1 /** @file 2 Compiler intrinsic to return the number of trailing zeros, ported from LLVM code. 3 4 Copyright (c) 2008 - 2009, Apple Inc. All rights reserved.<BR> 5 This program and the accompanying materials 6 are licensed and made available under the terms and conditions of the BSD License 7 which accompanies this distribution. The full text of the license may be found at 8 http://opensource.org/licenses/bsd-license.php 9 10 THE PROGRAM IS DISTRIBUTED UNDER THE BSD LICENSE ON AN "AS IS" BASIS, 11 WITHOUT WARRANTIES OR REPRESENTATIONS OF ANY KIND, EITHER EXPRESS OR IMPLIED. 12 13 **/ 14 /** 15 University of Illinois/NCSA 16 Open Source License 17 18 Copyright (c) 2003-2008 University of Illinois at Urbana-Champaign. 19 All rights reserved. 20 21 Developed by: 22 23 LLVM Team 24 25 University of Illinois at Urbana-Champaign 26 27 http://llvm.org 28 29 Permission is hereby granted, free of charge, to any person obtaining a copy of 30 this software and associated documentation files (the "Software"), to deal with 31 the Software without restriction, including without limitation the rights to 32 use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies 33 of the Software, and to permit persons to whom the Software is furnished to do 34 so, subject to the following conditions: 35 36 * Redistributions of source code must retain the above copyright notice, 37 this list of conditions and the following disclaimers. 38 39 * Redistributions in binary form must reproduce the above copyright notice, 40 this list of conditions and the following disclaimers in the 41 documentation and/or other materials provided with the distribution. 42 43 * Neither the names of the LLVM Team, University of Illinois at 44 Urbana-Champaign, nor the names of its contributors may be used to 45 endorse or promote products derived from this Software without specific 46 prior written permission. 47 48 THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR 49 IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS 50 FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE 51 CONTRIBUTORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER 52 LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, 53 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS WITH THE 54 SOFTWARE. 55 **/ 56 57 58 #include "Llvm_int_lib.h" 59 60 // Returns: the number of trailing 0-bits 61 62 // Precondition: a != 0 63 64 INT32 65 __ctzsi2(INT32 a) 66 { 67 UINT32 x = (UINT32)a; 68 INT32 t = ((x & 0x0000FFFF) == 0) << 4; // if (x has no small bits) t = 16 else 0 69 x >>= t; // x = [0 - 0xFFFF] + higher garbage bits 70 UINT32 r = t; // r = [0, 16] 71 // return r + ctz(x) 72 t = ((x & 0x00FF) == 0) << 3; 73 x >>= t; // x = [0 - 0xFF] + higher garbage bits 74 r += t; // r = [0, 8, 16, 24] 75 // return r + ctz(x) 76 t = ((x & 0x0F) == 0) << 2; 77 x >>= t; // x = [0 - 0xF] + higher garbage bits 78 r += t; // r = [0, 4, 8, 12, 16, 20, 24, 28] 79 // return r + ctz(x) 80 t = ((x & 0x3) == 0) << 1; 81 x >>= t; 82 x &= 3; // x = [0 - 3] 83 r += t; // r = [0 - 30] and is even 84 // return r + ctz(x) 85 // The branch-less return statement below is equivalent 86 // to the following switch statement: 87 // switch (x) 88 // { 89 // case 0: 90 // return r + 2; 91 // case 2: 92 // return r + 1; 93 // case 1: 94 // case 3: 95 // return r; 96 // } 97 return r + ((2 - (x >> 1)) & -((x & 1) == 0)); 98 } 99
