I've recently learnt about XADD and I had no luck at looking up on google. Does an instruction like XADD that also adds the carry flag exist? (ex. XADC) Or am I forced to do it with two different instructions?
I'm on Ubuntu and I'm using NASM in 64 bits mode.
No. xadd and adc are two different niche versions of addition that don't normally overlap, so it's not surprising that x86 doesn't have an instruction for it.
xadd is usually used with a memory destination (and often a lock prefix) for atomic_fetch_addadc is usually used for extended-precision stuff, for the high parts of an integer wider than a registerIf you did lock xadd / lock xadc (hypothetical), you wouldn't be doing a single double-width atomic fetch_add, you'd be doing two separate atomic additions on the two halves of a wider number. So it's not really useful; if you want to atomic_fetch_add an __int128, you need a lock cmpxchg16b retry loop, not xadd / "xadc".
If you want to do fetch_add(&mem, reg) + CF or fetch_add(&mem, reg+CF), you can do that manually with a combination of adc and xadd in some order. (And maybe some branching to handle the case where the +CF itself produces a carry if that's a concern).
That's a niche enough need that x86 didn't choose to spend an opcode on it. There are only a limited number of opcodes, and each one would require transistors in the decoders, and at least microcode to implement it.