The optimizers are much stronger nowadays. They rewrite the program, so that the resulting assembly might have nothing to do with the code you wrote.
Especially if undefined behavior is invalid. Decades ago you did not need to care about undefined behavior. You write a + b, and you know the compiler emits an ADD instruction for +, and that ADD of x86 does not distinguish between signed and unsigned instructions, and you get the same result for signed and unsigned numbers, regardless of overflow. But nowadays the optimizer comes, says, wait, signed overflow is undefined, I will optimize the entire function away.
Especially if undefined behavior is invalid. Decades ago you did not need to care about undefined behavior. You write a + b, and you know the compiler emits an ADD instruction for +, and that ADD of x86 does not distinguish between signed and unsigned instructions, and you get the same result for signed and unsigned numbers, regardless of overflow. But nowadays the optimizer comes, says, wait, signed overflow is undefined, I will optimize the entire function away.