SDCC will indeed work with the 80C31 and many other 80x51 variants, the processor variant is defined in a xxx.h file, so you could define your own processor.

I have seen from the previous thread on this subject that it is claimed to generate large and slow code. I currently have a project using an IAR compiler, which given time is being re-compiled using SDCC as a test to see the differences between a $lots compiler and a FREE! one.

This project deserves to be supported as many of us cannot afford a commercial C compiler for our own projects.

I suspect that it is better to use SDCC on a Linux platform as the Windows cludge is just that.