A 1980 paper called "Register Allocation via Coloring" contains something that looks like source code:
P: PROC(MODE); DCL MODE BIT(1), (A1,A2,A3,A4,A5,A6,A7,A8,A9,A10, B1,B2,B3,B4,B5,B6,B7,B8,B9,B10, SUM) FIXED BIN(15) AUTO, (U(10),V(10)) FIXED BIN(15) STATIC EXT; IF MODE THEN DO; A1=U(1); A2=U(2); A3=U(3); A4=U(4); A5=U(5); A6=U(6); A7=U(7); A8=U(8); A9=U(9); A10=U(10); END; ELSE DO; B1=V(1); B2=V(2); B3=V(3); B4=V(4); B5=V(5); B6=V(6); B7=V(7); B8=V(8); B9=V(9); B10=V(10); END; LABEL:; IF MODE
I couldn't find what programming language this is. Is it a real historical programming language, or is it some type of pseudo-code?
The beginning of the paper mentions that the research is based on a PL/I Compiler. So maybe it's the programming language that the compiler was built on. I couldn't locate the paper where it talks about the compiler on the Internet.