[mlir][Math] Add constant folder for ErfOp.

[jacquesguan]

Summary

This patch adds constant folder for ErfOp by using erf/erff of libm.

mlir/include/mlir/Dialect/Math/IR/MathOps.td

let description = [{
 
     Example:
 
     ```mlir
     // Scalar error function value.
     %a = math.erf %b : f64
     ```
   }];
+  let hasFolder = 1;
 }
 
 
 //===----------------------------------------------------------------------===//
 // ExpOp
 //===----------------------------------------------------------------------===//
 
 def Math_ExpOp : Math_FloatUnaryOp<"exp"> {

mlir/lib/Dialect/Math/IR/MathOps.cpp

 
 OpFoldResult math::CtPopOp::fold(ArrayRef<Attribute> operands) {
   return constFoldUnaryOp<IntegerAttr>(operands, [](const APInt &a) {
     return APInt(a.getBitWidth(), a.countPopulation());
   });
 }
 
 //===----------------------------------------------------------------------===//
+// ErfOp folder
+//===----------------------------------------------------------------------===//
+
+OpFoldResult math::ErfOp::fold(ArrayRef<Attribute> operands) {
+  return constFoldUnaryOpConditional<FloatAttr>(
+      operands, [](const APFloat &a) -> Optional<APFloat> {
+        switch (a.getSizeInBits(a.getSemantics())) {
+        case 64:
+          return APFloat(erf(a.convertToDouble()));
+        case 32:
+          return APFloat(erff(a.convertToFloat()));
+        default:
+          return {};
+        }
+      });
+}
+
+//===----------------------------------------------------------------------===//
 // IPowIOp folder
 //===----------------------------------------------------------------------===//
 
 OpFoldResult math::IPowIOp::fold(ArrayRef<Attribute> operands) {
   return constFoldBinaryOpConditional<IntegerAttr>(
       operands, [](const APInt &base, const APInt &power) -> Optional<APInt> {
         unsigned width = base.getBitWidth();
         auto zeroValue = APInt::getZero(width);

mlir/test/Dialect/Math/canonicalize.mlir

 // CHECK-LABEL: @sin_fold_vec
 // CHECK-NEXT: %[[cst:.+]] = arith.constant dense<[0.000000e+00, 0.84{{[0-9]+}}, 0.000000e+00, 0.84{{[0-9]+}}]> : vector<4xf32>
 // CHECK-NEXT:   return %[[cst]]
 func.func @sin_fold_vec() -> (vector<4xf32>) {
   %v1 = arith.constant dense<[0.0, 1.0, 0.0, 1.0]> : vector<4xf32>
   %0 = math.sin %v1 : vector<4xf32>
   return %0 : vector<4xf32>
 }
+
+// CHECK-LABEL: @erf_fold
+// CHECK-NEXT: %[[cst:.+]] = arith.constant 0.84{{[0-9]+}} : f32
+// CHECK-NEXT:   return %[[cst]]
+func.func @erf_fold() -> f32 {
+  %c = arith.constant 1.0 : f32
+  %r = math.erf %c : f32
+  return %r : f32
+}
+
+// CHECK-LABEL: @erf_fold_vec
+// CHECK-NEXT: %[[cst:.+]] = arith.constant dense<[0.000000e+00, 0.84{{[0-9]+}}, 0.000000e+00, 0.84{{[0-9]+}}]> : vector<4xf32>
+// CHECK-NEXT:   return %[[cst]]
+func.func @erf_fold_vec() -> (vector<4xf32>) {
+  %v1 = arith.constant dense<[0.0, 1.0, 0.0, 1.0]> : vector<4xf32>
+  %0 = math.erf %v1 : vector<4xf32>
+  return %0 : vector<4xf32>
+}