AVR Freaks

Think I'll give it a while to "mature" then.

It only took avr-gcc about 5 years to get pretty good!

I just tried 11.0.0-rc5.  It appears avr code generation is working, creating .o files in elf format.  However the linker doesn't seem to support avr:

clang --target=avr -mmcu=atmega328 main.c -Os -o main.elf
clang-11: warning: no avr-gcc installation can be found on the system, cannot link standard libraries [-Wavr-rtlib-linking-quirks]
clang-11: warning: standard library not linked and so no interrupt vector table or compiler runtime routines will be linked [-Wavr-rtlib-linking-quirks]

 

Since the warning says "no avr-gcc installation", I thought it was trying to use avr-gcc(avr-ld) for linking.  It really should say "no avr-libc installation".

I fired up my debian vm again and installed avr-libc (apt install avr-libc), and now can get a binary elf with "clang --target=avr -mmcu=atmega328 -Os main.c -o main.elf".

The only target I've found that works is the m328.  Trying various attinies, m8, and even m168 gives the following warning, and generates a 0-byte output file.

clang-11: warning: support for linking stdlibs for microcontroller 'atmega168' is not implemented [-Wavr-rtlib-linking-quirks]

Attempting to use LTO gives the following error:

no lto support: clang-11: error: 'avr': unable to pass LLVM bit-code files to linker

llvm puts a reference to __do_copy_data and __do_clear_bss in every .o, even when there are no global or static variables declared.  avr-gcc only does this when required.

To use avr/io.h, the include path /usr/lib/avr/include needs to be passed to clang.  If you use util/delay.h, __DELAY_BACKWARD_COMPATIBLE__ must be defined because llvm does not implement __builtin_avr_delay_cycles.

With -Os, clang's function-level optimization seems on par with avr-gcc 7.3.0 for the few tests I tried.  For example it knows to compile "PORTB |= 1;" to "sbi PORTB, 0".

So it sort of works, if your only target is the m328, and you don't want link-time optimization.  I suspect adding target support for the rest of the avr-libc devices isn't overly complicated, but learning llvm internals isn't on my priority list.

p.s. I also noticed llvm-objdump doesn't recognize the jmp instruction, and can't calculate target offsets for the rjmp and branch instructions.

b2: ff cf         rjmp    <unknown>

b2: ff cf         rjmp    <unknown>

/usr/lib/avr/include

clang --target=avr -mmcu=atmega328 -Os main.c -o main.elf

clang --target=avr -mmcu=atmega328 -Os main.c -o main.elf

That seems to be a reasonable conclusion.

Google has not been my friend.

What does FORTRAN need?

I've seen pages descibing how to install a FORTRAN runtime,

but nothing about what it does.