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

[Philip Reames via llvm-commits]

================
@@ -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">;
----------------
preames wrote:

I don't have a strong preference here, and would tend to bias towards support what we know of now, then change later if multiple options show up.  

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