According to this page, LLVM-MOS seems to be pretty soundly beaten in performance of generated code by Oscar64.

https://thred.github.io/c-bench-64/

I think the ideal compiler for 6502, and maybe any of the memory-poor 8-bit systems would be one that supported both native code generation where speed is needed as well as virtual machine code for compactness. Ideally would also support inline assembler.

The LLVM-MOS approach of reserving some of zero page as registers is a good start, but given how valuable zero page is, it would also be useful to be able to designate static/global variables as zero page or not.