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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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25 changes: 23 additions & 2 deletions mlir/lib/Dialect/Vector/IR/VectorOps.cpp
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Original file line number Diff line number Diff line change
Expand Up @@ -5992,8 +5992,29 @@ OpFoldResult ShapeCastOp::fold(FoldAdaptor adaptor) {

// shape_cast(constant) -> constant
if (auto splatAttr =
llvm::dyn_cast_if_present<SplatElementsAttr>(adaptor.getSource()))
return splatAttr.reshape(getType());
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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Original file line number Diff line number Diff line change
Expand Up @@ -1037,6 +1037,18 @@ 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 •
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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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@MengmSun MengmSun Jul 15, 2025

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Yes it's OK. Updated.

// CHECK: %[[CONST:.*]] = llvm.mlir.constant
// CHECK-NEXT: return %[[CONST]]
func.func @canonicalize_extract_shapecast_different_element_type()->vector<12xi8> {
%0 = llvm.mlir.constant(dense<0.000000e+00> : vector<12xf8E4M3FN>) : vector<12xi8>
%1 = vector.shape_cast %0 : vector<12xi8> to vector<1x12xi8>
%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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