Does gcc support 128-bit int on amd64?

void f(__int128* res, __int128* op1, __int128* op2)
{
    *res = *op1 + *op2;
}

Save to test.c and compile with:

$ gcc -c -O3 test.c
$ objdump -d -M intel test.o

You get:

mov    rcx, rdx
mov    rax, [rsi]
mov    rdx, [rsi+0x8]

add    rax, [rcx]
adc    rdx, [rcx+0x8]

mov    [rdi], rax
mov    [rdi+0x8], rdx

As you can see the __int128 type is supported by keeping two 64-bits in sequence and then operating on them with the typical big int pattern of using two instructions, for example ADD and then ADC (add with carry)