BadShiftAmount
Shift by an amount that is out of range

Category
Severity
ERROR
Maturity

The problem

For shift operations on int types, only the five lowest-order bits of the shift amount are used as the shift distance. This means that shift amounts that are not in the range 0 to 31, inclusive, are silently mapped to values in that range. For example, a shift of an int by 32 is equivalent to shifting by 0, i.e., a no-op.

See JLS 15.19, “Shift Operators”, for more details.

Suppression

Suppress false positives by adding an @SuppressWarnings("BadShiftAmount") annotation to the enclosing element.


Positive examples

BadShiftAmountPositiveCases.java

/*
 * Copyright 2013 Google Inc. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.errorprone.bugpatterns.testdata;

/** @author Bill Pugh (bill.pugh@gmail.com) */
public class BadShiftAmountPositiveCases {

  public void foo() {
    int x = 0;
    long result = 0;

    // BUG: Diagnostic contains: (long) x >> 32
    result += x >> 32;
    // BUG: Diagnostic contains: (long) x << 32
    result += x << 32;
    // BUG: Diagnostic contains: (long) x >>> 32
    result += x >>> 32;
    // BUG: Diagnostic contains: (long) x >> 40
    result += x >> 40;
    // BUG: Diagnostic contains: (long) (x & 255) >> 40
    result += (x & 255) >> 40;

    // BUG: Diagnostic contains: 1L << 48
    result += 1 << 48;

    // BUG: Diagnostic contains: x >> 4
    result += x >> 100;
    // BUG: Diagnostic contains: x >> 31
    result += x >> -1;

    byte b = 0;
    char c = 'a';

    // BUG: Diagnostic contains: (long) b >> 32
    result += b >> 32;
    // BUG: Diagnostic contains: (long) b << 32
    result += b << 32;
    // BUG: Diagnostic contains: (long) c >> 32
    result += c >> 32;
    // BUG: Diagnostic contains: (long) c >>> 32
    result += c >>> 32;
  }
}

Negative examples

BadShiftAmountNegativeCases.java

/*
 * Copyright 2013 Google Inc. All Rights Reserved.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.google.errorprone.bugpatterns.testdata;

/** @author Bill Pugh (bill.pugh@gmail.com) */
public class BadShiftAmountNegativeCases {

  public void foo() {
    int x = 0;
    long result = 0;

    result += (long) x >> 3;
    result += x << 3;
    result += x >>> 3;
    result += (long) (x & 0xff) >> 40;
  }
}