They could have very well used lots or reusable parts of GCC. It was possible, and were many projects trying to change GCC.
gcc was specifically designed to not have many reusable parts to avoid people ripping them out. modularity only happened after LLVM and clang specifically started eating its lunch.
alt Hacker News
gcc was specifically designed to not have many reusable parts to avoid people ripping them out. modularity only happened after LLVM and clang specifically started eating its lunch.