fix comment

Author: Zhang Yan

mlir/include/mlir/Dialect/Arith/IR/ArithOps.td

@@ -1209,7 +1209,9 @@ def Arith_ExtFOp : Arith_FToFCastOp<"extf", [DeclareOpInterfaceMethods<ArithFast
   let hasVerifier = 1;
   let hasFolder = 1;
 
-  let arguments = (ins FloatLike:$in, OptionalAttr<Arith_FastMathAttr>:$fastmath);
+  let arguments = (ins FloatLike:$in,
+                       DefaultValuedOptionalAttr<
+                         Arith_FastMathAttr, "::mlir::arith::FastMathFlags::none">:$fastmath);
   let results = (outs FloatLike:$out);
 
   let assemblyFormat = [{ $in (`fastmath` `` $fastmath^)?
@@ -1256,7 +1258,8 @@ def Arith_TruncFOp :
        DeclareOpInterfaceMethods<CastOpInterface>]>,
     Arguments<(ins FloatLike:$in,
                    OptionalAttr<Arith_RoundingModeAttr>:$roundingmode,
-                   OptionalAttr<Arith_FastMathAttr>:$fastmath)>,
+                   DefaultValuedOptionalAttr<
+                     Arith_FastMathAttr, "::mlir::arith::FastMathFlags::none">:$fastmath)>,
     Results<(outs FloatLike:$out)> {
   let summary = "cast from floating-point to narrower floating-point";
   let description = [{

mlir/lib/Dialect/Arith/IR/ArithOps.cpp

@@ -1392,13 +1392,10 @@ LogicalResult arith::ExtSIOp::verify() {
 OpFoldResult arith::ExtFOp::fold(FoldAdaptor adaptor) {
   if (auto truncFOp = getOperand().getDefiningOp<TruncFOp>()) {
     if (truncFOp.getOperand().getType() == getType()) {
-      arith::FastMathFlags truncFMF =
-          truncFOp.getFastmath().value_or(arith::FastMathFlags::none);
+      arith::FastMathFlags truncFMF = truncFOp.getFastmath();
       bool isTruncContract =
           bitEnumContainsAll(truncFMF, arith::FastMathFlags::contract);
-      arith::FastMathFlags extFMF =
-          getFastmath().value_or(arith::FastMathFlags::none);
-      ;
+      arith::FastMathFlags extFMF = getFastmath();
       bool isExtContract =
           bitEnumContainsAll(extFMF, arith::FastMathFlags::contract);
       if (isTruncContract && isExtContract) {