[Mlir-commits] [mlir] [mlir][vector] Disallow vector broadcast along scalable dim (PR #212197)

[llvmlistbot at llvm.org]

https://github.com/Brythzz created https://github.com/llvm/llvm-project/pull/212197

Broadcasts from vectors to higher rank vectors get lowered recursively by iterating over leading dimensions to create "size(dimension)" insert ops. This cannot be done for scalable dimensions whose size is unknown. This PR prevents such transforms from occurring by stopping if the leading dimension is scalable.

>From 4bb1542ea0ba6302cbdadb526c9993deeab6f2ed Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Matt=C3=A9o=20Rizza=20Murgier?=
 <matteo.rizza-murgier at sipearl.com>
Date: Wed, 22 Jul 2026 11:50:58 +0200
Subject: [PATCH] [mlir][vector] Disallow vector broadcast along scalable dim

---
 .../Vector/Transforms/LowerVectorBroadcast.cpp       |  2 ++
 .../test/Conversion/VectorToLLVM/vector-to-llvm.mlir | 12 ++++++++++++
 2 files changed, 14 insertions(+)

diff --git a/mlir/lib/Dialect/Vector/Transforms/LowerVectorBroadcast.cpp b/mlir/lib/Dialect/Vector/Transforms/LowerVectorBroadcast.cpp
index a78c31367ee28..0e31b6934e239 100644
--- a/mlir/lib/Dialect/Vector/Transforms/LowerVectorBroadcast.cpp
+++ b/mlir/lib/Dialect/Vector/Transforms/LowerVectorBroadcast.cpp
@@ -73,6 +73,8 @@ class BroadcastOpLowering : public OpRewritePattern<vector::BroadcastOp> {
     //   %x = [%b,%b,%b,%b] : n-D
     if (srcRank < dstRank) {
       // Duplication.
+      if (dstType.getScalableDims()[0])
+        return failure();
       VectorType resType = VectorType::Builder(dstType).dropDim(0);
       Value bcst =
           vector::BroadcastOp::create(rewriter, loc, resType, op.getSource());
diff --git a/mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir b/mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir
index 094353119e493..3da33dad91efc 100644
--- a/mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir
+++ b/mlir/test/Conversion/VectorToLLVM/vector-to-llvm.mlir
@@ -242,6 +242,18 @@ func.func @broadcast_vec2d_from_vec1d_scalable(%arg0: vector<[2]xf32>) -> vector
 
 // -----
 
+// TODO: Add support for scalable vectors
+
+func.func @broadcast_vec2d_from_vec1d_scalable_leading(%arg0: vector<2xf32>) -> vector<[4]x2xf32> {
+  %0 = vector.broadcast %arg0 : vector<2xf32> to vector<[4]x2xf32>
+  return %0 : vector<[4]x2xf32>
+}
+// CHECK-LABEL: @broadcast_vec2d_from_vec1d_scalable_leading
+// CHECK-SAME:  %[[A:.*]]: vector<2xf32>)
+// CHECK: vector.broadcast %[[A]] : vector<2xf32> to vector<[4]x2xf32>
+
+// -----
+
 func.func @broadcast_vec2d_from_index_vec1d(%arg0: vector<2xindex>) -> vector<3x2xindex> {
   %0 = vector.broadcast %arg0 : vector<2xindex> to vector<3x2xindex>
   return %0 : vector<3x2xindex>