I know that for arbitrary precision arithmetic Mathematica uses GMP. But apart from just providing arbitrary precision, Mathematica also does precision tracking for its arbitrary-precision numbers, so that e.g.
1.2345`5 - 1.2300`5 // Precision
2.2614836912846. Among GNU packages of the GMP family there is MPFI. But I haven't found any references that Mathematica does use it: there's no such library in installation directory (unlike
libgmp.so.10), and it doesn't seem mentioned anywhere.
So does Mathematica actually implement precision tracking on its own, without using any well-known package for this?