  This article is about twofold arithmetic. Here I introduce algorithms and
experimental code for twofold variant of C/C++ standard functions exp() and
log(), and expm1() and log1p(). Twofold function $y_0+y_1 \approx f(x_0+x_1)$
is nearly 2x-precise so can assess accuracy of standard one. Performance allows
assessing on-fly: twofold texp() over double is ~10x times faster than expq()
by GNU quadmath.

Mathematical Software