x86-64 long double precision

x86-64 precision is the same as regular x86. Extended double is 80 bits, using the x87 ISA, with 6 padding bytes added. There is no 128-bit FP hardware.

A software implementation of quad or extended quad precision might benefit from the x86-64 64x64 => 128 integer multiply instruction, though.

I would recommend using MPFR. It is a more sophisticated multiple-precision floating point library that is built on top of GMP.

There is a good chance that it's 64 bit for both (depending on the compiler and OS), because the compiler is emitting scalar SSE2 instead of x87 instructions.

x86 doesn't support higher precision than 80 bits, but if you really need more than 64 bits for a FP algorithm most likely you should check your numerics instead of solving the problem with brute force.