Re: Using progmem on the reduced Tiny core?

On 13.07.2016 19:10, Georg-Johann Lay wrote:
> Currently progmem attribute works the same way on the reduced core (no LPM, 16
> GPRs) and on the ordinary cores (with LPM, 32 GPRs).
>
> As we have a linearized memory model on reduced core and flash memory is
> visible in the RAM address range (starting at 0x4000), it would make sense if
> the compiler added that offset.

...same question arises for the __flash address space:

It's currently accepted for rTiny but generates nonsensical code:  No offset is 
added to symbols and LPM instructions are generated or a silly libcall.

We could handle __flash but there is no benefit for the user, instead it makes 
things more complicated because qualified pointers would have to be passed 
around.  And it would make the avr backend even more complicated.

Actually progmem makes much more sense here because it is about handling of 
symbols and their values and about different code needed to access flash vs. RAM.


> Attached is a tentative patch that adds this feature to avr-gcc together with a
> small test program that demonstrates some use cases.
>
> If this does not break anything (e.g. because the current practice is to use
> progmem together with hand-crafted offsets and / or inline assembly) and the
> feedback is positive, I would propose the change to gcc-patches.
>
> Johann
>
> gcc/
>     * doc/extend.texi (AVR Variable Attributes) [progmem]: Add
>     documentation how it works on reduced Tiny cores.
>     * avr.c (AVR_SYMBOL_FLAG_TINY_PM): New macro.
>     (avr_address_tiny_pm_p): New static function.
>     (avr_print_operand_address) [AVR_TINY]: Add AVR_TINY_PM_OFFSET
>     if the address is in progmem.
>     (avr_assemble_integer): Same.
>     (avr_encode_section_info) [AVR_TINY]: Set AVR_SYMBOL_FLAG_TINY_PM
>     for symbol_ref in progmem.
>     * avr.h (AVR_TINY_PM_OFFSET): New macro.
>     * avr-c.c (avr_cpu_cpp_builtins): Use it instead of magic 0x4000
>     when built-in def'ing __AVR_TINY_PM_BASE_ADDRESS__.