A Forgotten Principle of Compiler Design

That a clean system for separately compiled modules appeared in Modula-2, a programming language designed by Niklaus Wirth in 1978, but not in the 2011 C++ standard...hmmm, no further comment needed. But the successor to Modula-2, Oberon, is even more interesting.

With Oberon, Wirth removed features from Modula-2 while making a few careful additions. It was a smaller language overall. Excepting the extreme minimalism of Forth, this is the first language I'm aware of where simplicity of the implementation was a concern. For example, nested modules were rarely used in Modula-2, but they were disproportionately complex to compile, so they were taken out of Oberon.

That simplicity carried over to optimizations performed by the compiler. Here's Michael Franz:

Optimizing compilers tend to be much larger and much slower than their straightforward counterparts. Their designers usually do not follow Oberon's maxim of making things "as simple as possible", but are inclined to completely disregard cost (in terms of compiler size, compilation speed, and maintainability) in favor of code-quality benefits that often turn out to be relatively marginal. Trying to make an optimizing compiler as simple as possible and yet as powerful as necessary requires, before all else, a measurement standard, by which both simplicity and power can be judged. For a compiler that is written in the language it compiles, two such standards are easily found by considering first the time required for self-compilation, and then the size of the resulting object program. With the help of these benchmarks, one may pit simplicity against power, requiring that every new capability added to the compiler "pays its own way" by creating more benefit than cost on account of at least one of the measures.

The principle is "compiler optimizations should pay for themselves."

Clearly it's not perfect (the Oberon compiler doesn't make heavy use of floating point math, for example, so floating point optimizations may not speed it up or make it smaller), but I like the spirit of it.

(If you liked this, you might enjoy Papers from the Lost Culture of Array Languages.)

permalink April 25, 2012

previously