[SOLVED] Are there ARM intrinsics for add-with-carry in C?

Are there ARM intrinsics for add-with-carry in C?

Do there exist intrinsics for ARM C compilers to do add-with-carry operations, or is it necessary to use assembly language?

On x86, there is _addcarry_u64 for add-with-carry. (There's also the newer _addcarryx_u64 for special purposes.)

Solution

There is no intrinsic with current versions of gcc (gcc5 was released the year this question was asked). An issue is that communication of the 'carry flag'. However, the ARM backend does know and define a set of ADC primitives such as addsi3_carryin.

For example,

unsigned long long big_inc(unsigned long long x)
{
  return ++x;
}

Is translated to,

big_inc(unsigned long long):
        @ args = 0, pretend = 0, frame = 0
        @ frame_needed = 0, uses_anonymous_args = 0
        @ link register save eliminated.
        adds    r0, r0, #1
        adc     r1, r1, #0
        bx      lr

It is always instructive to look at open source multi-precision libraries when you have a question like this. There is OpenSSL bignum and GNU MP libraries without any research. As the intrinsic doesn't exist a more definitive answer (for your work) depends on exactly what it is you want to achieve; prime factors, multiply, add, etc. You can always use assembler or more powerfully use a script that generates assembler for your particular integer length.