_FLAGS) $(LTO_FLAGS)
     NVCCFLAGS += -O3
     HIPCCFLAGS += -O3
 endif

From c5369d98a33d35629b8fe7571af1138a1fd1c7a7 Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@google.com>
Date: Tue, 20 Jan 2026 20:47:37 +0000
Subject: [PATCH 4/5] Revert change made in incorrect branch

---
 Makefile | 12 ++----------
 1 file changed, 2 insertions(+), 10 deletions(-)

diff --git a/Makefile b/Makefile
index cf5c60e94..dc87de73d 100644
--- a/Makefile
+++ b/Makefile
@@ -38,7 +38,7 @@ CXX ?= g++
 NVCC ?= nvcc
 HIPCC ?= hipcc
 
-BASE_CXXFLAGS := -std=c++17
+BASE_CXXFLAGS := -std=c++17 -fopenmp
 BASE_NVCCFLAGS := -std c++17 -Wno-deprecated-gpu-targets
 BASE_HIPCCFLAGS :=
 
@@ -52,21 +52,13 @@ ifeq ($(USING_CLANG),true)
     LTO_FLAGS := -flto
 endif
 
-# Test if OpenMP header files are available and we can link with the library.
-OMP_CHECK_CMD := echo "int main() { return 0; }" | \
-    $(CXX) -fopenmp -x c++ - -o /dev/null 2>/dev/null
-HAVE_OPENMP := $(shell $(OMP_CHECK_CMD) && echo "true")
-ifeq ($(HAVE_OPENMP),true)
-    OPENMP_FLAGS := -fopenmp
-endif
-
 ifdef DEBUG
     DEBUG_FLAGS := -g -O0
     CXXFLAGS += $(DEBUG_FLAGS)
     NVCCFLAGS += $(DEBUG_FLAGS)
     HIPCCFLAGS += $(DEBUG_FLAGS)
 else
-    CXXFLAGS += -O3 $(OPENMP_FLAGS) $(LTO_FLAGS)
+    CXXFLAGS += -O3 $(LTO_FLAGS)
     NVCCFLAGS += -O3
     HIPCCFLAGS += -O3
 endif

From da24b586e638693fe98216818cd52971e2b73b18 Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@google.com>
Date: Wed, 21 Jan 2026 15:14:53 +0000
Subject: [PATCH 5/5] Fix incorrect elseif()
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

As noted by Sergei, `elseif(LINUX)` → `else()`.
---
 pybind_interface/avx2/CMakeLists.txt | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/pybind_interface/avx2/CMakeLists.txt b/pybind_interface/avx2/CMakeLists.txt
index 417d1f147..f79bc0738 100644
--- a/pybind_interface/avx2/CMakeLists.txt
+++ b/pybind_interface/avx2/CMakeLists.txt
@@ -20,7 +20,7 @@ if(MSVC)
     # Add /O2 to any configuration that is NOT Debug.
     # This prevents a conflict with /RTC1 in DEBUG builds.
     add_compile_options($<$<NOT:$<CONFIG:Debug>>:/O2>)
-elseif(LINUX)
+else()
     add_compile_options(-mavx2 -mfma -O3)
     execute_process(
         COMMAND bash --noprofile -c "grep -qs bmi2 /proc/cpuinfo"