1 /*
   2  * Copyright (c) 2013, 2016, 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 package org.graalvm.compiler.replacements.aarch64;
  24 
  25 import org.graalvm.compiler.api.replacements.ClassSubstitution;
  26 import 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(Long.class)
  33 public class AArch64LongSubstitutions {
  34 
  35     @MethodSubstitution
  36     public static int bitCount(long 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 11 instructions.
  41         long x = value;
  42         x = x - ((x & 0xaaaaaaaaaaaaaaaaL) >>> 1);
  43         x = (x & 0x3333333333333333L) + ((x & 0xccccccccccccccccL) >>> 2);
  44         x = (x + (x >>> 4)) & 0x0f0f0f0f0f0f0f0fL;
  45         x = x + (x >>> 8);
  46         x = x + (x >>> 16);
  47         x = x + (x >>> 32);
  48         return (int) x & 0x7f;
  49     }
  50 
  51 }