This test can not be optimized to always true:

    if (count > 0)
        total += unit_price(volume, count);

Are you confusing it with the opposite scenario:

    total += unit_price(volume, count);
    if (count == 0)
        printf("oops\n");

That test might be optimized out because divide by zero is undefined behavior.

I probably am confusing them. I don’t program in C or C++, so I’m not aware of all the “gotchas”, and can easily confuse them (as shown here). Not to mention that compilers have been getting more aggressive in optimizing undefined behavior.

What the compiler assumes depends on the compilation options.

Unfortunately most compilers have too permissive default options.

C and C++ programs should always be compiled, at least during development, with strict options, e.g.:

-fsanitize=undefined -fsanitize-undefined-trap-on-error

or equivalent options, which eliminate all the horror stories with unexpected behavior caused by integer divide-by-zero or overflow, pointer overflow, out-of-bounds access and many others.

Because unlike integer operations the FP operations are standardized, there is no need for compiler options, the behavior for exceptions can be controlled from the program with the functions defined in <fenv.h> in C or <cfenv> in C++.