The Shape of Code

The specification of ALGOL 60 was published in May 1960. Unlike today, where the creators of a new language release the source of a corresponding compiler, people were expected to write their own compiler. The June 1962 paper: The Replies to the AB14 Questionnaire lists implementation details on 21’ish compilers (it’s not clear whether some are dialects or languages very similar to Algol 60; 1963: list of 32 Algol compilers/versions).

Compiler writing was a hot leading edge research topic in the 1960s; at the start of this decade all the techniques we take for granted today had not yet been invented (Knuth invented LR parsing in 1965, and algorithms for optimal code generation started appearing in 1970). The 1960s was the period of the Cambrian explosion for programming languages.

Implementors not only had to deal with all the unknowns of writing a compiler, they also had to do the work using systems whose memory was measured in tens of kilobytes, computer interaction probably via punched card or punched tape, or if lucky, the luxury of teletype input/output. It’s no surprise that fourteen of the implementations considered themselves to be a “true subset” (which I take to mean that everything implemented was as per the specification). Compilers for earlier languages probably had the benefit of the language not supporting anything that was hard to implement.

Compiler implementation know-how received a major boost in 1964 with the publication of the book ALGOL 60 Implementation.

The plot below shows the number of compilers having a given reported implementation time (code+data):

The median implementation effort is 2 man-years. Is this the result of a few good people working off the clock to create software, or management supporting the creation of a product that customers are not clamouring for?

The 0.25 man-year implementation looks like a port of an existing compiler to a different version of the same hardware. The 10 man-year implementation time was for what looks like a full implementation, plus extensions. The 80 man-year implementation time was reported by SDC (a large defence contractor) for a range of JOVIAL compilers (derived from Algol 58) targetting five different hardware platforms.

Were the implementors of Algol compilers different from the implementors of other languages? It’s not possible to say, although the language was created by a distinct group of people. The definition of Algol 60 was created by a committee composed of computing academics and like-minded people, while Fortran was dominated by the major computer company of the day, IBM (1963: list of 51 Fortran compilers; 1964: at least 43 Fortran compilers/versions), and COBOL was designed to be used by those strange business people (1963: list of 37 COBOL implementations/versions).