[LLVMdev] : Predication on SIMD architectures and LLVM

[Tom Stellard]

On Wed, Oct 31, 2012 at 09:13:43PM +0100, Bjorn De Sutter wrote:
> Hi all,
> 
> I am working on a CGRA backend (something like a 2D VLIW), and we also absolutely need predication. I extended the IfConversion pass to allow it to be executed multiple times and to predicate already predicated code. This is necessary to predicate code with nested conditional statements. At this point, we support or, and, and conditional predicates (see Scott Mahlke's papers on this issue) as supported by the OpenIMPACT compiler (=Trimaran). If anyone is interested, I can show some of the code. It is rather ad-hoc, however, so it is not at all ready for integration in the trunk (I think). 
>

I would be interested in looking at the code, since Southern Islands
GPUs also require predicates for nested conditions.

-Tom

> The problem we are still facing is that this predication works post instruction selection and post register allocation. This is problematic because some of the earlier optimizations such as loop unrolling should ideally be applied on if-converted code, on which it is easier to judge the opportunities for, e.g., modulo scheduling and initiation interval constraints (such as ResMII, RecMII).
> 
> In my view, the ideal would be to have very generic, full (OpenIMPACT-like) predication support throughout LLVM, with the option of enabling/skipping early if-conversion just like one can enable or disable aggressive inlining. 
> 
> Best,
> 
> Bjorn De Sutter
> Computer Systems Lab
> Ghent University
> 
> 
> 
> 
> 
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