指定-O0仍有变量被优化，如何在CMake中禁用LTO？

问题根源分析 #

你遇到的核心问题是MPI编译器包装器（mpicxx）默认注入了LTO相关编译标志，或者CMake设置的优先级/顺序不对，导致CMAKE_INTERPROCEDURAL_OPTIMIZATION FALSE未生效。以下是针对性的解决步骤：

解决方案1：调整CMake设置顺序与目标级LTO禁用 #

将全局LTO禁用指令移到find_package(MPI)之后，并为每个目标单独显式禁用LTO（避免MPI配置覆盖全局设置）：
修改你的CMakeLists.txt关键部分：

cmake_minimum_required(VERSION 3.21)
project(Patch CXX)

set(CMAKE_C_COMPILER mpicc)
set(CMAKE_CXX_COMPILER /usr/bin/mpicxx)
set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g -DDEBUG")
set(CMAKE_CXX_STANDARD 17)
set(CMAKE_DEBUG_POSTFIX d)

# 先加载MPI，再设置全局LTO禁用
find_package(MPI REQUIRED)
set(CMAKE_INTERPROCEDURAL_OPTIMIZATION OFF)
# 针对Debug模式单独强制禁用
set(CMAKE_INTERPROCEDURAL_OPTIMIZATION_DEBUG OFF)

set(MPI_INCLUDE_PATH /usr/include/mpich)
message(STATUS "Run: ${MPIEXEC} ${MPIEXEC_NUMPROC_FLAG} ${MPIEXEC_MAX_NUMPROCS} ${MPIEXEC_PREFLAGS} EXECUTABLE ${MPIEXEC_POSTFLAGS} ARGS")

link_directories(${CMAKE_SOURCE_DIR}/libs)
include_directories(${CMAKE_SOURCE_DIR}/include, ${MPI_INCLUDE_PATH})

ADD_LIBRARY(PATCH_NODES src/Patch.cpp src/Node.cpp)
target_include_directories(PATCH_NODES PUBLIC include ${MPI_INCLUDE_PATH})
# 为库目标显式禁用LTO
set_property(TARGET PATCH_NODES PROPERTY INTERPROCEDURAL_OPTIMIZATION OFF)
set_property(TARGET PATCH_NODES PROPERTY INTERPROCEDURAL_OPTIMIZATION_DEBUG OFF)
target_compile_options(PATCH_NODES PUBLIC -O0 -g -Wall -Wextra -Wpedantic)
target_link_libraries(PATCH_NODES PUBLIC MPI::MPI_CXX)

add_executable(TEST src/main.cpp)
# 为可执行目标显式禁用LTO
set_property(TARGET TEST PROPERTY INTERPROCEDURAL_OPTIMIZATION OFF)
set_property(TARGET TEST PROPERTY INTERPROCEDURAL_OPTIMIZATION_DEBUG OFF)
target_compile_options(TEST PUBLIC -O0 -g -Wall -Wextra -Wpedantic)
target_include_directories(TEST PUBLIC include)
target_link_libraries(TEST PUBLIC MPI::MPI_CXX PATCH_NODES)

解决方案2：手动过滤MPI编译器自带的LTO标志 #

如果MPI编译器包装器（mpicxx）默认添加了LTO参数，需要在CMake中显式移除：
在CMakeLists.txt中添加以下代码（放在编译选项设置之前）：

# 从Debug模式编译标志中移除LTO相关参数
string(REPLACE "-flto=auto" "" CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG}")
string(REPLACE "-ffat-lto-objects" "" CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG}")

# 也可以针对单个目标的编译选项做过滤
target_compile_options(PATCH_NODES PUBLIC "$<$<CONFIG:DEBUG>:-O0 -g -Wall -Wextra -Wpedantic>")
target_compile_options(TEST PUBLIC "$<$<CONFIG:DEBUG>:-O0 -g -Wall -Wextra -Wpedantic>")

解决方案3：清理CMake缓存并重新构建 #

之前的构建缓存可能保留了旧的LTO配置，必须彻底清理后重新生成：

# 假设你的构建目录是当前目录下的build文件夹
rm -rf build/*
# 重新生成CMake配置
/usr/bin/cmake --no-warn-unused-cli -DCMAKE_BUILD_TYPE:STRING=Debug -DCMAKE_EXPORT_COMPILE_COMMANDS:BOOL=TRUE -S./ -B./build -G "Unix Makefiles"
# 重新编译
make -C build

验证结果 #

重新编译后，查看compile_commands.json中的编译命令，确认-flto=auto和-ffat-lto-objects已被移除，此时调试时变量不会被优化。

内容的提问来源于stack exchange，提问作者adm0n