lib/builtins/ctzsi2.c

   1 /* ===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------===
   2  *
   3  *                     The LLVM Compiler Infrastructure
   4  *
   5  * This file is dual licensed under the MIT and the University of Illinois Open
   6  * Source Licenses. See LICENSE.TXT for details.
   7  *
   8  * ===----------------------------------------------------------------------===
   9  *
  10  * This file implements __ctzsi2 for the compiler_rt library.
  11  *
  12  * ===----------------------------------------------------------------------===
  13  */
  14
  15 #include "int_lib.h"
  16
  17 /* Returns: the number of trailing 0-bits */
  18
  19 /* Precondition: a != 0 */
  20
  21 COMPILER_RT_ABI si_int
  22 __ctzsi2(si_int a)
  23 {
  24     su_int x = (su_int)a;
  25     si_int t = ((x & 0x0000FFFF) == 0) << 4;  /* if (x has no small bits) t = 16 else 0 */
  26     x >>= t;           /* x = [0 - 0xFFFF] + higher garbage bits */
  27     su_int r = t;       /* r = [0, 16]  */
  28     /* return r + ctz(x) */
  29     t = ((x & 0x00FF) == 0) << 3;
  30     x >>= t;           /* x = [0 - 0xFF] + higher garbage bits */
  31     r += t;            /* r = [0, 8, 16, 24] */
  32     /* return r + ctz(x) */
  33     t = ((x & 0x0F) == 0) << 2;
  34     x >>= t;           /* x = [0 - 0xF] + higher garbage bits */
  35     r += t;            /* r = [0, 4, 8, 12, 16, 20, 24, 28] */
  36     /* return r + ctz(x) */
  37     t = ((x & 0x3) == 0) << 1;
  38     x >>= t;
  39     x &= 3;            /* x = [0 - 3] */
  40     r += t;            /* r = [0 - 30] and is even */
  41     /* return r + ctz(x) */
  42
  43 /*  The branch-less return statement below is equivalent
  44  *  to the following switch statement:
  45  *     switch (x)
  46  *    {
  47  *     case 0:
  48  *         return r + 2;
  49  *     case 2:
  50  *         return r + 1;
  51  *     case 1:
  52  *     case 3:
  53  *         return r;
  54  *     }
  55  */
  56     return r + ((2 - (x >> 1)) & -((x & 1) == 0));
  57 }