-O0 doesn't change what the compiler code generation is allowed to do around UB. Optimizing tends to expose UB by producing undesired operation, but the optimizer is only allowed to make transformations that result in the code still complying with the standard. So code generation without the optimizer could still produce undesired operation, and turning the optimizer off is no guarantee that it'll do what you want.
If you want code to be run line by line as written in source file I think you want -Og. O0 means optimize compilation speed.
Wouldn’t recommend relying on either of them for making your build correct. It breeds the false accusation that it’s the optimizations that break your code, when the code is in fact already broken and might fail for other reasons.
At least for GCC/Clang this isn't what O0 means. Excerpt from the GCC manual:
> Most optimizations are completely disabled at -O0 or if an -O level is not set on the command line, even if individual optimization flags are specified.
And
> Optimize debugging experience. -Og should be the optimization level of choice for the standard edit-compile-debug cycle, offering a reasonable level of optimization while maintaining fast compilation and a good debugging experience. It is a better choice than -O0 for producing debuggable code because some compiler passes that collect debug information are disabled at -O0.
(Og isn't really implemented in clang yet, but the mindset is the same)
