Adjust offset of array reference in named address space

[Tucker Kern <tuckkern@gmail.com>]

Hi,

I'm implementing named addresses spaces for a Harvard architecture machine
to support copying data from instruction memory to data memory. This is
achieved via a special instruction. e.g. think AVR and progmem/__flash.

However, the instruction memory is narrower than the data memory (12 vs 16
bits) on this machine. So a single data word is split across 2 instruction
words. When copied from IMEM to DMEM the two parts are combined via SHIFT +
OR patterns.

This is all working fine for regular variables (i.e. int som_var), but it
falls apart for array references (i.e. some_array[1]). Since the data is
stored across 2 IMEM words, I need to scale the calculated offset of each
array reference by 2. e.g. array[0] is actually stored in imem[0] & imem[1]
and array[1] is stored in imem[2] & imem[3].

e.g.
static __imem int imem_array[2];
return imem_array[1];

// needs to generate a symbol reference like
&imem_array.869+2

Similarly if the array index was a function parameter, I need to scale the
parameter by 2.
__imem int imem_array[2];
int some_func(int a)
{
  // a needs to be scaled by 2 when generating RTL/ASM
  return imem_array[a];
}

I haven't found any target hooks that would allow me to override the offset
calculation. Originally I thought I could handle it in a splitter but this
approach didn't work for the function parameter example as I ended up
scaling the entire address instead of just the offset.

I had another thought of using a combo of
TARGET_ADDR_SPACE_LEGITIMIZE_ADDRESS and
TARGET_ADDR_SPACE_LEGITIMATE_ADDRESS_P to scale the offset and mark it as
adjusted but I don't think this combo will work in the end.

Is there any way to achieve this?

Thanks,
Tucker