1/*
2 * Copyright (c) 2013, 2015, 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 */
23package org.graalvm.compiler.replacements.aarch64;
24
25import org.graalvm.compiler.api.replacements.ClassSubstitution;
26import org.graalvm.compiler.api.replacements.MethodSubstitution;
27
28/**
29 * AArch64 ISA offers a count leading zeros instruction which can be used to implement
30 * numberOfLeadingZeros more efficiently than using BitScanReverse.
31 */
32@ClassSubstitution(Integer.class)
33public class AArch64IntegerSubstitutions {
34
35    @MethodSubstitution
36    public static int bitCount(int value) {
37        // Based on Warren, Hacker's Delight, slightly adapted to profit from Aarch64 add + shift
38        // instruction.
39        // Assuming the peephole optimizer optimizes all x - y >>> z into a single instruction
40        // this takes 10 instructions.
41        int x = value;
42        x = x - ((x & 0xaaaaaaaa) >>> 1);
43        x = (x & 0x33333333) + ((x & 0xcccccccc) >>> 2);
44        x = (x + (x >>> 4)) & 0x0f0f0f0f;
45        x = x + (x >>> 8);
46        x = x + (x >>> 16);
47        return x & 0x3f;
48    }
49}
50