[llvm] [LoongArch] Lower build_vector to broadcast load if possible (PR #135896)

[via llvm-commits]

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@@ -1876,6 +1876,51 @@ static bool isConstantOrUndefBUILD_VECTOR(const BuildVectorSDNode *Op) {
   return false;
 }
 
+// Lower BUILD_VECTOR as broadcast load (if possible).
+// For example:
+//   %a = load i8, ptr %ptr
+//   %b = build_vector %a, %a, %a, %a
+// is lowered to :
+//   (VLDREPL_B $a0, 0)
+static SDValue lowerBUILD_VECTORAsBroadCastLoad(BuildVectorSDNode *BVOp,
+                                                const SDLoc &DL,
+                                                SelectionDAG &DAG) {
+  MVT VT = BVOp->getSimpleValueType(0);
+  int NumOps = BVOp->getNumOperands();
+
+  assert((VT.is128BitVector() || VT.is256BitVector() || VT.is512BitVector()) &&
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heiher wrote:

Just wondering if `is512BitVector()` is actually needed here?

https://github.com/llvm/llvm-project/pull/135896