I'm actually working on an LLVM backend. I'd read the Dragon Book before, long before, but I think my real education was the LLVM Developer Meeting tutorials.
LLVM is pass oriented but broadly there are front ends (Clang, Rust, Fortran), a middle end (LLVM IR SSA optimizations) and multiple in-tree and out-of-tree backends. I was a little resentful of having to write all of 5 lines or so inside of Clang. Otherwise, I happily live in the backend. Doing a merge with a new release is an hour or so of conflict resolution.
The pass structure importantly means that you can test your backend code, even at the GlobalISel subpass level, without needing to understand Clang, LLVM, .... Pretty sweet if you ask me.
LLVM is pass oriented but broadly there are front ends (Clang, Rust, Fortran), a middle end (LLVM IR SSA optimizations) and multiple in-tree and out-of-tree backends. I was a little resentful of having to write all of 5 lines or so inside of Clang. Otherwise, I happily live in the backend. Doing a merge with a new release is an hour or so of conflict resolution.
The pass structure importantly means that you can test your backend code, even at the GlobalISel subpass level, without needing to understand Clang, LLVM, .... Pretty sweet if you ask me.