[llvm-dev] [RFC] Being explicit in the cost model

Sam Parker via llvm-dev llvm-dev at lists.llvm.org
Tue Apr 28 08:13:48 PDT 2020


I have been working in the TargetTransformInfo layer for the last few weeks with several ongoing goals:
> Remove ambiguity.
> Improve cost modelling performance, accuracy and utility.
> Help empower the backends.
> Reduce the surface area of the API.
> Reduce the dependencies between the layers - of which there are many!

My latest patch is an NFC to help address the issue of ambiguity, I have uploaded the patch here: https://reviews.llvm.org/D79002. It's a biggy, adding TargetCostKind as an argument to almost all the get*Cost methods. My hope is that, as well as clarity, it will allow some backends to re-use and correlate costs if they wish. It would also allow adding a hook to query the backend which cost kind is the most important to it, which could then be passed around.

But as the patch currently stands, even as an NFC, it could still lead users astray because nothing else is setup to differentiate and return a specific cost. I'd like to hear from the backend people to hear what kind of costs they were modelling for and how we should handle the 'default' behaviour. For instance, whether they're happy to return the same cost for all cost kinds, or whether a check should be performed first on the cost kind first and only return something specific if its the expected cost kind. There's also the issue of the value of default arguments - I've used RecipThroughput for the calls used by the vectorizers and SizeAndLatency for the rest, but I wonder if default values should be used at all.


Sam Parker

Compilation Tools Engineer | Arm

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