[LLVMdev] Predication on SIMD architectures and LLVM

[Ralf Karrenberg]

Hi Marcello,

I am sure I've seen some postings on the list concerning architectures 
that support predicated execution and how to map that to LLVM IR, I'm 
just not sure anymore when and who was involved :).

I have implemented your first suggestion for targets that do not have 
predicated instructions (where control flow to data flow conversion with 
explicit maintaining of masks and blend operations is the only option 
you have for SIMD vectorization). However, I agree with you that this is 
not a good solution for you since you would basically ignore one of the 
strengths of your platform. Should you still need some code or help with 
this, feel free to drop me a message.

Cheers,
Ralf

On 19.10.2012 17:38, Marcello Maggioni wrote:
> Hello,
> I'm working on a compiler based on LLVM for a SIMD architecture that
> supports instruction predication. We would like to implement branching
> on this architecture using predication.
> As you know the LLVM-IR doesn't support instruction predication, so I'm
> not exactly sure on what is the best way to implement it.
> We came up with some ways to do it in LLVM:
>
> - Do not add any predication in the IR (except for load and stores
> through intrinsics), linearize the branches and substitute PHI nodes
> with selects for merging values . In the backend then we would custom
> lower the select instruction to produce a predicated mov to choose the
> right version of the value. I think this option doesn't make use of the
> possible benefits of the architecture we are targeting at all.
>
> - Another way could be adding intrinsics for all instructions in the
> target to make them support predication, still linearize all the
> branches, but use instruction predication instead of generating cmovs .
> The backend then would custom lower almost any instruction into
> predicated custom nodes that are matched through tablegen patterns. We
> could generate these intrinsics in the same IR pass that linearizes
> branches.
>
> - Make a custom backend that actually directly outputs predicated
> instructions (we really mainly only need one type of predicate , so
> every instruction could use that kind of predicate ...) but I think this
> is a nasty solution ...
>
> Did someone already tried to do this in LLVM and if yes what solution/s
> did you use to solve the problem?
>
> Regards,
> Marcello
>