[MLIR] Add `num_elements` to the shape dialect

[frgossen]

Summary

The operation num_elements determines the number of elements for a given
shape.
That is the product of its dimensions.
A tensor of the given shape holds this many elements.

Depends On D80280

[herhut]

Thanks!

[jpienaar]

Thanks

mlir/include/mlir/Dialect/Shape/IR/ShapeOps.td

289

Seems redundant with the previous line?

290

Already documented in arguments and results

[frgossen]

Remove redundant documentation

[frgossen]

Rebase

mlir/include/mlir/Dialect/Shape/IR/ShapeOps.td

let description = [{
     - lhs * rhs = unknown if either lhs or rhs unknown;
     - lhs * rhs = (int)lhs * (int)rhs if both known;
   }];
 
   let arguments = (ins Shape_SizeType:$lhs, Shape_SizeType:$rhs);
   let results = (outs Shape_SizeType:$result);
 }
 
+def Shape_NumElementsOp : Shape_Op<"num_elements", [
+    NoSideEffect,
+    DeclareOpInterfaceMethods<InferTypeOpInterface>]> {
+
+  let summary = "Returns the number of elements for a given shape";
+  let description = [{
+    Returns the number of elements for a given shape which is the product of its
+    dimensions.
+  }];
+
+  let arguments = (ins Shape_ShapeType:$shape);
+  let results = (outs Shape_SizeType:$result);
+
+  let assemblyFormat = "attr-dict $shape";
+
+  let hasFolder = 1;
+}
+
 def Shape_ReduceOp : Shape_Op<"reduce", []> {
   let summary = "Returns an expression reduced over a shape";
   let description = [{
     An operation that takes as input a shape, number of initial values and has a
     region/function that is applied repeatedly for every dimension of the shape.
 
     Conceptually this op performs the following reduction:
 

mlir/lib/Dialect/Shape/IR/Shape.cpp

OpFoldResult GetExtentOp::fold(ArrayRef<Attribute> operands) {
   uint64_t dimToGet = dim().getLimitedValue();
   // TODO: Constant fold this to some kind of constant error.
   if (dimToGet >= (uint64_t)elements.getNumElements())
     return nullptr;
   return elements.getValue({dimToGet});
 }
 
 //===----------------------------------------------------------------------===//
+// NumElementsOp
+//===----------------------------------------------------------------------===//
+
+OpFoldResult NumElementsOp::fold(ArrayRef<Attribute> operands) {
+
+  // Fold only when argument constant.
+  Attribute shape = operands[0];
+  if (!shape)
+    return {};
+
+  APInt product(64, 1);
+  for (auto value : shape.cast<DenseIntElementsAttr>())
+    product *= value;
+  Builder builder(getContext());
+  return builder.getIndexAttr(product.getLimitedValue());
+}
+
+LogicalResult NumElementsOp::inferReturnTypes(
+    MLIRContext *context, Optional<Location> location, ValueRange operands,
+    DictionaryAttr attributes, RegionRange regions,
+    SmallVectorImpl<Type> &inferredReturnTypes) {
+  inferredReturnTypes.push_back(SizeType::get(context));
+  return success();
+}
+
+//===----------------------------------------------------------------------===//
 // ShapeOfOp
 //===----------------------------------------------------------------------===//
 
 LogicalResult
 ShapeOfOp::inferReturnTypes(MLIRContext *context, Optional<Location> location,
                             ValueRange operands, DictionaryAttr attributes,
                             RegionRange regions,
                             SmallVectorImpl<Type> &inferredReturnTypes) {

mlir/test/Dialect/Shape/canonicalize.mlir

 // CHECK-LABEL: func @nonfoldable_index_to_size
 func @nonfoldable_index_to_size(%ci : index) -> !shape.size {
   // CHECK: shape.index_to_size
   %cs = shape.index_to_size %ci
   return %cs : !shape.size
 }
 
 // -----
+// Fold number of elements computation.
+// CHECK-LABEL: func @num_elements
+func @num_elements() -> !shape.size {
+  // CHECK-NOT: shape.const_shape
+  %shape = shape.const_shape [4, 5, 6]
+  // CHECK-NOT: shape.num_elements
+  %num_elements = shape.num_elements %shape
+  // CHECK: %[[NUM:.*]] = shape.const_size 120
+  // CHECK-NEXT: return %[[NUM]] : !shape.size
+  return %num_elements : !shape.size
+}
+
+// -----
+// No folding.
+// CHECK-LABEL: func @nonfoldable_num_elements
+func @nonfoldable_num_elements(%shape : !shape.shape) -> !shape.size {
+  // CHECK-NOT: shape.const_{{.*}}
+  %num_elements = shape.num_elements %shape
+  return %num_elements : !shape.size
+}
+
+// -----
 
 // Canonicalization of shape.get_extent
 
 // Basic folding.
 // CHECK-LABEL: func @basic
 func @basic() -> !shape.size {
   // CHECK: shape.const_size 2
   %0 = shape.const_shape [0, 1, 2]