RISC-V backend does not support PreserveMost calling convention

[CaiWeiran]

The RISC-V backend currently does not support the PreserveMost calling convention. Attempting to compile code that uses it results in a fatal error:

fatal error: error in backend: Unsupported calling convention

A minimal example can be seen on Compiler Explorer:
https://godbolt.org/z/h7e5WT7eM

This issue raises a few questions:

• Should the RISC-V backend implement support for PreserveMost, similar to x86, ARM, and AArch64?
• Alternatively, should the frontend (or mid-end) automatically lower unsupported calling conventions like PreserveMost to a more compatible one such as C?

It would be helpful to clarify whether support is planned or if users should explicitly avoid using this calling convention on RISC-V targets.

Thanks!

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: None (CaiWeiran)

The RISC-V backend currently does not support the `PreserveMost` calling convention. Attempting to compile code that uses it results in a fatal error:

> fatal error: error in backend: Unsupported calling convention

A minimal example can be seen on Compiler Explorer:
https://godbolt.org/z/h7e5WT7eM

This issue raises a few questions:

• Should the RISC-V backend implement support for PreserveMost, similar to x86, ARM, and AArch64?
• Alternatively, should the frontend (or mid-end) automatically lower unsupported calling conventions like PreserveMost to a more compatible one such as C?

It would be helpful to clarify whether support is planned or if users should explicitly avoid using this calling convention on RISC-V targets.

Thanks!

[folkertdev]

I'll add that many more targets support (or at least do not error on) preserve_mostcc (wasm, s390x, powerpc) making riscv kind of the odd one out.

This came up in rust-lang/rust#143319.

[wangpc-pp]

Adding the support should be easy, but I am not sure which registers we should preserve, maybe x5-x31?