Symbols missing on AVR target

[jonahbron]

When compiling to the avr-unknown-unknown platform using the compiler fork, I get errors about the following missing symbols.

• __ashlsi3
• __lshrsi3
• __divmodti4
• __ashrsi3

I'm trying to track down information on how to fix this. Someone in the original ticket I opened suggested I fork compiler-builtins and implement these intrinsics.

avr-rust/rust-legacy-fork#149 (comment)

I'd love any direction anyone can offer on whether this is really the correct approach, and how to add these, this is all super new to me. Thanks in advance.

[alexcrichton]

Thanks for the report! These intrinsics haven't yet been ported to Rust yet, presumably because they haven't been needed by other targets. They can be added at any time though! You'll probably be editing src/int/*.rs, and there should be examples throughout there of how to write the intrinsics.

You'll need to consult gcc/clang though to see what the expected signature of these intrinsics are.

[jonahbron]

@alexcrichton I've run into some very interesting behavior. As soon as I make compiler-builtins a dependency with Xargo, my program suddenly compiles. This is odd, considering those missing symbols are not provided by compiler-builtins right now. However, the area of my application affected by these errors does not behave as expected still, despite now compiling.

[alexcrichton]

Ah sorry I'm not entirely sure what's happening in that case, most things having to do with compiler-builtins tend to be very-specific bugs or difficult to reproduce.

[jonahbron]

@alexcrichton It seems like things are in fact working (bugs in my own code it seems). I still have no idea why adding the compiler-builtins dependency magically fixes the problems when it doesn't have the symbols that the compiler is complaining about, but if it compiles I'm happy 🤷‍♂️