[mlir][Math] Add constant folder for Exp2Op.

[jacquesguan]

Summary

This patch adds constant folder for Exp2Op which only supports single and double precision floating-point.

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

let description = [{
 
     Example:
 
     ```mlir
     // Scalar natural exponential.
     %a = math.exp2 %b : f64
     ```
   }];
+  let hasFolder = 1;
 }
 
 //===----------------------------------------------------------------------===//
 // ExpM1Op
 //===----------------------------------------------------------------------===//
 
 def Math_ExpM1Op : Math_FloatUnaryOp<"expm1"> {
   let summary = "base-e exponential of the specified value minus 1";

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

return constFoldUnaryOpConditional<FloatAttr>(
         case 32:
           return APFloat(expf(a.convertToFloat()));
         default:
           return {};
         }
       });
 }
 
+//===----------------------------------------------------------------------===//
+// Exp2Op folder
+//===----------------------------------------------------------------------===//
+
+OpFoldResult math::Exp2Op::fold(ArrayRef<Attribute> operands) {
+  return constFoldUnaryOpConditional<FloatAttr>(
+      operands, [](const APFloat &a) -> Optional<APFloat> {
+        switch (a.getSizeInBits(a.getSemantics())) {
+        case 64:
+          return APFloat(exp2(a.convertToDouble()));
+        case 32:
+          return APFloat(exp2f(a.convertToFloat()));
+        default:
+          return {};
+        }
+      });
+}
+
 /// Materialize an integer or floating point constant.
 Operation *math::MathDialect::materializeConstant(OpBuilder &builder,
                                                   Attribute value, Type type,
                                                   Location loc) {
   return builder.create<arith::ConstantOp>(loc, value, type);
 }

mlir/test/Dialect/Math/canonicalize.mlir

 // CHECK-LABEL: @exp_fold_vec
 // CHECK-NEXT: %[[cst:.+]] = arith.constant dense<[2.71828175, 7.3890562, 20.085537, 54.5981483]> : vector<4xf32>
 // CHECK-NEXT:   return %[[cst]]
 func.func @exp_fold_vec() -> (vector<4xf32>) {
   %v1 = arith.constant dense<[1.0, 2.0, 3.0, 4.0]> : vector<4xf32>
   %0 = math.exp %v1 : vector<4xf32>
   return %0 : vector<4xf32>
 }
+
+// CHECK-LABEL: @exp2_fold
+// CHECK-NEXT: %[[cst:.+]] = arith.constant 4.000000e+00 : f32
+// CHECK-NEXT:   return %[[cst]]
+func.func @exp2_fold() -> f32 {
+  %c = arith.constant 2.0 : f32
+  %r = math.exp2 %c : f32
+  return %r : f32
+}
+
+// CHECK-LABEL: @exp2_fold_vec
+// CHECK-NEXT: %[[cst:.+]] = arith.constant dense<[2.000000e+00, 4.000000e+00, 8.000000e+00, 1.600000e+01]> : vector<4xf32>
+// CHECK-NEXT:   return %[[cst]]
+func.func @exp2_fold_vec() -> (vector<4xf32>) {
+  %v1 = arith.constant dense<[1.0, 2.0, 3.0, 4.0]> : vector<4xf32>
+  %0 = math.exp2 %v1 : vector<4xf32>
+  return %0 : vector<4xf32>
+}