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?
