Module rust_cheri_compressed_cap::c_funcs [−][src]
Import C functions for CC64,128.
We used to do this separately in cc64.rs and cc128.rs for the 64 and 128 bit versions. This seemed to cause multiple-definition link errors, so now we link all of them in one place.
Normally Rust warns us about using u128, i128 in FFI function arguments, because there isn’t a well-defined ABI.
We initially tried to create a repr(C) struct that matched the ABI, but this seems impossible.
Clang (as of LLVM-13) defines u128,i128 as built-in types which are passed in two registers.
This cannot be replicated exactly by passing a struct - see clang/lib/CodeGen/TargetInfo.cpp: X86_64ABIInfo::classify(),
and this godbolt example.
Therefore, we instead use Rust native u128, i128 under the assumption that
- Rust is being compiled under LLVM
- Rust native u128, i128 are represented with LLVM’s built-in 128-bit types
- The C code is also compiled with LLVM, using the built-in 128-bit types.
- The versions of LLVM used to compile Rust, C are ABI-compatible for 128-bit types.
Assumption 3 may not be necessary - maintaining compatibility with GCC is in Clang’s interest, so GCC-compiled C may work just as well. Assumption 4 is difficult to check, we’re taking it on faith that LLVM doesn’t do anything silly in the future.
Under all these assumptions, we can safely ignore Rust’s “improper ctypes” warning.