[mlir][spirv] Add folding for SelectOp (#85430)

Add missing constant propogation folder for spirv.Select

Implement additional folding when both selections are equivalent or the
condition is a constant Scalar/SplatVector.

Allows for constant folding in the IndexToSPIRV pass.

Part of work #70704

Author: inbelic

mlir/include/mlir/Dialect/SPIRV/IR/SPIRVLogicalOps.td

@@ -800,6 +800,8 @@ def SPIRV_SelectOp : SPIRV_Op<"Select",
   // These ops require dynamic availability specification based on operand and
   // result types.
   bit autogenAvailability = 0;
+
+  let hasFolder = 1;
 }
 
 // -----

mlir/lib/Dialect/SPIRV/IR/SPIRVCanonicalization.cpp

@@ -797,6 +797,49 @@ OpFoldResult spirv::LogicalOrOp::fold(FoldAdaptor adaptor) {
   return Attribute();
 }
 
+//===----------------------------------------------------------------------===//
+// spirv.SelectOp
+//===----------------------------------------------------------------------===//
+
+OpFoldResult spirv::SelectOp::fold(FoldAdaptor adaptor) {
+  // spirv.Select _ x x -> x
+  Value trueVals = getTrueValue();
+  Value falseVals = getFalseValue();
+  if (trueVals == falseVals)
+    return trueVals;
+
+  ArrayRef<Attribute> operands = adaptor.getOperands();
+
+  // spirv.Select true  x y -> x
+  // spirv.Select false x y -> y
+  if (auto boolAttr = getScalarOrSplatBoolAttr(operands[0]))
+    return *boolAttr ? trueVals : falseVals;
+
+  // Check that all the operands are constant
+  if (!operands[0] || !operands[1] || !operands[2])
+    return Attribute();
+
+  // Note: getScalarOrSplatBoolAttr will always return a boolAttr if we are in
+  // the scalar case. Hence, we are only required to consider the case of
+  // DenseElementsAttr in foldSelectOp.
+  auto condAttrs = dyn_cast<DenseElementsAttr>(operands[0]);
+  auto trueAttrs = dyn_cast<DenseElementsAttr>(operands[1]);
+  auto falseAttrs = dyn_cast<DenseElementsAttr>(operands[2]);
+  if (!condAttrs || !trueAttrs || !falseAttrs)
+    return Attribute();
+
+  auto elementResults = llvm::to_vector<4>(trueAttrs.getValues<Attribute>());
+  auto iters = llvm::zip_equal(elementResults, condAttrs.getValues<BoolAttr>(),
+                               falseAttrs.getValues<Attribute>());
+  for (auto [result, cond, falseRes] : iters) {
+    if (!cond.getValue())
+      result = falseRes;
+  }
+
+  auto resultType = trueAttrs.getType();
+  return DenseElementsAttr::get(cast<ShapedType>(resultType), elementResults);
+}
+
 //===----------------------------------------------------------------------===//
 // spirv.IEqualOp
 //===----------------------------------------------------------------------===//

mlir/test/Conversion/SPIRVToLLVM/misc-ops-to-llvm.mlir

@@ -43,18 +43,18 @@ spirv.func @composite_insert_vector(%arg0: vector<3xf32>, %arg1: f32) "None" {
 //===----------------------------------------------------------------------===//
 
 // CHECK-LABEL: @select_scalar
-spirv.func @select_scalar(%arg0: i1, %arg1: vector<3xi32>, %arg2: f32) "None" {
+spirv.func @select_scalar(%arg0: i1, %arg1: vector<3xi32>, %arg2: vector<3xi32>, %arg3: f32, %arg4: f32) "None" {
   // CHECK: llvm.select %{{.*}}, %{{.*}}, %{{.*}} : i1, vector<3xi32>
-  %0 = spirv.Select %arg0, %arg1, %arg1 : i1, vector<3xi32>
+  %0 = spirv.Select %arg0, %arg1, %arg2 : i1, vector<3xi32>
   // CHECK: llvm.select %{{.*}}, %{{.*}}, %{{.*}} : i1, f32
-  %1 = spirv.Select %arg0, %arg2, %arg2 : i1, f32
+  %1 = spirv.Select %arg0, %arg3, %arg4 : i1, f32
   spirv.Return
 }
 
 // CHECK-LABEL: @select_vector
-spirv.func @select_vector(%arg0: vector<2xi1>, %arg1: vector<2xi32>) "None" {
+spirv.func @select_vector(%arg0: vector<2xi1>, %arg1: vector<2xi32>, %arg2: vector<2xi32>) "None" {
   // CHECK: llvm.select %{{.*}}, %{{.*}}, %{{.*}} : vector<2xi1>, vector<2xi32>
-  %0 = spirv.Select %arg0, %arg1, %arg1 : vector<2xi1>, vector<2xi32>
+  %0 = spirv.Select %arg0, %arg1, %arg2 : vector<2xi1>, vector<2xi32>
   spirv.Return
 }
 

mlir/test/Dialect/SPIRV/Transforms/canonicalize.mlir

@@ -1346,6 +1346,52 @@ func.func @convert_logical_or_true_false_vector(%arg: vector<3xi1>) -> (vector<3
 
 // -----
 
+//===----------------------------------------------------------------------===//
+// spirv.Select
+//===----------------------------------------------------------------------===//
+
+// CHECK-LABEL: @convert_select_scalar
+// CHECK-SAME: %[[ARG1:.+]]: i32, %[[ARG2:.+]]: i32
+func.func @convert_select_scalar(%arg1: i32, %arg2: i32) -> (i32, i32) {
+  %true = spirv.Constant true
+  %false = spirv.Constant false
+  %0 = spirv.Select %true, %arg1, %arg2 : i1, i32
+  %1 = spirv.Select %false, %arg1, %arg2 : i1, i32
+
+  // CHECK: return %[[ARG1]], %[[ARG2]]
+  return %0, %1 : i32, i32
+}
+
+// CHECK-LABEL: @convert_select_vector
+// CHECK-SAME: %[[ARG1:.+]]: vector<3xi32>, %[[ARG2:.+]]: vector<3xi32>
+func.func @convert_select_vector(%arg1: vector<3xi32>, %arg2: vector<3xi32>) -> (vector<3xi32>, vector<3xi32>) {
+  %true = spirv.Constant dense<true> : vector<3xi1>
+  %false = spirv.Constant dense<false> : vector<3xi1>
+  %0 = spirv.Select %true, %arg1, %arg2 : vector<3xi1>, vector<3xi32>
+  %1 = spirv.Select %false, %arg1, %arg2 : vector<3xi1>, vector<3xi32>
+
+  // CHECK: return %[[ARG1]], %[[ARG2]]
+  return %0, %1: vector<3xi32>, vector<3xi32>
+}
+
+// CHECK-LABEL: @convert_select_vector_extra
+// CHECK-SAME: %[[CONDITIONS:.+]]: vector<2xi1>, %[[ARG1:.+]]: vector<2xi32>
+func.func @convert_select_vector_extra(%conditions: vector<2xi1>, %arg1: vector<2xi32>) -> (vector<2xi32>, vector<2xi32>) {
+  %true_false = spirv.Constant dense<[true, false]> : vector<2xi1>
+  %cvec_1 = spirv.Constant dense<[42, -132]> : vector<2xi32>
+  %cvec_2 = spirv.Constant dense<[0, 42]> : vector<2xi32>
+
+  // CHECK: %[[RES:.+]] = spirv.Constant dense<42>
+  %0 = spirv.Select %true_false, %cvec_1, %cvec_2: vector<2xi1>, vector<2xi32>
+
+  %1 = spirv.Select %conditions, %arg1, %arg1 : vector<2xi1>, vector<2xi32>
+
+  // CHECK: return %[[RES]], %[[ARG1]]
+  return %0, %1: vector<2xi32>, vector<2xi32>
+}
+
+// -----
+
 //===----------------------------------------------------------------------===//
 // spirv.IEqual
 //===----------------------------------------------------------------------===//