[PATCH] D83088: Introduce CfgTraits abstraction

Wed Aug 19 14:11:29 PDT 2020

nhaehnle added a comment.

In D83088#2225415 <https://reviews.llvm.org/D83088#2225415>, @dblaikie wrote:

>>> But I guess coming back to the original/broader design: What problems is this intended to solve? The inability to write non-template algorithms over graphs? What cost does that come with? Are there algorithms that are a bit too complicated/unwieldy when done as templates? 
>>> If it's specifically the static/dynamic dispatch issue - I'm not sure the type erasure and runtime overhead may be worth the tradeoff here, though if it is - it'd be good to keep the non-dynamic version common, rather than now having GraphTraits and CfgTraits done a bit differently, etc.
>>
>> It's not just over graphs, but taking SSA values into account as well -- that is the key distinction between GraphTraits and CfgTraits.
>
> Not sure I follow - could you give an example of a graph where the GraphTraits concept of the Graph and the CfgTraits concept of the graph (or, perhaps more importantly - features of the graph/API surface area/properties you can expose through the CFG API/concept/thing but not through GraphTraits?

See below.

>> The most immediate problem is divergence analysis, which is extremely complex and difficult to get right. If I had tried to fight the accidental complexity that comes with attempting to write such an algorithm as C++ templates in addition to the inherent complexity of the algorithm at the same time, I'm not sure I would have been able to produce anything workable at all.
>>
>> Frankly, I suspect that our dominator tree implementation also suffer because of this, though at least dominator trees are much more well studied in the academic literature, so that helps keep the inherent complexity under control.
>
> I'm totally open to discussing making APIs more usable, for sure - though I'm thinking it's likely a concept (like containers in the C++ standard library) might be the better direction.
>
> Perhaps some code samples showing how one would interact (probably not whole algorithms - maybe something simple like generating a dot diagram for a graph) with these things given different APIs (traits, concepts, and runtime polymorphism) - and implementations of each kind too.

Take a look here for example: https://github.com/nhaehnle/llvm-project/blob/715450fa7f968ceefaf9c3b04b47066866c97206/llvm/lib/Analysis/GenericConvergenceUtils.cpp#L499 -- this is obviously still fairly simple, but it's an example of printing out the results of an analysis in a way that's generic over the underlying CFG and SSA form. A statically polymorphic wrapper is here: https://github.com/nhaehnle/llvm-project/blob/715450fa7f968ceefaf9c3b04b47066866c97206/llvm/include/llvm/Analysis/GenericConvergenceUtils.h#L569

The simple example might be bearable writing as a template, precisely because it's simple -- so only looking at simple examples is unlikely to really capture the motivation. Really what the motivation boils down to is stuff like this: https://github.com/nhaehnle/llvm-project/blob/controlflow-wip-v7/llvm/lib/Analysis/GenericUniformAnalysis.cpp -- I don't fancy writing all this as a template.

Thid motivation would essentially go away if C++ could type-check against traits in the way that Rust and other languages like it can -- but it can't, so here we are.

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https://reviews.llvm.org/D83088