x86-64 long double precision

Question

What is the actual precision of long double on Intel 64-bit platforms? is it 80 bits padded to 128 or actual 128 bit?

if former, besides going gmp, is there another option to achieve true 128 precision?

Answer 1

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.

Answer 2

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