/*
* Copyright (c) 2013, 2015, Oracle and/or its affiliates. All rights reserved.
* DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
*
* This code is free software; you can redistribute it and/or modify it
* under the terms of the GNU General Public License version 2 only, as
* published by the Free Software Foundation.
*
* This code is distributed in the hope that it will be useful, but WITHOUT
* ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
* FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License
* version 2 for more details (a copy is included in the LICENSE file that
* accompanied this code).
*
* You should have received a copy of the GNU General Public License version
* 2 along with this work; if not, write to the Free Software Foundation,
* Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
*
* Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
* or visit www.oracle.com if you need additional information or have any
* questions.
*/
package org.graalvm.compiler.replacements.aarch64;
import org.graalvm.compiler.api.replacements.ClassSubstitution;
import org.graalvm.compiler.api.replacements.MethodSubstitution;
/**
* AArch64 ISA offers a count leading zeros instruction which can be used to implement
* numberOfLeadingZeros more efficiently than using BitScanReverse.
*/
@ClassSubstitution(Integer.class)
public class AArch64IntegerSubstitutions {
@MethodSubstitution
public static int bitCount(int value) {
// Based on Warren, Hacker's Delight, slightly adapted to profit from Aarch64 add + shift
// instruction.
// Assuming the peephole optimizer optimizes all x - y >>> z into a single instruction
// this takes 10 instructions.
int x = value;
x = x - ((x & 0xaaaaaaaa) >>> 1);
x = (x & 0x33333333) + ((x & 0xcccccccc) >>> 2);
x = (x + (x >>> 4)) & 0x0f0f0f0f;
x = x + (x >>> 8);
x = x + (x >>> 16);
return x & 0x3f;
}
}