XPL

A small dialect of PL/I used for compiler writing from

Stanford, 1967-69. 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, Univac1100, ICL System 4, CDC6000 and Cyber series, XDSSigma-5 and Sigma-7 and DEC PDP-10.

An optimising XPL compiler (version 1) by Robin Vowels

[email protected] 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].

(1993-08-07)