[llvm] [RISCV] Tune flag for fast vrgather.vv (PR #124664)

[Petr Penzin via llvm-commits]

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@@ -2848,7 +2848,13 @@ InstructionCost RISCVTargetLowering::getLMULCost(MVT VT) const {
 /// is generally quadratic in the number of vreg implied by LMUL.  Note that
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ppenzin wrote:

Addressed

https://github.com/llvm/llvm-project/pull/124664