[llvm] [RISCV][CG]Use processShuffleMasks for per-register shuffles (PR #120803)

[Alexey Bataev via llvm-commits]

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@@ -5120,58 +5119,70 @@ static SDValue lowerShuffleViaVRegSplitting(ShuffleVectorSDNode *SVN,
 
   MVT ElemVT = VT.getVectorElementType();
   unsigned ElemsPerVReg = *VLen / ElemVT.getFixedSizeInBits();
-  unsigned VRegsPerSrc = NumElts / ElemsPerVReg;
-
-  SmallVector<std::pair<int, SmallVector<int>>>
-    OutMasks(VRegsPerSrc, {-1, {}});
-
-  // Check if our mask can be done as a 1-to-1 mapping from source
-  // to destination registers in the group without needing to
-  // write each destination more than once.
-  for (unsigned DstIdx = 0; DstIdx < Mask.size(); DstIdx++) {
-    int DstVecIdx = DstIdx / ElemsPerVReg;
-    int DstSubIdx = DstIdx % ElemsPerVReg;
-    int SrcIdx = Mask[DstIdx];
-    if (SrcIdx < 0 || (unsigned)SrcIdx >= 2 * NumElts)
-      continue;
-    int SrcVecIdx = SrcIdx / ElemsPerVReg;
-    int SrcSubIdx = SrcIdx % ElemsPerVReg;
-    if (OutMasks[DstVecIdx].first == -1)
-      OutMasks[DstVecIdx].first = SrcVecIdx;
-    if (OutMasks[DstVecIdx].first != SrcVecIdx)
-      // Note: This case could easily be handled by keeping track of a chain
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alexey-bataev wrote:

Do you expect that the multi-shuffles may cause perf degradation? Do you suggest to add some extra analysis and if it requires more than 3 2-vector shuffles just early exit?

https://github.com/llvm/llvm-project/pull/120803