Re: Adjust offset of array reference in named address space

[Richard Biener <richard.guenther@gmail.com>]

On Tue, Jan 12, 2021 at 4:29 PM Tucker Kern <tuckkern@gmail.com> wrote:
>
> > 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"?
>
> Effectively yes. The assembler handles dividing constants into their LSB and MSB components. I have insn patterns and splitters defined that emit the correct instructions to read and "pack" the value into a register or generic memory location.
>
> All I really need at this point is a means to augment how addresses (e.g. array offsets or struct members) are calculated in a non-generic address space. This doesn't feel like a far fetched idea as GCC currently supports address space specific legitimization and modes.

I think you'd need to hook this up in structure layouting which then
runs into the issue that
the address space is a qualifier and GCC internals expect layouts to
be compatible between
qualified and unqualified types.  Also all target hooks in the layout
code mostly deal with
bitfields only.

The offset is computed via get_inner_reference from expand_expr_real_*
so I don't
see a good way to handle this correctly.  But maybe Joseph has an idea.

Richard.

> On Mon, Jan 11, 2021 at 12:50 AM Richard Biener <richard.guenther@gmail.com> wrote:
>>
>> 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