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

Question

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.)

Answer 1

There is no intrinsic with current versions of gcc. 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.