G++/CMake Profile Guided Optimization cannot find generated .gcda files

Question

I am compiling an application using CMake 3.16.3 and G++ 9.3 on Ubuntu 20.04. This is the current source directory:

. (this is called MyApp)
├── src
│   ├── /* source files */
│   └── CMakeLists.txt
├── tests
│   ├── /* source files */
│   └── CMakeLists.txt
├── build-release
│   └── pgo /* folder for .gcda files */
├── build_release.sh
└── CMakeLists.txt*

I am setting the flags in CMakeLists.txt (the root one also marked with asterisk) as follows:

set(MYAPP_PGO "-fprofile-dir=${MYAPP_PGO} -fprofile-generate=${MYAPP_PGO}")

// apply flags
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNDEBUG -O3 -march=native -flto ${MYAPP_PGO}")
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${MYAPP_PGO}")
set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${MYAPP_PGO}")
set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${MYAPP_PGO}")

add_subdirectory(src)
add_subdirectory(tests)

No flags are being set in src/CMakeLists.txt and tests/CMakeLists.txt. MYAPP_PGO is set in bash and is the absolute path of build-release/pgo. I have a script (build_release.sh) that builds the program:

# excluded other lines and flags for brevity

cd build-release
cmake -DMYAPP_PGO="$(pwd)/pgo/" ..
make -j1

After the profile run, I see that there are multiple files in build-release/pgo, one for each cpp file in src and test, such as

#home#johndoe#MyApp#build-release#src#CMakeFiles#MYAPPLICATION_myapp.dir#sample_source.cpp.gcda

However, when compiling again with

// this is a multithreaded program
set(MYAPP_PGO "-fprofile-dir=${MYAPP_PGO} -fprofile-use=${MYAPP_PGO} -fprofile-correction")

// apply flags
set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -DNDEBUG -O3 -march=native -flto ${MYAPP_PGO}")
set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS} ${MYAPP_PGO}")
set(CMAKE_SHARED_LINKER_FLAGS "${CMAKE_SHARED_LINKER_FLAGS} ${MYAPP_PGO}")
set(CMAKE_MODULE_LINKER_FLAGS "${CMAKE_MODULE_LINKER_FLAGS} ${MYAPP_PGO}")

add_subdirectory(src)
add_subdirectory(tests)

I get a warning stating that there are missing profile files:

/home/johndoe/MyApp/src/sample_source.cpp: In function ‘_GLOBAL__sub_I_sample_source.cpp’:
/home/johndoe/MyApp/src/sample_source.cpp:215:1: warning: ‘/home/johndoe/MyApp/src/sample_source.cpp/build-release/pgo//#home#johndoe#MyApp#build-release#src#CMakeFiles#MYAPPLICATION_myapp.dir#sample_source.cpp.gcda’ profile count data file not found [-Wmissing-profile]

215:1 refers to the last character in this source file.

I have tested the speed of the program with and without PGO and have seen no improvement. Although PGO is not guaranteed to bring any speed ups, I tend to believe that PGO did not work as expected here. Am I doing something wrong?

Answer 1

For anyone wondering...

I did not write the original build system of the project. I discovered that it compiled one executable and one library from the same source. Whilst the executable was profiled and recompiled successfully (PGO was done flawlessly), the library was not used anywhere. Hence the missing profile files. Because the profiler outputs extremely long names, I thought the error came from the executable. Thank you all for the help.