We have played around with having "hints" in the original source code that will be preserved through the IR, but the vast majority of the optimizations and scheduling decisions we need to do are happening once it has been converted into machine code by the backend. I may be slightly off (I'm the dumb CEO/hardware architect), but I'm fairly sure 85%-90% of our compiler work has been done on the backend... I just remember being told some tweaks were made to clang due to some weird issue.

It's really three stages, not frontend and backend.

The first stage is the LLVM front end, Clang and other language parsers. They generate IR.

The middle stage is the LLVM optimizer, opt, which takes IR and runs a variety of optimization passes. Opt takes and gives IR.

http://llvm.org/docs/Passes.html

The third is the backend code generator. It lowers IR to the target ISA. There's a tablegen description, there's are MI scheduler models which abstract architectural information as well.

http://llvm.org/docs/WritingAnLLVMBackend.html

I think you really meant can you write an optimizing pass specifically for your target? Yes. But I don't think that modifying the real front end Clang is going to get you much of anything. Intrinsics are different; that's the programmer inserting ISA specific instructions rather than Clang extracting ISA specific information. Really, modifying Clang isn't going to get you anything.

Thomas, what were the 'small' Clang specific tweaks?