What are mechanisms of selection of emulation mode for a linker? - c++

Question

I am trying to compile arduino code under codeblocks with avr-gnu compiler. I was getting this error:
avr-ld.exe unrecognized emulation mode: mcu=atmega328p.
Supported emulations:avr2 avr1 avr25 avr3 avr31 avr35 avr4 avr5 avr51 avr6 avrxmega1 avrxmega2 avrxmega3 avrxmega4 avrxmega5 avrxmega6 avrxmega7 avrtiny
I found this page: https://gcc.gnu.org/onlinedocs/gcc/AVR-Options.html which suggests that it should be linked under avr5 sim mode, so I added -mmcu=avr5 to linker options. Now I am getting:
avr-ld.exe unrecognized emulation mode: mcu=avr5
Supported emulations:avr2 avr1 avr25 avr3 avr31 avr35 avr4 avr5 avr51 avr6 avrxmega1 avrxmega2 avrxmega3 avrxmega4 avrxmega5 avrxmega6 avrxmega7 avrtiny
How is avr5 unrecognized if it is supported?o0
I tried few different toolchains from few different years but it seems that linker cant interpret this emulation mode correctly. Newer toolchains had support for 328p but still somehow dont work. I also tried creating CodeBlocks project dedicated for Arduino but the difference is that it passes -mmcu=atmega328p itself and still doesnt work on this or newer toolchains.
Update:
There is something that I am totally missing here. I changed linker to a standard x86 32 ld.exe from Mingw and passed -mmcu=i386pe and I am getting the similar error:
ld.exe unrecognized emulation mode: mcu=i386pe
Supported emulations:i386pe
So this only leads to my lack of understanding of mechanisms how the linker emulation mode is selected/configured.

Answer 1

In almost any case, you do not want to call the linker (ld resp. avr-ld) by hand!
Use avr-gcc to link your programs!
Notice that avr-gcc is not a compiler by itself, it's just a driver program that calls sub-programs like compiler proper (cc1 for C, cc1plus for C++, lto1 for LTO-compiled byte-code), assembler and linker depending on command-line options and input files provided on the command line.
avr-gcc knows how to provide the respective tools with appropriate command line options like: emulation, libraries, paths for includes and libraries, startup-code, sub-options, etc.
In order to see which tools are called by avr-gcc, add the -v option. For example, for a compile command as simple as
> avr-gcc -v main.c -mmcu=atmega328p -save-temps
You'll see somethinh like (here for avr-gcc v8.5.0):
...
COLLECT_GCC_OPTIONS='-v' '-save-temps' '-specs=device-specs/specs-atmega328p' '-mmcu=avr5'
$AVRGCC_HOME/bin/../libexec/gcc/avr/8.5.0/collect2 <many options> -plugin-opt=-pass-through=-latmega328p -mavr5 -Tdata 0x800100 $AVRGCC_HOME/bin/../lib/gcc/avr/8.5.0/../../../../avr/lib/avr5/crtatmega328p.o -L$AVRGCC_HOME/bin/../lib/gcc/avr/8.5.0/avr5 -L$AVRGCC_HOME/bin/../lib/gcc/avr/8.5.0/../../../../avr/lib/avr5 -L$AVRGCC_HOME/bin/../lib/gcc/avr/8.5.0 -L$AVRGCC_HOME/bin/../lib/gcc -L$AVRGCC_HOME/bin/../lib/gcc/avr/8.5.0/../../../../avr/lib main.o --start-group -lgcc -lm -lc -latmega328p --end-group
collect2 is just some other tool that wraps the call to ld. If you want to see the real thing, use avr-gcc ... -v -Wl,-v.
The correct emulation for the linker is worked out by the device-specific spec file device-specs/specs-atmega328p located at ./lib/gcc/avr/$version in the compiler's install directory. The specs "language" is a means to describe option mappings; in particular the bits that are responsible for mapping `-mmcu=atmega328p' to the right emulation are
*link_arch:
%{mmcu=*:-m%*}
...
*self_spec:
%{!mmcu=avr*: %<mmcu=* -mmcu=avr5} %<mshort-calls %<msp8
self_spec removes -mmcu=* from the command line and sets -mmcu=avr5 which is the core family ATmega328P belongs to. Then as part of the linker invocation, link_arch maps -mmcu=avr5 to -mavr5 which is the linker emulation to be used.