The Shape of Code

What is the oldest compiler still in production use?

A CHILL compiler I worked on a long time ago has probably been in production use for 30 years now.

Code gets added and deleted from production software all the time, how might ‘oldest’ be measured? I propose using the mean age of every line of code, including comments, where the age of a line is reset to zero when it is modified in any way (excluding code formatting).

The following are two environmental factors that enable a production compiler to get very old:

• a relatively obscure language: popular languages have new compilers written for them (compiler death through competition) or have new features added to them (requiring new lines of code which could even displace ‘aged’ code),
• a very long-lived application associated with the language: obscure languages tend to be very quickly abandoned in the dust of history unless they have a symbiotic relationship with an important application,
• very long-lived host hardware and target processor: changing either often requires substantial new code or a move to a newer compiler. For ancient the only candidate is the IBM 370 and just really old the Intel 80×80, Zilog Z80.

Virtual machines provide a mechanism to be host hardware independent. The Micro Focus Cobol had a rewrite in the early 1990s (it might have had others since) and I don’t think UCSD Pascal I.5 is still used for production work.

Fortran is an evolving language and very popular in some application domains. I doubt there are any (mean age) old Fortran compilers in production use.

Why do I put forward the ITT (in its International Telephone & Telegraph days, these bits subsequently sold off) CHILL compiler as potentially the oldest compiler currently in production use?

• Obscure language and long-lived application (telephone switching software),
• host hardware was IBM 370 family, target processor Intel 8086 (later updated to support 80386),
• large development team and very small support team (i.e., lots of old code and small changes over the years),
• single customer, i.e., no push to add features to attract new customers or keep existing ones.

My last conversation with anybody associated with this compiler was a chance meeting over 10 years ago, so I might be a bit out of date.

30+ year old source code for compilers can be downloaded (e.g., the original PDP 11 C compiler) but these compilers are not in production use (forgotten about military installations anybody?)

I welcome other proposals for the oldest compiler currently in production use.