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

[Mehdi Amini via llvm-dev]

> On Oct 24, 2016, at 4:15 PM, Nicolai Hähnle <nhaehnle at gmail.com> wrote:
> 
> On 25.10.2016 01:11, Nicolai Hähnle wrote:
>> On 24.10.2016 21:54, Mehdi Amini wrote:
>>>> On Oct 24, 2016, at 12:38 PM, Nicolai Hähnle via llvm-dev
>>>> <llvm-dev at lists.llvm.org> wrote:
>>>> 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.
>> 
>> Yeah, that's actually the key problem I've been struggling with.
>> 
>> The first example I sent shows the gist of it. It also shows that the
>> concept can't be expressed in terms of the CFG, which makes this tricky.
>> 
>> In a way it's the data-flow analog of the convergent attribute: the
>> argument cannot be changed to have additional dependencies in its
>> computation. That captures the basic intention, but I'm not completely
>> sure that it works.
> 
> One big question is whether there are transformations in LLVM today that replace a value %a with a value %b, where %b "has additional dependencies in its computation". I can't think of anything obvious where that would be the case, but I'm not sure.

What do you mean by “additional dependencies in its computation" here? 

— 
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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