Why `-Og` over `-O0` ?

[antoine-bussy]

Hi 👋

When building with CMake in Debug mode, the qpp library sets the compiler optimization flag to -Og rather than -O0. For debugging, -O0 is generally preferred because it disables all optimizations and makes debugging more predictable. Using -Og can sometimes interfere with breakpoints and variable inspection in debuggers like GDB.

Moreover, this flag propagates to all consumer projects that link to qpp. As a result, the entire project (not just qpp) is built with -Og in Debug mode.

Is there a specific reason for choosing -Og over -O0 for Debug builds?

[vsoftco]

@antoine-bussy thanks for raising this. Do you have any example where -Og breaks debugging in some debugger like GDB (i.e., variables are not visible etc?). As far as I am aware of, -Og is recommended to -O0, as the latter can be painfully slow.

[antoine-bussy]

I had a dimension mismatch exception in QEngineState::reset (exception::DimsNotEqual) and was not able to set a breakpoint with gdb at the line I wanted because it was optimized out.

I can understand that you'd want -Og as a default while developing qpp. I just found out that it is recommended by the gcc documentation (https://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html) :

-Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable blend of optimization, fast compilation and debugging experience especially for code with a high abstraction penalty.

But it feels wrong to me to propagate it to consumer projects. It is the default of both gcc and cmake. Besides it can be hard to pinpoint that -Og is the cause of the difficulties to debug and that it comes from qpp.

[vsoftco]

I see... so you're saying that everything that compiles with qpp has -Og propagated into... Any ideas how to propagate a different flag to consumer projects? I'd prefer to not propagate -O0 instead.

[antoine-bussy]

It depends on the intent. Do you want to apply -Og to

1. libqpp
2. the targets that depend on it
3. its dependencies

If it's only 1 and 2, I would add

target_compile_options(libqpp INTERFACE $<BUILD_INTERFACE:$<$<CONFIG:Debug>:-Og>>)

or something similar, next to the add_library(libqpp ...)

If it's 1, 2 and 3, maybe the best solution would be to add

if (NOT DEFINED CMAKE_FIND_PACKAGE_NAME)
  set(CMAKE_CXX_FLAGS_DEBUG "${CMAKE_CXX_FLAGS_DEBUG} -Og")
endif()

in qpp_dependencies.

The second solution would be closer to the current implementation.