Is there hardware support for 128bit integers in modern processors?

Question

Do we still need to emulate 128bit integers in software, or is there hardware support for them in your average desktop processor these days?

Answer 1

The x86-64 instruction set can do 64-bit*64-bit to 128-bit using one instruction (mul for unsigned imul for signed each with one operand) so I would argue that to some degree that the x86 instruction set does include some support for 128-bit integers.

If your instruction set does not have an instruction to do 64-bit*64-bit to 128-bit then you need several instructions to emulate this.

This is why 128-bit * 128-bit to lower 128-bit operations can be done with few instructions with x86-64. For example with GCC

__int128 mul(__int128 a, __int128 b) {
    return a*b;
}

produces this assembly

imulq   %rdx, %rsi
movq    %rdi, %rax
imulq   %rdi, %rcx
mulq    %rdx
addq    %rsi, %rcx
addq    %rcx, %rdx

which uses one 64-bit * 64-bit to 128-bit instructions, two 64-bit * 64-bit to lower 64-bit instructions, and two 64-bit additions.