[Mlir-commits] [mlir] [MLIR][Conversion] Vector to LLVM: Remove unneeded vector shuffle (PR #162946)

[Sang Ik Lee]

https://github.com/silee2 created https://github.com/llvm/llvm-project/pull/162946

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

>From c44404b91ff2e99b56fecc613ce21268f6f43109 Mon Sep 17 00:00:00 2001
From: "Lee, Sang Ik" <sang.ik.lee at intel.com>
Date: Sat, 11 Oct 2025 00:09:38 +0000
Subject: [PATCH] [MLIR][Conversion] Vector to LLVM: Remove unneeded
 vectorshuffle if vector.broadcast source is a scalar and target is a single
 element 1D vector.

---
 .../Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp   |  6 +++++-
 mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir | 11 +++++++++++
 2 files changed, 16 insertions(+), 1 deletion(-)

diff --git a/mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp b/mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp
index 5355909b62a7f..2bab94f82723e 100644
--- a/mlir/lib/Conversion/VectorToLLVM/ConvertVectorToLLVM.cpp
+++ b/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.
diff --git a/mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir b/mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir
index 2d33888854ea7..f704b8dba5eed 100644
--- a/mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir
+++ b/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>