From dc1446da9ed857e63e85bdfd09035fdebbcf4ffa Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@caltech.edu>
Date: Mon, 5 Jan 2026 21:57:04 +0000
Subject: [PATCH 1/4] Fix handling of user CXXFLAGS and add a DEBUG flag

The way `CXXFLAGS` and other flags were being set meant that users had
to add the default values (`-std=c++17 -fopenmp -O3` etc) if they set
the flags at all. The new approach here adds the user values to the
existing ones, so that they can override values more selectively.

This also adds a DEBUG flag that switches between using `-g -O0` and the
regular `-O3` option.
---
 Makefile | 30 ++++++++++++++++++++++++------
 1 file changed, 24 insertions(+), 6 deletions(-)

diff --git a/Makefile b/Makefile
index 4b37317c2..2cf6b1428 100644
--- a/Makefile
+++ b/Makefile
@@ -16,11 +16,12 @@
 EIGEN_PREFIX = "3bb6a48d8c171cf20b5f8e48bfb4e424fbd4f79e"
 EIGEN_URL = "https://gitlab.com/libeigen/eigen/-/archive/"
 
-# Default build targets. Additional may be added conditionally below.
+# Default build targets. Additional ones are added conditionally below.
 TARGETS = qsim
 TESTS = run-cxx-tests
 
 # By default, we also build the pybind11-based Python interface.
+# Can be overriden via env variables or command-line flags
 PYBIND11 ?= true
 
 ifeq ($(PYBIND11), true)
@@ -31,14 +32,31 @@ endif
 # Default options for Pytest (only used if the pybind interface is built).
 PYTESTFLAGS ?= -v
 
-# Default C++ compilers and compiler flags. Can be overriden via env variables.
+# Default compilers and compiler flags.
+# Can be overriden via env variables or command-line flags.
 CXX ?= g++
 NVCC ?= nvcc
 HIPCC ?= hipcc
 
-CXXFLAGS ?= -O3 -std=c++17 -fopenmp -flto=auto
-NVCCFLAGS ?= -O3 --std c++17 -Wno-deprecated-gpu-targets
-HIPCCFLAGS ?= -O3
+BASE_CXXFLAGS := -std=c++17 -fopenmp
+BASE_NVCCFLAGS := -std c++17 -Wno-deprecated-gpu-targets
+BASE_HIPCCFLAGS :=
+
+CXXFLAGS := $(BASE_CXXFLAGS) $(CXXFLAGS)
+NVCCFLAGS := $(BASE_NVCCFLAGS) $(NVCCFLAGS)
+HIPCCFLAGS := $(BASE_HIPCCFLAGS) $(HIPCCFLAGS)
+
+ifdef DEBUG
+    # Debug build: Add debug symbols & disable optimizations.
+    DEBUG_FLAGS := -g -O0
+    CXXFLAGS += $(DEBUG_FLAGS)
+    NVCCFLAGS += $(DEBUG_FLAGS)
+    HIPCCFLAGS += $(DEBUG_FLAGS)
+else
+    CXXFLAGS += -O3 -flto=auto
+    NVCCFLAGS += -O3
+    HIPCCFLAGS += -O3
+endif
 
 # For compatibility with CMake, if $CUDAARCHS is set, use it to set the
 # architecture options to nvcc. Otherwise, default to the "native" option,
@@ -91,7 +109,7 @@ ifneq (,$(strip $(CUQUANTUM_ROOT)))
     ifneq (,$(strip $(wildcard $(CUQUANTUM_ROOT)/.)))
         CUSVFLAGS =  -I$(CUQUANTUM_ROOT)/include
         CUSVFLAGS += -L${CUQUANTUM_ROOT}/lib -L$(CUQUANTUM_ROOT)/lib64
-        CUSVFLAGS += -lcustatevec -lcublas
+	    CUSVFLAGS += -Xcompiler \"-Wl,-rpath=${CUQUANTUM_ROOT}/lib\"
         CUSTATEVECFLAGS ?= $(CUSVFLAGS)
         TARGETS += qsim-custatevec
         TARGETS += qsim-custatevecex

From 3298cdc348e0c28c43e452c6de18429a60702934 Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@caltech.edu>
Date: Mon, 5 Jan 2026 21:58:45 +0000
Subject: [PATCH 2/4] Fix an accidental line deletion

---
 Makefile | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index 2cf6b1428..2bdf59065 100644
--- a/Makefile
+++ b/Makefile
@@ -109,7 +109,7 @@ ifneq (,$(strip $(CUQUANTUM_ROOT)))
     ifneq (,$(strip $(wildcard $(CUQUANTUM_ROOT)/.)))
         CUSVFLAGS =  -I$(CUQUANTUM_ROOT)/include
         CUSVFLAGS += -L${CUQUANTUM_ROOT}/lib -L$(CUQUANTUM_ROOT)/lib64
-	    CUSVFLAGS += -Xcompiler \"-Wl,-rpath=${CUQUANTUM_ROOT}/lib\"
+        CUSVFLAGS += -lcustatevec -lcublas
         CUSTATEVECFLAGS ?= $(CUSVFLAGS)
         TARGETS += qsim-custatevec
         TARGETS += qsim-custatevecex

From 292d5dfb94d2e72911960b86c4fc29eaabc617d8 Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@caltech.edu>
Date: Tue, 6 Jan 2026 04:10:04 +0000
Subject: [PATCH 3/4] Remove trivial comment

---
 Makefile | 1 -
 1 file changed, 1 deletion(-)

diff --git a/Makefile b/Makefile
index 2bdf59065..cb1d851d6 100644
--- a/Makefile
+++ b/Makefile
@@ -47,7 +47,6 @@ NVCCFLAGS := $(BASE_NVCCFLAGS) $(NVCCFLAGS)
 HIPCCFLAGS := $(BASE_HIPCCFLAGS) $(HIPCCFLAGS)
 
 ifdef DEBUG
-    # Debug build: Add debug symbols & disable optimizations.
     DEBUG_FLAGS := -g -O0
     CXXFLAGS += $(DEBUG_FLAGS)
     NVCCFLAGS += $(DEBUG_FLAGS)

From 6c33861bda36d171dd62ad9a132e73db51963dee Mon Sep 17 00:00:00 2001
From: mhucka <mhucka@caltech.edu>
Date: Tue, 6 Jan 2026 04:50:42 +0000
Subject: [PATCH 4/4] Use correct version of -flto flag for Clang

Clang doesn't understand `-flto=auto` and G++ doesn't understand `-flto`.
---
 Makefile | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/Makefile b/Makefile
index cb1d851d6..3e470c21f 100644
--- a/Makefile
+++ b/Makefile
@@ -46,13 +46,19 @@ CXXFLAGS := $(BASE_CXXFLAGS) $(CXXFLAGS)
 NVCCFLAGS := $(BASE_NVCCFLAGS) $(NVCCFLAGS)
 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
+endif
+
 ifdef DEBUG
     DEBUG_FLAGS := -g -O0
     CXXFLAGS += $(DEBUG_FLAGS)
     NVCCFLAGS += $(DEBUG_FLAGS)
     HIPCCFLAGS += $(DEBUG_FLAGS)
 else
-    CXXFLAGS += -O3 -flto=auto
+    CXXFLAGS += -O3 $(LTO_FLAGS)
     NVCCFLAGS += -O3
     HIPCCFLAGS += -O3
 endif