Re: 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 <a...@gjlay.de> wrote:
Ralph Doncaster schrieb:
On Sun, Jan 3, 2016 at 1:22 PM, Georg-Johann Lay <a...@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.