lib/builtins/clzsi2.c

   1 /* ===-- clzsi2.c - Implement __clzsi2 -------------------------------------===
   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 __clzsi2 for the compiler_rt library.
  11  *
  12  * ===----------------------------------------------------------------------===
  13  */
  14
  15 #include "int_lib.h"
  16
  17 /* Returns: the number of leading 0-bits */
  18
  19 /* Precondition: a != 0 */
  20
  21 COMPILER_RT_ABI si_int
  22 __clzsi2(si_int a)
  23 {
  24     su_int x = (su_int)a;
  25     si_int t = ((x & 0xFFFF0000) == 0) << 4;  /* if (x is small) t = 16 else 0 */
  26     x >>= 16 - t;      /* x = [0 - 0xFFFF] */
  27     su_int r = t;       /* r = [0, 16] */
  28     /* return r + clz(x) */
  29     t = ((x & 0xFF00) == 0) << 3;
  30     x >>= 8 - t;       /* x = [0 - 0xFF] */
  31     r += t;            /* r = [0, 8, 16, 24] */
  32     /* return r + clz(x) */
  33     t = ((x & 0xF0) == 0) << 2;
  34     x >>= 4 - t;       /* x = [0 - 0xF] */
  35     r += t;            /* r = [0, 4, 8, 12, 16, 20, 24, 28] */
  36     /* return r + clz(x) */
  37     t = ((x & 0xC) == 0) << 1;
  38     x >>= 2 - t;       /* x = [0 - 3] */
  39     r += t;            /* r = [0 - 30] and is even */
  40     /* return r + clz(x) */
  41 /*     switch (x)
  42  *     {
  43  *     case 0:
  44  *         return r + 2;
  45  *     case 1:
  46  *         return r + 1;
  47  *     case 2:
  48  *     case 3:
  49  *         return r;
  50  *     }
  51  */
  52     return r + ((2 - x) & -((x & 2) == 0));
  53 }