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

XPL

    A small dialect of PL/I developed at Stanford
   in 1967-69, used for compiler writing.  XPL has
   one-dimensional arrays.  I/O is achieved with character
   pseudo-variable INPUT and OUTPUT, e.g.

   	OUTPUT = 'This is a line';

   It has inline machine code.  "Programmers are given all the
   rope they ask for.  Novices tend to hang themselves fairly
   frequently."  XPL has been implemented on IBM 360, Univac
   1100, ICL System 4, CDC 6000 and Cyber series, XDS
   Sigma-5 and Sigma-7, DEC PDP-10, IA32, FreeBSD and
   Linux.

   An optimising XPL compiler (version 1) by Robin Vowels
    is a standard implementation of XPL
   and is based on McKeeman, Horning, and Wortman's improved
   XCOM (which employs hashed symbol table generation).  It
   includes the extra built-in function COREHALFWORD.

   The following areas have been optimised: procedures calls when
   the argument and corresponding parameter are of the same type,
   and when the argument is a constant; constant subscripts; use
   of CORELHALFWORD and COREWORD; string constants of length one;
   iterative DO statements by transferring code to the end of the
   loop.

   String constants of length one do not require a descriptor,
   hence more descriptors are available for string variables.
   Comparison operations are treated as commutative, and an
   improved Commute algorithm is used.  Halfword instructions are
   generated for BIT(16) variables.

   These areas have been improved or re-written: calls on OUTPUT,
   catenation, integer-to-string conversion, multiply, divide,
   and MOD.  An emitter for SS-type instructions has been added.
   The compiler achieves an 11% reduction in object code
   compiling itself, an 11% increase in compilation rate, a 55%
   increase in compilation speed when the $E toggle is set.
   Special treatment for catenating a string to an integer
   substantially decreases consumption of the free string area,
   and decreases string moves.  The latter improvement is most
   noticeable on small core machines.

   Core requirements: less than the improved XCOM on which it is
   based (approx. 98000 bytes).  Symbol table size is 468.
   Ported to IBM System 370.  The compiler is written in XPL.
   The code generators are machine-specific.

   ["A Compiler Generator," W.M. McKeeman et al, P-H 1970].

   [JCC, AFIPS 1968].

   (2017-09-17)