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

[Craig Topper 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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topperc wrote:

x280 cost is linear in VL. Later x280 is constant time for VL<= (VLEN/2)/SEW (single DLEN), but linear in VL for everything else. Later x280 is quadratic in LMUL*2 except for fractional LMUL or when element at a time would be faster (i.e. there are less elements in a destination DLEN than the number of possible source DLENs for them).

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