[MLIR][Conversion] Vector to LLVM: Remove unneeded vectorshuffle

if vector.broadcast source is a scalar and target is a single element 1D vector.

Author: silee2

mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp

@@ -1723,12 +1723,16 @@ struct VectorBroadcastScalarToLowRankLowering
       return success();
     }
 
-    // For 1-d vector, we additionally do a `vectorshuffle`.
+    // For 1-d vector, we additionally do a `vectorshuffle` if vector width > 1.
     auto v =
         LLVM::InsertElementOp::create(rewriter, broadcast.getLoc(), vectorType,
                                       poison, adaptor.getSource(), zero);
 
     int64_t width = cast<VectorType>(broadcast.getType()).getDimSize(0);
+    if (width == 1) {
+      rewriter.replaceOp(broadcast, v);
+      return success();
+    }
     SmallVector<int32_t> zeroValues(width, 0);
 
     // Shuffle the value across the desired number of elements.

mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir

@@ -76,6 +76,17 @@ func.func @broadcast_vec1d_from_f32(%arg0: f32) -> vector<2xf32> {
 
 // -----
 
+func.func @broadcast_single_elem_vec1d_from_f32(%arg0: f32) -> vector<1xf32> {
+  %0 = vector.broadcast %arg0 : f32 to vector<1xf32>
+  return %0 : vector<1xf32>
+}
+// CHECK-LABEL: @broadcast_single_elem_vec1d_from_f32
+// CHECK-SAME:  %[[A:.*]]: f32)
+// CHECK:       %[[T0:.*]] = llvm.insertelement %[[A]]
+// CHECK:       return %[[T0]] : vector<1xf32>
+
+// -----
+
 func.func @broadcast_vec1d_from_f32_scalable(%arg0: f32) -> vector<[2]xf32> {
   %0 = vector.broadcast %arg0 : f32 to vector<[2]xf32>
   return %0 : vector<[2]xf32>