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

[Pengcheng Wang via llvm-commits]

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@@ -1365,6 +1365,10 @@ def FeatureUnalignedVectorMem
                       "true", "Has reasonably performant unaligned vector "
                       "loads and stores">;
 
+def TuneFastVRGather
+   : SubtargetFeature<"fast-vrgather", "HasFastVRGather",
+                      "true", "Has vrgather.vv with LMUL*log2(LMUL) latency">;
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wangpc-pp wrote:

Not blocking, but what if the `vrgather.vv` is not with `LMUL*log2(LMUL)` complexity?

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