avr-gcc doesn't know address space wraps?

[Georg-Johann Lay]

Ralph Doncaster schrieb:
> On Sun, Jan 3, 2016 at 2:36 PM, Georg-Johann Lay <avr@gjlay.de> wrote:
>> Ralph Doncaster schrieb:
>>
>>> On Sun, Jan 3, 2016 at 1:22 PM, Georg-Johann Lay <avr@gjlay.de> wrote:
>>>> Ralph Doncaster schrieb:
>>>>> avr-gcc 4.9.2 doesn't seem to know that the address space wraps, so
>>>>> that an rjmp in the last 2KB of the address space can reach code in
>>>>> the first 2KB.
> [...]
>> Well, you are using RJMP with an address that's out of scope, hence the
>> linker/locator complains.  The compiler would not generate code like
>>
>>>>>     rjmp ResetVector
> [...]
>> Third approach is to link with --pmem-wrap-around= so that no explicit
>> offset and no explicit RJMP is needed in the code.
> 
> Thanks for the reference to the --pmem-wrap-around= option.  I
> understand what's going on now; even though the compiler knows the MCU
> (-mmcu=), that does not get passed to the linker.  Since the linker
> doesn't know the exact flash size of the part it is linking for, that
> has to be specified with the --pmem-wrap-around option.  In this case,
> with a m328p, adding "-Wl,--relax -Wl,--pmem-wrap-around=32k" does the
> trick.

In the old days, the driver tried to deduce the flash size from -mmcu= 
by means of a specs trick.  Up to 4.9, avr.h reads:

#define LINK_SPEC "\
%{mrelax:--relax\
          %{mpmem-wrap-around:%{mmcu=at90usb8*:--pmem-wrap-around=8k}\
                              %{mmcu=atmega16*:--pmem-wrap-around=16k}\
                              %{mmcu=atmega32*|\
                                mmcu=at90can32*:--pmem-wrap-around=32k}\
                              %{mmcu=atmega64*|\
                                mmcu=at90can64*|\
                               mmcu=at90usb64*:--pmem-wrap-around=64k}}}\
  ..."

I ported this to the GCC 5 spec file approach of device support so that 
it should work in v5.  However the device names are much to diverse, and 
such an approach is not very sensible today (-mpmen-wrap-around isn't 
even documented).  A proper approach would add flash sizes to 
avr-mcus.def, and whereby eventually to device-specs.  The device-specs 
for your device, specs-atmega328p, reads

*link_pmem_wrap:
	%{mpmem-wrap-around: --pmem-wrap-around=32k}

*link_relax:
	%{mrelax:--relax %(link_pmem_wrap)}

so that it's a bit more convenient to let addresses wrap (link_relax is 
part of LINK_SPEC).  -mpmem-wrap-around is supposed to work with 4.9, 
but it does not for some reasons.