[llvm] [LoongArch] Split 256-bit build_vector to avoid using LASX element insertion (PR #154918)

[via llvm-commits]

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@@ -2647,6 +2646,43 @@ static SDValue lowerBUILD_VECTORAsBroadCastLoad(BuildVectorSDNode *BVOp,
   return SDValue();
 }
 
+// Sequentially insert elements from Ops into Vector, from low to high indices.
+// Note: Ops can have fewer elements than Vector.
+static void fillVector(ArrayRef<SDValue> Ops, SelectionDAG &DAG, SDLoc DL,
+                       const LoongArchSubtarget &Subtarget, SDValue &Vector,
+                       EVT ResTy) {
+  assert(Ops.size() <= ResTy.getVectorNumElements());
+
+  SDValue Op0 = Ops[0];
+  if (!Op0.isUndef())
+    Vector = DAG.getNode(ISD::SCALAR_TO_VECTOR, DL, ResTy, Op0);
+  for (unsigned i = 1; i < Ops.size(); ++i) {
+    SDValue Opi = Ops[i];
+    if (Opi.isUndef())
+      continue;
+    Vector = DAG.getNode(ISD::INSERT_VECTOR_ELT, DL, ResTy, Vector, Opi,
+                         DAG.getConstant(i, DL, Subtarget.getGRLenVT()));
+  }
+}
+
+// Build a ResTy subvector from Node, taking NumElts elements starting at index
+// 'first'.
+static SDValue fillSubVectorFromBuildVector(BuildVectorSDNode *Node,
+                                            SelectionDAG &DAG, SDLoc DL,
+                                            const LoongArchSubtarget &Subtarget,
+                                            EVT ResTy, unsigned first) {
+  unsigned NumElts = ResTy.getVectorNumElements();
+
+  assert(first >= 0 &&
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zhaoqi5 wrote:

Thank you for catching this. This assert `first >= 0` can be safely removed. I am currently on National Day holiday and unable to submit the patch. It would be very helpful if you could contribute the fix. If you are unavailable, I will be pleased to submit it once I am back.

https://github.com/llvm/llvm-project/pull/154918