Directed rounding as a thread-local state (or global state), which is what the first 25 years of IEEE-754 processors supported, turns out to be not a very usefu

Directed rounding as a thread-local state (or global state), which is what the first 25 years of IEEE-754 processors supported, turns out to be not a very useful model (except for debugging tricks like estimating stability, which is dear to Kahan); that's a big part of why directed rounding isn't widely used or supported. Several newer hardware designs have added per-instruction stateless directed rounding (the rounding direction is part of the opcode); this is a much more useful model (and its presence makes dynamic rounding modes more usable as well!). This feature was added to ieee-754 in the 2008 revision, too late for language bindings to be added to c[++]11, but there's already language support in OpenCL and other compute languages, and bindings have been proposed to the C committee. So, progress is being made, slowly but surely.