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

From old documentation (as old as gcc 5 !!!!)

https://gcc.gnu.org/onlinedocs/gcc/Integer-Overflow-Builtins.html https://gcc.gnu.org/onlinedocs/gcc-5.3.0/gcc/Integer-Overflow-Builtins.html

Both clang and GCC do implement these builtins, and I verified the generated code is optimal on both x86_64 and aarch64 targets

#include <stdint.h>

typedef unsigned __int128 uint128_t;


// carry_out = a + b + carry_in
uint8_t my_addcarry_u64(uint8_t carry_in, uint64_t a, uint64_t b, uint64_t * sum)
{
        bool c;
        uint64_t res;
        c = __builtin_uaddll_overflow (a, b, (long long unsigned *)&res);
        c |= __builtin_uaddll_overflow (res, carry_in, (long long unsigned *)&res);
        *sum = res;
        return c;
}

// carry_out = a + b + carry_in
uint8_t my_addcarry_u128(uint8_t carry_in, uint128_t a, uint128_t b, uint128_t * sum)
{
        bool c;
        uint64_t res_lo, res_hi;
        c = __builtin_uaddll_overflow (a, b, (long long unsigned *)&res_lo);
        c |= __builtin_uaddll_overflow (carry_in, res_lo, (long long unsigned *)&res_lo);
        c = __builtin_uaddll_overflow (a >> 64, c, (long long unsigned *)&res_hi);
        c |= __builtin_uaddll_overflow (b >> 64, res_hi, (long long unsigned *)&res_hi);
        *sum = ((uint128_t)res_hi << 64) + res_lo;
        return c;
}

Even if the original post is old, I provide a solution to the original question, in case someone reads this thread again