On Sat, Jan 9, 2021 at 12:24 AM Tucker Kern via Gcc <gcc@gcc.gnu.org> wrote:
>
> 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].
So you are saying that a 16bit data word in IMEM is actually two 12bit
data words (supposedly only the lower 8 bits used in each) and thus the
array contains "padding"? That's not really supported and is also not
the scope of named address spaces. I'd suggest you go down the route
of providing intrinsics for the transfer of data instead which could resort
to target specific builtin functions.
> 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
