Missing support for GCC's attribute access (IR attributes with size index?)

[davidbolvansky]

https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes

access (access-mode, ref-index)
access (access-mode, ref-index, size-index)

The access attribute specifies that a function to whose by-reference arguments the attribute applies accesses the referenced object according to access-mode. The access-mode argument is required and must be one of four names: read_only, read_write, write_only, or none. The remaining two are positional arguments.

We have IR attributes read_only and write_only, so mapping for these two modes could be easy. In terms of size-index, maybe we could extend existing IR attributes to optionally take size-index?

cc @jdoerfert (IR attributes)
cc @fhahn (DSE)
cc @nikic (Rust side; can this generalization of IR attributes be useful for Rust?)

[llvmbot]

@llvm/issue-subscribers-clang-codegen

[davidbolvansky]

cc @nickdesaulniers for usage in Linux kernel..

[jdoerfert]

I would recommend not to add size to the existing ones but instead replace them as whole. That said, mapping all but the size argument to existing ones would be great and pretty straight forward (IMHO).

[kees]

I would love to start using this for the Linux kernel. Tracking it here: KSPP/linux#208
cc @gwelymernans