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Since compilers (and by "compilers" I mean gcc mostly) quietly break your code behind your back, you have no way of telling whether you really fixed things or not.

Compile your test suite with -fsanitize=undefined.

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That will catch a tiny part of undefined behavior.

Why doesn't the compiler emit a warning for all UB it finds while compiling? Or do regular programs rely on this too much to be feasible? It must be something like that, or there'd not be much performance gains to be had by exploiting UB right?

Any time you add two (signed) integers, that's potentially undefined behaviour.

It's not necessarily detectable statically.

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