Symbol Mangling

[philberty]

Compiling Generics requires us to use symbol mangling to avoid the duplicate symbol error in linking.
This could also be part of a cleanup effort for the backend's handling for mangling in general.

• Legacy
• Legacy with actual sip-128 hash from https://github.com/rust-lang/rust/blob/506e75cbf8cb5305e49a41326307004ca3976029/compiler/rustc_data_structures/src/sip128.rs
• V0 https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html

[bjorn3]

The hash that is part of the symbol name includes both the crate name and value passed in using -Cmetadata if any (always done by cargo). This metadata value allows for multiple versions of the same crate without symbol conflicts. Rustc will error when trying to link multiple versions of the same crate with the same -Cmetadata but different SVH (semantic version hash, derived from all inputs of the compilation process) in a single crate graph. The crate types bin, staticlib and cdylib are always the root of a crate graph. The crate types rlib and dylib are not, but rustc still checks for conflicts as far as I know as any conflicts present there must also be present in the final crate graph.

[philberty]

Thanks for the heads up I didn't realise about -Cmetadata which is a really neat idea.

[philberty]

Rust has 2 forms of symbol mangling:

1. Legacy which is what is in stable
2. V0 which is unstable and defined in https://rust-lang.github.io/rfcs/2603-rust-symbol-name-mangling-v0.html

I think we will need to support both at some point for now lets implement legacy in the short term. Down the line when rust-lang/rust#60705 is stableized we can implement that which would be ideal.

[alex]

This is putting the cart way way way before the horse, but FYI: The Rust-for-Linux project uses the unstable v0 (though I have no recollection of why :-)). We're very interested in the progress of gccrs, for hopefully obvious reasons, so I figured I'd let you know.

[philberty]

Thanks for the heads up @alex

[philberty]

For the generics milestone right now I will be implementing basic legacy with a dummy hash for now.

[philberty]

This PR implements the first part for legacy symbol mangling #425

[philberty]

Moving this ticket out of the generics milestone, since we have a basic implementation of the legacy form will be enough for that milestone to be completed for now.

[ojeda]

The Rust-for-Linux project uses the unstable v0 (though I have no recollection of why :-)).

The reason is that the "legacy" mangling scheme uses invalid characters for C identifiers (like $) which is a problem when reusing the kernel's EXPORT_SYMBOL_* C macros. Since I noticed v0 does not, I simply went for that. :)