Read the linked content in the post about undefined behavior. Signed overflow is undefined, not implementation defined. Clang and GCC treat it as undefined such without -fwrapv. That means they assume it cannot happen and feed information into optimization passes and code generation based on that assumption. It's worse than the result potentially being different: a program with signed overflow may crash, corrupt data or worse and it happens in practice. One common example is overflow checks often being optimized out if they do it by trying the operation and then checking for overflow. As the compilers get smarter, the problems will grow. They barely do any integer range analysis right now...