This intrinsic helps with supporting the various AMD & NVIDIA libraries like rocBLAS or cuBLAS.
They provide functions which must be called from the host, but require a mixture of host and device pointers.
This offload_args intrinsic maps our host allocations to device allocations and transfers memory as required.
It reuses the whole infrastructure which we already have for the main offload intrinsic.
Unlike the main offload intrinsic, this also already fully works with std. I also got it to work with a single cargo invocation:
RUSTFLAGS="-L native=/opt/rocm-6.4.0/lib -l dylib=rocblas -l dylib=amdhip64 -l dylib=omp -l dylib=omptarget -Zoffload=Args -Zunstable-options" cargo +offload run -r

To be cleaned up.

TODO: handle mangled fnc names. Done

I updated compiler/rustc_monomorphize/src/collector/autodiff.rs, it now works without no_mangle, otherwise the function won't be codegen'ed. It also works without lto=fat if we only have main.rs
If we put and use stuff in lib.rs and call it in main.rs, then it currently trips the verifier. I guess that should be easy to fix:

thread 'rustc' (494962) panicked at compiler/rustc_monomorphize/src/collector.rs:468:13:
assertion failed: tcx.should_codegen_locally(instance)
stack backtrace:

cc @kevinsala @Sa4dUs