[llvm] [LV] Factor costInterleaveGatherScatter (NFC) (PR #215857)

[Ramkumar Ramachandra via llvm-commits]

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@@ -2400,6 +2456,13 @@ bool LoopVectorizationCostModel::isLegalMaskedLoadOrStore(
                                          getLoadStoreAddressSpace(I));
 }
 
+bool LoopVectorizationCostModel::isLegalGatherOrScatter(Instruction *I,
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artagnon wrote:

Yes, we're trying our level best to move away from the legacy cost model. However, we must preserve legacy behavior in https://github.com/llvm/llvm-project/pull/215862, as a starting point: several other VPlan-based widening routines already use stuff from the legacy CM via CostCtx, and I do the same in the gather-scatter migration. At the moment, this refactoring is necessary to re-use the code in the gather-scatter widening: the function is private anyway, so I don't think it matters?

https://github.com/llvm/llvm-project/pull/215857