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

[Chris Tetreault via llvm-dev]

It might also make sense to have functions that return costs for vector ops return some sort of `LinearPolyBase` with `ScalarTy` being `InstructionCost`.

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Vineet Kumar via llvm-dev
Sent: Monday, November 16, 2020 11:24 PM
To: David Sherwood <David.Sherwood at arm.com>; llvm-dev at lists.llvm.org
Subject: [EXT] Re: [llvm-dev] [Proposal] Introducing the concept of invalid costs to the IR cost model


Hi David,

This would be a very useful upgrade to the cost model.

One thing I want to add is that we need to be mindful of the cases where the cost is proportional (or inversely proportional) to the VF, for instance in the LoopVectorizationCostModel::selectVectorizationFactor(ElementCount MaxVF), there is a point where expected cost is divided by the VF. I believe there are other places where the instruction cost is dependent on the actual number of elements in the vector. While this is not a problem for fixed vectors, for scalable vectors we need to account for the vscale component of the VF.  I guess using the polynomial type for the base cost type might work.

Also, for scalable vectors, when making the cost based decision of whether to vectorize or not, we will need to consider the special case of comparing the scalable vectorization cost with the scalar loop cost, which is considered to be a fixed vector cost of VF=1. A simple solution might be to always assume that for scalable vectors, vectorization is always beneficial. Another option might be to assume that the vscale value is always at least 2, and compare the cost with the scalar loop based on that.

Thanks and Regards,

Vineet


On 2020-11-05 9:12 p.m., David Sherwood via llvm-dev 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.

Kind Regards,
David Sherwood.




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