The Rust Calling Convention We Deserve

2 days ago

The article criticizes the C ABI for being inefficient in passing complex types and suggests the Go register ABI as a better alternative.
Rust currently uses a conservative calling convention that mimics the C ABI, leading to suboptimal code generation for simple functions.
The author proposes a new calling convention for Rust, `-Zcallconv=fast`, which would optimize argument passing by utilizing registers more effectively.
The proposed calling convention involves complex heuristics for register allocation, including handling of structs, unions, and enums to maximize register usage.
The article discusses the technical challenges of implementing such a calling convention in LLVM, including the need for function prologues and epilogues to handle register-based argument passing.
The author suggests that Rust could leverage its ability to inspect function bodies to optimize ABIs further, such as by eliminating unused arguments or passing hot arguments in registers.
The conclusion highlights the potential for significant performance improvements but acknowledges the complexity and expertise required to implement these changes in Rust's compiler.

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