ArmPkg/Library/CompilerIntrinsicsLib/Arm/ctzsi2.c

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