llvm dev - Jan 2020 - Alignment parameter attributes

I have a question about how to communicate alignment via function
parameters.  The IR supports an attribute specifying the alignment of a
pointer parameter:

define void @func(double* noalias align 64 %p)

This says that %p is 64-byte aligned.  Great!

Is there any way to convey the alignment of a pointer pointed to by a **
argument?  I'd like something like this:

define void @func(double* align 64 * noalias %p)

Or even this to add a noalias attribute:

define void @func(double* noalias align 64 * noalias %p)

I don't think either of these is legal today.

Has anyone else run into this need?  If so, any advice would be much
appreciated!

                  -David

On 01/14, Greene, David via llvm-dev wrote:
> I have a question about how to communicate alignment via function
> parameters.  The IR supports an attribute specifying the alignment of a
> pointer parameter:
> 
> define void @func(double* noalias align 64 %p)
> 
> This says that %p is 64-byte aligned.  Great!
> 
> Is there any way to convey the alignment of a pointer pointed to by a **
> argument?  I'd like something like this:
> 
> define void @func(double* align 64 * noalias %p)
> 
> Or even this to add a noalias attribute:
> 
> define void @func(double* noalias align 64 * noalias %p)
> 
> I don't think either of these is legal today.
> 
> Has anyone else run into this need?  If so, any advice would be much
> appreciated!
If you have this knowledge during code generation you can associate
align metadata to the loads from %p.

We could use a custom tag on `llvm.assume`, as an extension of
https://reviews.llvm.org/D72475, but that is not yet implemented.

Cheers,
  Johannes
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"Doerfert, Johannes via llvm-dev" <llvm-dev at lists.llvm.org>
writes:
> We could use a custom tag on `llvm.assume`, as an extension of
> https://reviews.llvm.org/D72475, but that is not yet implemented.
I had thought about using llvm.assume but was wondering if there is a
better way.  Tagging the loads with metadata really seems about the same
amount of effort.  Both require major surgery to code generation.

Thanks for your help Johannes!

                  -David