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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * 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).
 *
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 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
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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.
/**
 * 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;
    }
}