1 /*

     2  * Copyright (c) 2013, 2018, Oracle and/or its affiliates. All rights reserved.

     3  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.

     4  *

     5  * This code is free software; you can redistribute it and/or modify it

     6  * under the terms of the GNU General Public License version 2 only, as

     7  * published by the Free Software Foundation.

     8  *

     9  * This code is distributed in the hope that it will be useful, but WITHOUT

    10  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or

    11  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License

    12  * version 2 for more details (a copy is included in the LICENSE file that

    13  * accompanied this code).

    14  *

    15  * You should have received a copy of the GNU General Public License version

    16  * 2 along with this work; if not, write to the Free Software Foundation,

    17  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.

    18  *

    19  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA

    20  * or visit www.oracle.com if you need additional information or have any

    21  * questions.

    22  */

    23 

    24 

    25 package org.graalvm.compiler.replacements.aarch64;

    26 

    27 import org.graalvm.compiler.api.replacements.ClassSubstitution;

    28 import org.graalvm.compiler.api.replacements.MethodSubstitution;

    29 

    30 /**

    31  * AArch64 ISA offers a count leading zeros instruction which can be used to implement

    32  * numberOfLeadingZeros more efficiently than using BitScanReverse.

    33  */

    34 @ClassSubstitution(Integer.class)

    35 public class AArch64IntegerSubstitutions {

    36 

    37     @MethodSubstitution

    38     public static int bitCount(int value) {

    39         // Based on Warren, Hacker's Delight, slightly adapted to profit from Aarch64 add + shift

    40         // instruction.

    41         // Assuming the peephole optimizer optimizes all x - y >>> z into a single instruction

    42         // this takes 10 instructions.

    43         int x = value;

    44         x = x - ((x & 0xaaaaaaaa) >>> 1);

    45         x = (x & 0x33333333) + ((x & 0xcccccccc) >>> 2);

    46         x = (x + (x >>> 4)) & 0x0f0f0f0f;

    47         x = x + (x >>> 8);

    48         x = x + (x >>> 16);

    49         return x & 0x3f;

    50     }

    51 }