An important goal of the Gforth Project is availability across a wide range of personal machines. fig-Forth, and, to a lesser extent, F83, achieved this goal by manually coding the engine in assembly language for several then-popular processors. This approach is very labor-intensive and the results are short-lived due to progress in computer architecture.
Others have avoided this problem by coding in C, e.g., Mitch Bradley (cforth), Mikael Patel (TILE) and Dirk Zoller (pfe). This approach is particularly popular for UNIX-based Forths due to the large variety of architectures of UNIX machines. Unfortunately an implementation in C does not mix well with the goals of efficiency and with using traditional techniques: Indirect or direct threading cannot be expressed in C, and switch threading, the fastest technique available in C, is significantly slower. Another problem with C is that it is very cumbersome to express double integer arithmetic.
Fortunately, there is a portable language that does not have these
limitations: GNU C, the version of C processed by the GNU C compiler
(see Extensions to the C Language Family in GNU C Manual). Its labels as values feature (see Labels as Values in GNU C Manual) makes direct and indirect
threading possible, its long long type (see Double-Word Integers in GNU C Manual) corresponds to Forth’s
double numbers on many systems. GNU C is freely available on all
important (and many unimportant) UNIX machines, VMS, 80386s running
MS-DOS, the Amiga, and the Atari ST, so a Forth written in GNU C can run
on all these machines.
Writing in a portable language has the reputation of producing code that is slower than assembly. For our Forth engine we repeatedly looked at the code produced by the compiler and eliminated most compiler-induced inefficiencies by appropriate changes in the source code.
However, register allocation cannot be portably influenced by the
programmer, leading to some inefficiencies on register-starved
machines. We use explicit register declarations (see Variables in Specified Registers in GNU C Manual) to
improve the speed on some machines. They are turned on by using the
configuration flag --enable-force-reg (gcc switch
-DFORCE_REG). Unfortunately, this feature not only depends on the
machine, but also on the compiler version: On some machines some
compiler versions produce incorrect code when certain explicit register
declarations are used. So by default -DFORCE_REG is not used.