13d8ba7dea
There are a couple of places in the loop vectoriser where we want to calculate the cost of extracting the last lane in a vector. However, we wrongly assume that asking for the cost of extracting lane (VF.getKnownMinValue() - 1) is an accurate representation of the cost of extracting the last lane. For SVE at least, this is non-trivial as it requires the use of whilelo and lastb instructions. To solve this problem I have added a new getReverseVectorInstrCost interface where the index is used in reverse from the end of the vector. Suppose a vector has a given ElementCount EC, the extracted/inserted lane would be EC - 1 - Index. For scalable vectors this index is unknown at compile time. I've added a AArch64 hook that better represents the cost, and also a RISCV hook that maintains compatibility with the behaviour prior to this PR. I've also taken the liberty of adding support in vplan for calculating the cost of VPInstruction::ExtractLastElement.
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