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

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

In D79744#2040434 <https://reviews.llvm.org/D79744#2040434>, @jdoerfert wrote:

> In D79744#2040380 <https://reviews.llvm.org/D79744#2040380>, @rjmccall wrote:
>
> > In D79744#2040348 <https://reviews.llvm.org/D79744#2040348>, @jdoerfert wrote:
> >
> > > 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.
> >
> >
> > Oh, do we really not have a way to mark that memory is known to be truly immutable for a time?  That seems like a really bad limitation.  It should be doable with a custom alias analysis at least.
>
>
> `noalias` + `readone` on an argument basically implies immutable for the function scope. I think we have invariant intrinsics that could do the trick as well, though I'm not too familiar with those. I was eventually hoping for paired/scoped `llvm.assumes` which would allow `noalias` + `readnone` again. Then there is `invariant` which can be placed on a load instruction. Finally, TBAA has a "constant memory" tag (or something like that), but again it is a per-access thing. That are all the in-tree ways I can think of right now.
>
> Custom alias analysis can probably be used to some degree but except address spaces I'm unsure we have much that you can attach to a pointer and that "really sticks".
>
> >>> 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.
> > 
> > I thought that was finished a few years ago; is it really not considered usable yet?  Or does "we" not just mean LLVM here?
>
> Yes, I meant "we" = LLVM here. Maybe we talk about different things. I was referring to local restrict qualified variables, e.g., `double * __restrict Ptr = ...`. The proposal to not just ignore the restrict (see https://godbolt.org/z/jLzjR3) came last year, it was a big one and progress unfortunately stalled (partly my fault). Now we are just about to see a second push to get it done.
>  Is that what you meant too?


I thought I remembered Hal doing a lot of work on local restrict a few years ago.  I'm probably just misremembering, or I didn't realize that the work never landed or got pulled out later.

Okay.  So where we're at is that you'd like to add a new argument-passing convention that's basically "passed indirect but immutable", implying that the frontend has to copy it in order to create the mutable local parameter.  That's not actually a totally ridiculous convention in principle, although it has poor worst-case behavior (copies on both sides), and that happens to be what the frontend will often have to conservatively emit.  I would still prefer not to add new argument-passing conventions just to satisfy short-term optimization needs, though.  Are there any other reasonable options?


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