[PATCH] D79744: clang: Add address space to indirect abi info and use it for kernels

[Johannes Doerfert via Phabricator via cfe-commits]

jdoerfert added a comment.

Drive by, I haven't read the entire history.

In D79744#2040208 <https://reviews.llvm.org/D79744#2040208>, @rjmccall wrote:

> I don't understand why `noalias` is even a concern if the whole buffer passed to the kernel is read-only.  `noalias` is primarily about proving non-interference, but if you can tell IR that the buffer is read-only, that's a much more powerful statement.


The problem is that it is a "per-pointer" attribute and not "per-object". Given two argument pointers, where one is marked `readonly`, may still alias. Similarly, an access to a global, indirect accesses, ... can write the "readonly" memory. Hence, the `readonly` is pretty useless *in the callee* if other accesses can write the memory. The `readonly` is useful in the caller though, usually if we have basically `noalias` information there (e.g., it is an alloca). `Noalias` is useful in the callee regardless of `readonly` but even better with.

> Regardless, if you do need `noalias`, you should be able to emit the same IR that we'd emit if pointers to all the fields were assigned into `restrict` local variables and then only those variables were subsequently used.

We are still debating the local restrict pointer support. Once we merge that in it should be usable here.


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  https://reviews.llvm.org/D79744/new/

https://reviews.llvm.org/D79744