Correct Rounding in Double Extended Precision

The double extended precision format is an 80-bit floating-point format introduced in the 80x87 series of floating-point processors by Intel. Since the introduction of vector instructions in the x86 processors, its use has fallen due to speed concerns. We implement in CORE-MATH the first correctly-rounded routines for double extended precision. These implementations use modern microprocessor features and double-double arithmetic, avoiding x87-specific features, and achieve up to 2x speedup over state-of-the-art implementations which are not correctly rounded. This demonstrates that double extended precision could be viable as a large computational format.