🚀 go-pugleaf

In article <PCG.90Sep23181145@teachc.cs.aber.ac.uk> pcg@cs.aber.ac.uk (Piercarlo Grandi) writes:
> Maybe Rubin is a bit rough in expression, but I think that he is trying
> to say that he would dearly love to have extensible compilers,

I'd like to focus on this point. Upward extensibility through macros and
functions is pretty well understood. Its syntax may not be as well
understood, but that's not a serious problem. What would it take to make
a realistic language downward-extensible?

For example, many computers have a way to multiply 32 bits by 32 bits
and get each word of the 64-bit result, even if they don't have 64-bit
types. How can I take advantage of this in a program?

The simplest answer is to settle on a standard way of escaping to the
lowest levels of the machine. C has asm(), for example. But this isn't
enough. I have to explicitly define a compile-time symbol to choose
between the assembler version and the slower, high-level version. I have
to learn a new language and write a new routine for every computer where
I want a speedup. And if the compiler learns some new optimization
tricks or the CPU is upgraded, my ``fast'' code may be worthless.

It isn't hard to solve the last problem. The language just needs some
construct to say ``compile either this code or that code, whichever you
think will come out faster.'' I may want to reorganize the flow of my
program if one alternative is better, so the language should also define
a compile-time symbol for me saying which alternative it picked. This
also solves the first problem.

What's tricky is the second problem. How can downward extensions be
portable? A sufficiently weird hardware operation may not have any
sensible language equivalent, but most of the time it will. So the
problem reduces to teaching the compiler to recognize *something* in the
langauge that takes advantage of any given instruction. Then programmers
use the ``compile this or that'' form mentioned above; the more forms
they use, the better chance they'll have of matching the hardware on any
given computer. And their programs will still be portable.

Comments?

---Dan