[PATCH] D135991: [AArch64] Fix cost model for `udiv` instruction when one of the operands is a uniform constant

[Simon Pilgrim via Phabricator via llvm-commits]

RKSimon added inline comments.


================
Comment at: llvm/lib/Target/AArch64/AArch64TargetTransformInfo.cpp:2095
         // into scalar divisions of each pair of elements.
         Cost += getArithmeticInstrCost(Instruction::ExtractElement, Ty,
                                        CostKind, Op1Info, Op2Info);
----------------
dmgreen wrote:
> Using getArithmeticInstrCost here seems very odd.
> 
> I would expect the base cost to be `N*getVectorInstrCost(Instruction::ExtractElement) + N* getVectorInstrCost(Instruction::InsertElement) + N*getArithmeticInstrCost(Div)`. Sometimes we use a cheaper cross-register-bank copy cost than getVectorInstrCost would give.
> 
> I would expect the getVectorInstrCost's to become removed if one of the operations was uniform or constant. And maybe the Cost += Cost shouldn't be needed if the vector instructions have correct costs. All the code here seems to be trying to give a very high cost for vector divides, as they will just be scalarized.
You should be able to use getScalarizationOverhead to handle all of the extract/insert costs for you.


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  https://reviews.llvm.org/D135991/new/

https://reviews.llvm.org/D135991