[llvm] [RISCV] Handle fixed length vectors with exact VLEN in lowerINSERT_SUBVECTOR (PR #84107)

[Philip Reames via llvm-commits]

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@@ -9698,41 +9714,92 @@ SDValue RISCVTargetLowering::lowerINSERT_SUBVECTOR(SDValue Op,
     return DAG.getBitcast(Op.getValueType(), SubVec);
   }
 
-  unsigned SubRegIdx, RemIdx;
-  std::tie(SubRegIdx, RemIdx) =
-      RISCVTargetLowering::decomposeSubvectorInsertExtractToSubRegs(
-          VecVT, SubVecVT, OrigIdx, TRI);
+  MVT ContainerVecVT = VecVT;
+  if (VecVT.isFixedLengthVector()) {
+    ContainerVecVT = getContainerForFixedLengthVector(VecVT);
+    Vec = convertToScalableVector(ContainerVecVT, Vec, DAG, Subtarget);
+  }
 
-  RISCVII::VLMUL SubVecLMUL = RISCVTargetLowering::getLMUL(SubVecVT);
+  MVT ContainerSubVecVT = SubVecVT;
+  if (SubVecVT.isFixedLengthVector()) {
+    ContainerSubVecVT = getContainerForFixedLengthVector(SubVecVT);
+    SubVec = convertToScalableVector(ContainerSubVecVT, SubVec, DAG, Subtarget);
+  }
+
+  unsigned SubRegIdx;
+  ElementCount RemIdx;
+  // insert_subvector scales the index by vscale if the subvector is scalable,
+  // and decomposeSubvectorInsertExtractToSubRegs takes this into account. So if
+  // we have a fixed length subvector, we need to adjust the index by 1/vscale.
+  if (SubVecVT.isFixedLengthVector()) {
+    assert(VLen);
+    unsigned Vscale = *VLen / RISCV::RVVBitsPerBlock;
+    auto Decompose =
+        RISCVTargetLowering::decomposeSubvectorInsertExtractToSubRegs(
+            ContainerVecVT, ContainerSubVecVT, OrigIdx / Vscale, TRI);
+    SubRegIdx = Decompose.first;
+    RemIdx = ElementCount::getFixed((Decompose.second * Vscale) +
+                                    (OrigIdx % Vscale));
+  } else {
+    auto Decompose =
+        RISCVTargetLowering::decomposeSubvectorInsertExtractToSubRegs(
+            ContainerVecVT, ContainerSubVecVT, OrigIdx, TRI);
+    SubRegIdx = Decompose.first;
+    RemIdx = ElementCount::getScalable(Decompose.second);
+  }
+
+  RISCVII::VLMUL SubVecLMUL = RISCVTargetLowering::getLMUL(ContainerSubVecVT);
   bool IsSubVecPartReg = SubVecLMUL == RISCVII::VLMUL::LMUL_F2 ||
                          SubVecLMUL == RISCVII::VLMUL::LMUL_F4 ||
                          SubVecLMUL == RISCVII::VLMUL::LMUL_F8;
+  bool AlignedToVecReg = !IsSubVecPartReg;
----------------
preames wrote:

Again, not alignment in the naming here, and another instance of getting a fixed size from a scalable quantity.  

https://github.com/llvm/llvm-project/pull/84107