[llvm-dev] [Proposal] Introducing the concept of invalid costs to the IR cost model

[Chris Lattner via llvm-dev]

> On Nov 5, 2020, at 7:42 AM, David Sherwood via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Hi,
>  
> I'd like to propose a change to our cost interfaces so that instead of returning
> an unsigned value from functions like getInstructionCost, getUserCost, etc., we
> instead return a wrapper class that encodes an integer cost along with extra
> state. The extra state can be used to express:
>  
> 1. A cost as infinitely expensive in order to prevent certain optimisations
> taking place. For example, there are already examples in LLVM where the cost is
> set extremely high, but not so high that it would cause overflow. This might be to
> prevent vectorisation in cases where we would have to scalarize the operation,
> which is particularly relevant for scalable vectors, where scalarisation is
> not [yet] available. There isn't currently a standard value for something that
> constitutes very expensive and we can replace all the magic numbers with a
> single invalid state.
> 2. A cost as unknown, where the user is simply unable to determine an accurate
> cost for an operation.
>  
> This new wrapper class would work almost seamlessly with existing code as it
> would contain the full set of operators required for arithmetic and comparisons.
> This is in addition to the ability to create invalid costs and query the validity
> of an existing cost. Once a cost becomes invalid or unknown it will remain in that
> state regardless of any further arithmetic performed.

This sounds like a great step to me David,

-Chris


-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20201105/9013a862/attachment.html>