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

[Sjoerd Meijer via llvm-dev]

If people are happy with the idea in principle I can push a patch up soon that

shows what such a class could like so that people can comment on it?

Yeah, or if you got something already but needs a some more work, just upload it and tag it as a work-in-progress patch so people can have a sneak peek what's coming.


Cheers,

Sjoerd.

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of David Sherwood via llvm-dev <llvm-dev at lists.llvm.org>
Sent: 06 November 2020 08:17
To: Chris Tetreault <ctetreau at quicinc.com>; LLVM Dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [Proposal] Introducing the concept of invalid costs to the IR cost model


Hi Chris,



Thanks for the reply. My email was really to just ask if people were ok with the idea

in principle and I kind of deliberately avoided getting into too many details at this

point. If people are happy with the idea in principle I can push a patch up soon that

shows what such a class could like so that people can comment on it?



Regards,

David Sherwood.



From: Chris Tetreault <ctetreau at quicinc.com>
Sent: 05 November 2020 19:23
To: David Sherwood <David.Sherwood at arm.com>; LLVM Dev <llvm-dev at lists.llvm.org>
Subject: RE: [llvm-dev] [Proposal] Introducing the concept of invalid costs to the IR cost model



You’re being quite vague about what the extra state is. Is this because you haven’t yet settled on what it should be, or do you actually intend to let it be a template parameter:



template <typename T>

struct Cost {

  unsigned Val;

  T State;

}



Thanks,

   Christopher Tetreault



From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>> On Behalf Of David Sherwood via llvm-dev
Sent: Thursday, November 5, 2020 7:43 AM
To: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: [EXT] [llvm-dev] [Proposal] Introducing the concept of invalid costs to the IR cost model



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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