base + index register addressing

[Joern Rennecke]

> On some of TI's DSPs  index/base are very important as well.
> This is almost the inverse of above case - sets of registers are vital !
> Here to use a register as an index its hard-register number has to be 
> "related" to the hard register number of the base.
> Current GCC register allocation just cannot handle this, so I have
> "given up" (and just lurk here).

Why so you think gcc cannot handle this?  Just describe the relatedness
of the registers with register classes, and enforce it in the REG_OK_STRICT
version of GO_IF_LEGITIMATE_ADDRESS .