How do I use clang to compile for avr (arduino)

Question

When I search I found 7yr old results talking about a fork of clang instead of clang itself.

Using avr-gcc I can compile and upload my code with

avr-gcc a.cpp -DF_CPU=16000000 -mmcu=atmega2560 -Wall -Werror -Wextra -Os
avr-objcopy -j .text -j .data -O ihex a.out my.hex
sudo avrdude -patmega2560 -cwiring -P/dev/ttyACM0 -b115200 -D -Uflash:w:my.hex:i

I'd like to replace the first step with clang++. The changes I made here

avr-gcc to clang++
Added --target=avr
Added -nostdlib because I'll include it myself
Added -I/usr/avr/include/ because path wasn't implicit
Added -L/usr/avr/lib/avr6 -lc -latmega2560 so it has enough info to build an elf

I found device-specs at /usr/lib/gcc/avr/10.2.0/device-specs/specs-atmega2560 which mentions crtatmega2560.o and -latmega2560 which appears to be located at /usr/avr/lib/avr6/. So I came up with the following and got these errors. How should I be compiling so I can get a hex to upload using avrdude?

$ clang++ a.cpp -DF_CPU=16000000 -mmcu=atmega2560 -Wall -Werror -Wextra -Os --target=avr -I/usr/avr/include/ -nostdlib -L/usr/avr/lib/avr6 -lc -latmega2560
/usr/bin/avr-ld: skipping incompatible /usr/avr/lib/avr6/libc.a when searching for -lc
/usr/bin/avr-ld: cannot find -lc
/usr/bin/avr-ld: skipping incompatible /usr/avr/lib/avr6/libatmega2560.a when searching for -latmega2560
/usr/bin/avr-ld: cannot find -latmega2560

As we see in errors, linking is not going fine. But when you link manually, it links just fine. Are you sure that libc and libatmega2560 are the same when you link manually? I am a bit confused. — CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP, Jan 07 '21 at 03:03
Also I noticed that when I install `gcc-avr` and `avr-libc` from apt, they work just fine without directly specifying them, but clang freaks out to `atmega2560`. — CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP, Jan 07 '21 at 03:11
@CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP I'm not sure of anything tbh. I just know if I do `-c` and link with arv-gcc it doesn't complain but I dont know if the code is completely incorrect. I want to use a non C language on arduino who emits LLVM IR so I'm hoping I can get the code running later. — Eric Stotch, Jan 07 '21 at 16:26
Understood. Speaking about correctness, I am not sure if I can help :( I like playing with LLVM too, but probably you'll have to run the code to guess if it works correctly :( Does your version of Clang freaks out to `atmega2560` too? — CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP, Jan 07 '21 at 20:40
Generally, if it will be able to run correctly - problem is kinda solved? — CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP, Jan 07 '21 at 20:50
@CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP, did you compile LLVM to enable AVR target and other experimental tagets? — Juraj, Jan 08 '21 at 08:04
@juraj Anyway, compiler freaks out to linker. Not so important I guess. — CPPCPPCPPCPPCPPCPPCPPCPPCPPCPP, Jan 08 '21 at 09:43

score 1 · Answer 1 · answered Jan 06 '21 at 20:17

1

AVR target is experimental in LLVM compiler for which clang is the C and C++ front-end. To enable experimental targets you must compile LLVM from source. This Stack Overflow answer describes how to do it.

Looking at the bug tracker I see there are good reasons why it is experimental.

answered Jan 06 '21 at 20:17

Juraj

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~~Is there no work arounds? I figured out if I do `-c` in llvm and use `avr-gcc` to link them it seems to work (didn't execute the code but it compiled/linked without error).~~ From the bug tracker it seems like it just doesn't produce correct code? – Eric Stotch Jan 06 '21 at 21:27
@EricStotch, workaround what? do you thing it generated machine code for the AVR without support for it? – Juraj Jan 07 '21 at 06:58

score 0 · Answer 2 · answered Jan 08 '21 at 09:56

I am not sure what to answer finally.

Probably it is not the worst idea to compile .o file with clang, and link everything manually just like you wish.

I am not sure, if it is needed to enable any experiment features, due I tried to compile something to AVR, and it works fine with clang-12 when I use llvm repository apt.llvm.org.

How do I use clang to compile for avr (arduino)

2 Answers2