The Free On-line Dictionary of Computing (30 December 2018):

Register Transfer Language

   (RTL) 1. A kind of hardware description language (HDL) used
   in describing the registers of a computer or digital
   electronic system, and the way in which data is transferred
   between them.

   2.  An intermediate code for a machine with an infinite number
   of registers, used for machine-independent optimisation.
   RTL was developed by Chris Fraser  and
   J. Davidson  at the University of Arizona
   in the early 1980s.  RTL is used by the GNU C compiler,
   gcc and by Davidson's VPCC (Very Portable C compiler).

   ["Quick Compilers Using Peephole Optimisation", Davidson et
   al, Soft. Prac. & Exp. 19(1):79-97 (Jan 1989)].

   (1994-11-16)