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[MLIR][Vector]Add constraints to vector.shape_cast(constant) -> constant #147691

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[MLIR][Vector]Add constraints to vector.shape_cast(constant) -> constant #147691

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5e9aa6b
[mlir][Vector]Add constraints to vector.shape_cast(constant) -> constant
Jul 9, 2025
5604b26
Update based on comments
Jul 15, 2025
e54ba9b
Merge branch 'main' into mlir/fix/vector_shape_cast_fold
MengmSun Jul 15, 2025
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9 changes: 6 additions & 3 deletions mlir/lib/Dialect/Vector/IR/VectorOps.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5922,10 +5922,13 @@ OpFoldResult ShapeCastOp::fold(FoldAdaptor adaptor) {
return bcastOp.getSource();
}

// shape_cast(constant) -> constant
// shape_cast(constant) -> constant,
// if element type of the source and result are the same
if (auto splatAttr =
llvm::dyn_cast_if_present<SplatElementsAttr>(adaptor.getSource()))
return splatAttr.reshape(getType());
llvm::dyn_cast_if_present<SplatElementsAttr>(adaptor.getSource())) {
if (splatAttr.getElementType() == resultType.getElementType())
return splatAttr.reshape(getType());
}
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The code below will fold, but use the correct element type:

  // shape_cast(constant) -> constant
  if (auto splatAttr =
          llvm::dyn_cast_if_present<SplatElementsAttr>(adaptor.getSource())) {

    // The shape and 'scalable dims' of the new attribute must match the result
    // of the shape_cast:
    auto newShape = resultType.getShape();
    auto newScalableDims = resultType.getScalableDims();

    // The element type must be retained. Note that this is to handle currently
    // valid IR like
    //
    // ```
    // %0 = llvm.mlir.constant(dense<0.> : vector<1xf8E4M3FN>) : vector<1xi8>
    // %1 = vector.shape_cast %0 : vector<1xi8> to vector<1x1xi8>
    // ```
    //
    // where the element types of the attribute and result do not match.
    auto newElementType = splatAttr.getElementType();

    auto newAttr = VectorType::get(newShape, newElementType, newScalableDims);

    return DenseElementsAttr::get(newAttr,
                                  splatAttr.getSplatValue<Attribute>());
  }


// shape_cast(poison) -> poison
if (llvm::dyn_cast_if_present<ub::PoisonAttr>(adaptor.getSource())) {
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12 changes: 12 additions & 0 deletions mlir/test/Dialect/Vector/canonicalize.mlir
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Expand Up @@ -1002,6 +1002,18 @@ func.func @fold_broadcast_shapecast(%arg0: vector<4xf32>) -> vector<4xf32> {

// -----

// CHECK-LABEL: func @canonicalize_extract_shapecast_different_element_type
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@newling newling Jul 14, 2025 •
edited
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Something along the lines of:

// CHECK: [...] = llvm.mlir.constant
// CHECK-NEXT: return %[[CONST]]

will be more explicit.

Wondering if it is ok to mix llvm dialect here, I guess so?

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Yes it's OK. Updated.

func.func @canonicalize_extract_shapecast_different_element_type()->vector<12xi8> {
%0 = llvm.mlir.constant(dense<0.000000e+00> : vector<12xf8E4M3FN>) : vector<12xi8>
// CHECK-NOT: vector.shape_cast
%1 = vector.shape_cast %0 : vector<12xi8> to vector<1x12xi8>
// CHECK-NOT: vector.extract
%2 = vector.extract %1[0] : vector<12xi8> from vector<1x12xi8>
return %2 : vector<12xi8>
}

// -----

// CHECK-LABEL: func @canonicalize_broadcast_shapecast_scalar
// CHECK: vector.broadcast
// CHECK-NOT: vector.shape_cast
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