diff --git a/mlir/include/mlir/Dialect/Arith/IR/ArithOps.td b/mlir/include/mlir/Dialect/Arith/IR/ArithOps.td
--- a/mlir/include/mlir/Dialect/Arith/IR/ArithOps.td
+++ b/mlir/include/mlir/Dialect/Arith/IR/ArithOps.td
@@ -1047,6 +1047,7 @@
     When operating on vectors, casts elementwise.
   }];
   let hasVerifier = 1;
+  let hasFolder = 1;
 }
 
 //===----------------------------------------------------------------------===//
diff --git a/mlir/lib/Dialect/Arith/IR/ArithOps.cpp b/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
--- a/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
+++ b/mlir/lib/Dialect/Arith/IR/ArithOps.cpp
@@ -1224,6 +1224,16 @@
 // ExtFOp
 //===----------------------------------------------------------------------===//
 
+/// Always fold extension of FP constants.
+OpFoldResult arith::ExtFOp::fold(FoldAdaptor adaptor) {
+  auto constOperand = adaptor.getIn().dyn_cast_or_null<FloatAttr>();
+  if (!constOperand)
+    return {};
+
+  // Convert to target type via 'double'.
+  return FloatAttr::get(getType(), constOperand.getValue().convertToDouble());
+}
+
 bool arith::ExtFOp::areCastCompatible(TypeRange inputs, TypeRange outputs) {
   return checkWidthChangeCast<std::greater, FloatType>(inputs, outputs);
 }
diff --git a/mlir/test/Dialect/Arith/canonicalize.mlir b/mlir/test/Dialect/Arith/canonicalize.mlir
--- a/mlir/test/Dialect/Arith/canonicalize.mlir
+++ b/mlir/test/Dialect/Arith/canonicalize.mlir
@@ -502,6 +502,15 @@
   return %ext : vector<4xi16>
 }
 
+// CHECK-LABEL: @extFPConstant
+//       CHECK:   %[[cres:.+]] = arith.constant 1.000000e+00 : f64
+//       CHECK:   return %[[cres]]
+func.func @extFPConstant() -> f64 {
+  %cst = arith.constant 1.000000e+00 : f32
+  %0 = arith.extf %cst : f32 to f64
+  return %0 : f64
+}
+
 // CHECK-LABEL: @truncConstant
 //       CHECK:   %[[cres:.+]] = arith.constant -2 : i16
 //       CHECK:   return %[[cres]]