[lld] Fix R_AVR_7_PCREL and R_AVR_13_PCREL range checking (PR #92695)

Tue May 21 23:24:10 PDT 2024

benshi001 wrote:

> > I wonder how RJMP can jumps over 4kB on the ATtiny85 device. Is RJMP has a different encoding against other devices ?
> 
> I can't quickly find it in the instruction set manual, but I did find this:
> 
> > For AVR microcontrollers with Program memory not exceeding 4K words (8KB) this instruction can address the entire memory from every address location.
> 
> I assume it wraps around, because that's the only way I see that would make it possible to address the full 8kB of memory. So for example if there's a `rjmp .+4094` at address 6114, it will jump to (6114 + 4094 + 2) = 10210, which wraps around to address 2018. (The +2 is the size of the instruction itself).
> 
> I also found this in the [attiny85 datasheet](https://ww1.microchip.com/downloads/en/DeviceDoc/Atmel-2586-AVR-8-bit-Microcontroller-ATtiny25-ATtiny45-ATtiny85_Datasheet.pdf):
> 
> > The ATtiny25/45/85 Program Counter (PC) is 10/11/12 bits wide, thus addressing the 1024/2048/4096 Program memory locations
> 
> This means that the attiny85 program counter is 12 bits, which means that it will indeed wrap around (12 bits is 4096 words, is 8192 bytes).
> 
> For avr-ld, there's an option to enable/disable wraparound (but it appears that wraparound is enabled by default): https://sourceware.org/binutils/docs-2.30/as/AVR-Options.html. See `-mno-wrap`. I don't know whether we can add a flag like that to lld, I'd much prefer if we didn't need a special chip-specific flag for this (in my opinion a better solution would be two different relocations: one with wraparound and one without - but that would break compatibility with avr-ld).

I would agree that lld should support wraparound  as default, but could you please supplement some encoding tests for that in [lld/test/ELF/avr-reloc.s](https://github.com/llvm/llvm-project/pull/92695#diff-020c5c5113ee98bd23be19a24d0347a8cf5d149c4758a9ac5379861206637696), just like the normal encoding tests.

https://github.com/llvm/llvm-project/pull/92695