Compilers vary greatly in their complexity. In particular, optimization is a truly open-ended task, which can consume however much CPU time and memory as you'r

Compilers vary greatly in their complexity. In particular, optimization is a truly open-ended task, which can consume however much CPU time and memory as you're willing to throw at it. GCC and LLVM today are much more sophisticated than anything around in the 1970s or 1980s. They will chew up the entire program, transform it into an enormous (many megabyte or gigabyte) graph abstractly representing the program, and then spend hundreds of billions of cycles, exploring possible rewrites of this graph, that make the program faster or more efficient. In comparison, early compilers, including the first C compiler, usually worked on a single statement at a time, immediately translating it into equivalent machine code. It never examines the whole program together, as there wasn't enough RAM in those machines to do that. This produces inefficient code, but it's very fast to compile. In fact, accommodating the memory limits of machines like the PDP-11 influenced much of C's design. It's why everything has to be declared before use, for example. That way less state needs to be kept to resolve forward references.