I won't be able to use them in portable code anytime soon, but the new feature here that most interests me is __builtin_add_overflow, __builtin_sub_overflow and __builtin_mul_overflow

"These builtins have two integral arguments (which don't need to have the same type), the arguments are extended to infinite precision signed type, +, - or * is performed on those, and the result is stored in an integer variable pointed to by the last argument. If the stored value is equal to the infinite precision result, the built-in functions return false, otherwise true. The type of the integer variable that will hold the result can be different from the types of the first two arguments."

Handling all possible integer overflow with plain C is annoyingly tricky and for some critical paths in-efficient. These built-ins can often do a conditional jump based on the overflow bit in the flags register.