the frontend of sdcc has lots of features of mcs-51, so, in my opinion, 32bit addressing should a hard work.
I'm trying to find some way of compiling C code for the Z380, with 32bit linear addressing. The Z380 is compatible with the Z80, so any Z80 compiler could be a good start point.
Unfortunately, LLVM backends for the Z80 haven't reached production status. And SDCC seems focused in 8bit CPUs.
But I was wondering whether SDCC has indeed some imposed design limitation for not allowing 32bit pointers, or if there's no such limitation.
In other words, could the Z80 backend support 32bit pointers for full Z380 support, or would that be an incompatible task with the SDCC design and concept?