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

[Nicolai Hähnle via llvm-dev]

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.

Nicolai



>
> Thanks,
> Nicolai
>
>>
>> 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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>>> llvm-dev at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>