Bug 49467

On hardware with fixed width registers, it is possible for an integer math operation to overflow/wrap/underflow. For example, when working with 32 bit unsigned integers, UINT_MAX + 1 will result in 0, while UINT_MAX + UINT_MAX = 4294967294 (ie, UINT_MAX -1). Similarly, -1 + INT_MIN = 2147483647.

Due to the security implications of integer overflow, it would be great if GCC would provide an intrinsic for fetching relevant status flags. For signed integers on x86/x64 PCs, the flag of interest is OVERFLOW. For unsigned integers, the flag of interest is CARRY. Many [other] popular architectures, such as ARM processors, include similar flags (in the case of ARM, it is available in the Current Program Status Register (CPSR)). 

I envision one of two intrinsics. The first intrinsic would simply return the value of the flag (carry shown, overflow similar):

    uint32_t a, b;
    ....

    uint32_t r = a + b;
    unsigned int cy = __get_carry_flag(); 

A second version of the same would accept an lvaue:

    uint32_t a, b, r;

    unsigned int cy;
    r = a + b;
    __get_carry_flag(&cy);

With carry/overflow in hand, a programmer would be enabled to perform safe math on data, without the need/overhead for a big integer package. For example:

    uint32_t r = a + b;
    unsigned int cy = __get_carry_flag(); 

    if(cy) {
        fprintf(stderr, "Integer addition overflowed\n");
        abort();
    }

A third system would include integer math handlers. Upon program startup, the process would register 'failed add', 'failed subtract', etc, handlers. A program could perform safe integer math as follows:

    uint32_t r = a + b;
    test_uadd_result();

An executing program which was not interested in carry/overflow handling would simply call the operation without the subsequent test:

    uint32_t r = a + b;
    // No test, who cares?

Though this feature request was filed C, both C++ and Objective C would benefit from the intrinsic.