[llvm-dev] Unable to link AVR object files with lld? (unrecognized relocation R_AVR_LO8_LDI_GS etc)

[Fangrui Song via llvm-dev]

On 2021-01-20, Carl Peto wrote:
>Ok that makes sense.
>
>I’m happy to try and add the relocations. I’m pretty comfortable with hacking llvm and with AVR assembly and the AVR back end in llvm.
>
>One thing I don’t understand. R_AVR_7_PCREL Is saying out of range.
>
>Is that because the implementation is there but is buggy?

Unlikely a bug.

For such long branches, there are usually two approaches

* compiler: avoid short range branches if likely overflow
* assembler: if a branch instruction can be converted to a wider range form
* linker: range extension thunk mechanism

I know barely nothing about AVR... I guess the first two are more
possible and the LLVM backend hasn't implemented them.

>I thought about “branch relaxing code” issues, which is something I’ve heard of but never quite understood. However that’s a compile thing I think. Not a linker thing. The code links fine using the standard gnu linker so perhaps that’s a dead end?
>
>
>Secondly, are there any good resources, blogs, etc. That I can use to help learn how the linker works? Or is it just read the source code for lld and come back here if I get stuck?

There are some LLD talks giving high-level overview.  You may debug
Finding the review threads of various patches introducing some new
features is useful.  You may also debug lld/test/ELF tests. For newer
tests, we've been spent more time writing the tests and adding comments.

>Final question, when it comes time to submit a patch, I’ve never really done it properly before. I used to had them to the maintainer Dylan. Who did the work of formal submission. Is there some good getting started guides for how to use Phabricator etc. Do I raise a bug on there then submit a patch there somehow?

You can `git log -- avr-reloc.s` :)

In 2019 I added ppc32 support and largely improved RISC-V.  I hope
ppc32-*.s and riscv-*.s are well organized and representative for
targets we don't devote too much time:)

>Thanks for any help people can give!
>
>
>> On 19 Jan 2021, at 23:30, Fangrui Song <maskray at google.com> wrote:
>>
>> On 2021-01-19, Carl Peto via llvm-dev wrote:
>>> I built llvm with clang and lld with AVR as one of the target architectures and I’m trying to use them to build an AVR executable.
>>>
>>> llc is working well, making object files from llvm or files.
>>>
>>> I can link these just fine using gnu ld with this command:
>>>
>>> Carls-MacBook-Air:AVR2 carlpeto$ "/Users/carlpeto/compilers/extracted/bin/avr-ld" -L"/Users/carlpeto/compilers/extracted/lib/avr-libc/lib/avr5" -L"/Users/carlpeto/compilers/extracted/lib/avr-libgcc/avr5" -L"/Users/carlpeto/compilers/extracted/lib/avr-binutils/avr/lib/ldscripts" -Tdata 0x800100 -Tavr5.xn --gc-sections "/Users/carlpeto/compilers/extracted/lib/avr-libc/lib/avr5/crtatmega328p.o"  -o main.elf main.o reference.o uSwift/Runtime/missing_runtime_show_warning.o -L AVR -lAVR -L uSwift -lSwift --start-group -lgcc -lm -lc -latmega328p --end-group
>>> Carls-MacBook-Air:AVR2 carlpeto$
>>>
>>> No error message and the elf file is well structured and runs.
>>>
>>>
>>> But if I take the above command and just replace avr-ld with my version of lld, it fails to link with a bunch of errors about relocations...
>>>
>>> Carls-MacBook-Air:AVR2 carlpeto$ ~/compilers/llvm-project/llvm/build/bin/ld.lld -L"/Users/carlpeto/compilers/extracted/lib/avr-libc/lib/avr5" -L"/Users/carlpeto/compilers/extracted/lib/avr-libgcc/avr5" -L"/Users/carlpeto/compilers/extracted/lib/avr-binutils/avr/lib/ldscripts" -Tdata 0x800100 -Tavr5.xn --gc-sections "/Users/carlpeto/compilers/extracted/lib/avr-libc/lib/avr5/crtatmega328p.o"  -o main.elf main.o reference.o uSwift/Runtime/missing_runtime_show_warning.o -L AVR -lAVR -L uSwift -lSwift --start-group -lgcc -lm -lc -latmega328p --end-group
>>> ld.lld: warning: cannot find entry symbol _start; defaulting to 0x0
>>> ld.lld: error: vfprintf.c:(function vfprintf: .text.avr-libc+0x4): unrecognized relocation R_AVR_LO8_LDI_GS
>>> ld.lld: error: snprintf.c:(function snprintf: .text.avr-libc+0x4): unrecognized relocation R_AVR_LO8_LDI_GS
>>> ld.lld: error: serial.c:(function _setupSerial: .text._setupSerial+0xC): relocation R_AVR_7_PCREL out of range: 68 is not in [-64, 63]
>>> ld.lld: error: util.c:(function _stringAddWord: .text._stringAddWord+0x14): relocation R_AVR_7_PCREL out of range: 92 is not in [-64, 63]
>>> ld.lld: error: util.c:(function _stringCompare: .text._stringCompare+0x2): relocation R_AVR_7_PCREL out of range: 74 is not in [-64, 63]
>>> ld.lld: error: serial.c:(function __vector_24: .text.__vector_24+0x90): relocation R_AVR_7_PCREL out of range: -78 is not in [-64, 63]
>>> ld.lld: error: malloc.c:(function malloc: .text.avr-libc+0x36): relocation R_AVR_7_PCREL out of range: 120 is not in [-64, 63]
>>> ld.lld: error: /Users/carlpeto/compilers/extracted/lib/avr-libc/lib/avr5/libc.a(ultoa_invert.o):(function __ultoa_invert: .text.avr-libc+0x6): relocation R_AVR_7_PCREL out of range: 86 is not in [-64, 63]
>>> ld.lld: error: util.c:(function __vector_6: .text.__vector_6+0x40): relocation R_AVR_7_PCREL out of range: 82 is not in [-64, 63]
>>> ld.lld: error: util.c:(function __vector_11: .text.__vector_11+0x2C): relocation R_AVR_7_PCREL out of range: 96 is not in [-64, 63]
>>> ld.lld: error: util.c:(function _stringCompare: .text._stringCompare+0x8): relocation R_AVR_7_PCREL out of range: 68 is not in [-64, 63]
>>> ld.lld: error: serial.c:(function __vector_24: .text.__vector_24+0xB0): relocation R_AVR_7_PCREL out of range: 124 is not in [-64, 63]
>>> ld.lld: error: snprintf.c:(function snprintf: .text.avr-libc+0x6): unrecognized relocation R_AVR_HI8_LDI_GS
>>> ld.lld: error: vfprintf.c:(function vfprintf: .text.avr-libc+0x6): unrecognized relocation R_AVR_HI8_LDI_GS
>>> ld.lld: error: util.c:(function __vector_6: .text.__vector_6+0x48): relocation R_AVR_7_PCREL out of range: 74 is not in [-64, 63]
>>> ld.lld: error: util.c:(function __vector_9: .text.__vector_9+0x2C): relocation R_AVR_7_PCREL out of range: 74 is not in [-64, 63]
>>> ld.lld: error: util.c:(function _stringCompare: .text._stringCompare+0xC): relocation R_AVR_7_PCREL out of range: 64 is not in [-64, 63]
>>> ld.lld: error: malloc.c:(function malloc: .text.avr-libc+0x40): relocation R_AVR_7_PCREL out of range: 100 is not in [-64, 63]
>>> ld.lld: error: vfprintf.c:(function vfprintf: .text.avr-libc+0x64): relocation R_AVR_7_PCREL out of range: 70 is not in [-64, 63]
>>> ld.lld: error: serial.c:(function __vector_24: .text.__vector_24+0x13C): relocation R_AVR_7_PCREL out of range: 84 is not in [-64, 63]
>>> ld.lld: error: too many errors emitted, stopping now (use -error-limit=0 to see all errors)
>>> Carls-MacBook-Air:AVR2 carlpeto$
>>>
>>>
>>> Does anyone have any ideas what I’ve done wrong?
>>
>> The AVR port is incomplete. Some relocations are not handled.
>