Okay, somebody mentioned the Pastel compiler (Pascal-ish, compiled itself) that was the original basis for RMS' hacking. He wrote his own C frontend and that later became GCC proper.
It's very hard to find info on this, so I'm pasting here my recent findings for ya. (Note that I know less than probably anybody about this. I'm no knowledgeable insider or anything!) In particular, GCC's history and Wikipedia seem to omit mention of most of this.
Somebody should also probably e-mail RMS to ask if he still has a copy of the sources around somewhere (interesting to me, at least, and I'm just vaguely curious!).
"Jeff Broughton, then Amber project leader, implemented two extended Pascal compilers. The extended language was named Pastel (an off-color Pascal). A complete compiler targeted to the S-1 architecture was completed in several months. However, the first version of the compiler, which was implemented in slightly more than a weekend, produced PL/1 as its output! This was done to allow the continued use of Multics, which had no Pascal compiler, as the development site for Amber. At that point the project had a couple of man-years of code written in PL/1. The conversion of all PL/1 code to Pastel was accomplished in about 3 man months. (1998 historical note: at one point in the project Richard Stallman visited, and had the Pastel compiler explained to him. He left with a copy of the source, and used it to produce the Gnu C compiler. Most of the techniques that gave the Gnu C compiler its reputation for good code generation came from the Amber Pastel compiler.)"
http://www.mit.edu/~cbf/thesis.htm http://killian.com/earl/ http://www.lbl.gov/cs/CSnews/CSnews073109a.html http://en.wikipedia.org/wiki/PathScale