Make CMake only include debug symbols when appropriate

[bytzo]

Recently, I noticed that GDExtension shared libraries using godot-cpp that were built with CMake had significantly larger file sizes than the equivalent shared libraries built with SCons.

Pre-analysis

For the SCons project, I followed the GDExtension C++ example. To enable debug symbols, I added debug_symbols=True to the scons command. When debug symbols were disabled, I also set release optimizations through target=template_release since I used CMAKE_BUILD_TYPE=Release with CMake to avoid setting debug symbols through the default Debug build type.

For the CMake project, I used the same project but with this custom CMakeLists.txt file. To enable debug symbols, I configured the build with CMAKE_BUILD_TYPE=Debug. As noted above, when not building with debug symbols, I configured the build with CMAKE_BUILD_TYPE=Release as not setting anything would have fallen back to the default of CMAKE_BUILD_TYPE=Debug.

Build tool	Build type	Debug symbols	.so size
SCons	Release	No	5.9 MB
CMake	Release	No	27.6 MB
SCons	Debug	Yes	30.7 MB
CMake	Debug	Yes	27.6 MB

Notice how enabling debug symbols did not change the file size of the CMake-built shared library. It seems that, despite setting CMAKE_BUILD_TYPE=Release, debug symbols are still being added.

Analysis

Looking at the CMakeLists.txt file in this repository, I noticed that, at least for non-MSVC compilers, the -g flag was always being set regardless if the build was configured to be a debug build or not. This forces the project to build with debug symbols and ignores the CMAKE_BUILD_TYPE.

godot-cpp/CMakeLists.txt

Lines 101 to 109 in c1196a1

	else() # GCC/Clang
	set(GODOT_COMPILE_FLAGS "${GODOT_COMPILE_FLAGS} -g")
	if(CMAKE_BUILD_TYPE MATCHES Debug)
	set(GODOT_COMPILE_FLAGS "${GODOT_COMPILE_FLAGS} -fno-omit-frame-pointer -O0")
	else()
	set(GODOT_COMPILE_FLAGS "${GODOT_COMPILE_FLAGS} -O3")
	endif(CMAKE_BUILD_TYPE MATCHES Debug)
	endif()

To my knowledge, there isn't an easy way for others using godot-cpp's CMakeLists.txt to workaround this if they wish to build their project without debug symbols, unless they strip them from the shared library after the fact.

Changes

In order to allow projects to choose whether or not to build with debug symbols, I removed the line setting the -g flag. This is because CMake already sets the -g flag automatically based on the CMAKE_BUILD_TYPE.

However, I also noticed several other flags, such as -O0 and -O3, were being set. These flags are similarly automatically set by CMake depending on the value of CMAKE_BUILD_TYPE. My assumption here is that the godot-cpp CMake file is trying to be as explicit as possible. So, to maintain consistency, I added the -g flag alongside the other flags that are set when a Debug CMake build is run.

Results

Build tool	Build type	Debug symbols	.so size
SCons	Release	No	5.7 MB
CMake	Release	No	5.7 MB
SCons	Debug	Yes	30.7 MB
CMake	Debug	Yes	27.6 MB

Now, it is possible to build the shared library without debug symbols using CMake. That is about an 80% reduction in release shared library size for projects building with CMake, which also happens to be consistent with SCons.

[dsnopek]

Thanks!

I was able to reproduce the issue, and this PR fixes it in my testing.

[akien-mga]

Thanks!

[dsnopek]

Cherry-picking to 4.1 in #1373