[llvm-dev] RFC: (Co-)Convergent functions and uniform function parameters

[Mehdi Amini via llvm-dev]

> On Oct 24, 2016, at 12:38 PM, Nicolai Hähnle via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi all,
> 
> Some brain-storming on an issue with SPMD/SIMT backend support where I think some additional IR attributes would be useful. Sorry for the somewhat long mail; the short version of my current thinking is that I would like to have the following:
> 
> 1) convergent: a call to a function with this attribute cannot be moved to have additional control dependencies; i.e., moving it from A to B is only possible if B dominates or post-dominates A.
> 
> 2) co-convergent (divergent? for lack of a better name...): a call to a function with this attribute cannot be moved to have _fewer_ control dependencies; i.e., moving it from A to B is only possible if A dominates or post-dominates B.
> 
> 3) uniform (for function arguments): transformations are not allowed to introduce additional non-uniformity in this argument.

Can you describe it in terms that are non-SPMD/SIMT? 
I.e. I’m not sure that “uniformity” refers to an existing LLVM IR concept.

Thanks,

— 
Mehdi

> 
> I'd appreciate input on this proposal, e.g. if this can be solved in an easier way or if there are obvious problems with this approach.
> 
> Thanks,
> Nicolai
> 
> ..........
> In a nutshell, the problem that this intends to solve is that:
> 
>  %v1 = texelFetch(%sampler, %coord0)
>  %v2 = texelFetch(%sampler, %coord1)
>  %v = select i1 %cond, vType %v1, %v2
> 
> is logically equivalent to and could benefit from being transformed to,
> 
>  %coord = select i1 %cond, cType %coord0, %coord1
>  %v = texelFetch(%sampler, %coord)
> 
> but on the other hand
> 
>  %v1 = texelFetch(%sampler0, %coord)
>  %v2 = texelFetch(%sampler1, %coord)
>  %v = select i1 %cond, vType %v1, %v2
> 
> _must_not_ be transformed to
> 
>  %s = select i1 %cond, sType %sampler0, %sampler1
>  %v = texelFetch(%s, %coord)
> 
> because of uniformity restrictions on the first argument of texelFetch.
> 
> We currently have a shader that is mis-compiled in the wild because something much like the latter transform is done by SimplifyCFG.[1] There, the equivalent thing happens with phi nodes:
> 
> if:
>  %v.if = texelFetch(%sampler0, %coord)
>  br label %end
> 
> else:
>  %v.else = texelFetch(%sampler1, %coord)
>  br label %end
> 
> end:
>  %v = phi [ %v.if, %if ], [ %v.else, %else ]
> 
> becomes
> 
>  ...
> end:
>  %s = phi [ %sampler0, %if ], [ %sampler1, %else ]
>  %v = texelFetch(%s, %coord)
> 
> I have collected some more examples at [2]
> 
> The distinctions between all three attribute types mentioned above makes sense, because there are OpenGL shader intrinsics that naturally carry almost all of the possible combinations of those attributes.
> 
> That said, a pass that is not SPMD/SIMT-aware must treat every function call that has a uniform argument as if it were co-convergent, because the input to the uniform argument could have non-uniformity whose structure correlates with control dependencies; see [2] for an example. Furthermore, I have not yet found an operation that needs the co-convergent attribute without also having uniform arguments. So for the purposes of LLVM, it may be sufficient to add the 'uniform' attribute for function arguments.
> 
> 
> [1] https://bugs.freedesktop.org/show_bug.cgi?id=97988
> [2] http://nhaehnle.blogspot.de/2016/10/compiling-shaders-dynamically-uniform.html
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