compiler-rt/lib/builtins/ctzsi2.c

   1 //===-- ctzsi2.c - Implement __ctzsi2 -------------------------------------===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // This file implements __ctzsi2 for the compiler_rt library.
  10 //
  11 //===----------------------------------------------------------------------===//
  12
  13 #include "int_lib.h"
  14
  15 // Returns: the number of trailing 0-bits
  16
  17 // Precondition: a != 0
  18
  19 COMPILER_RT_ABI int __ctzsi2(si_int a) {
  20   su_int x = (su_int)a;
  21   si_int t = ((x & 0x0000FFFF) == 0)
  22              << 4; // if (x has no small bits) t = 16 else 0
  23   x >>= t;         // x = [0 - 0xFFFF] + higher garbage bits
  24   su_int r = t;    // r = [0, 16]
  25   // return r + ctz(x)
  26   t = ((x & 0x00FF) == 0) << 3;
  27   x >>= t; // x = [0 - 0xFF] + higher garbage bits
  28   r += t;  // r = [0, 8, 16, 24]
  29   // return r + ctz(x)
  30   t = ((x & 0x0F) == 0) << 2;
  31   x >>= t; // x = [0 - 0xF] + higher garbage bits
  32   r += t;  // r = [0, 4, 8, 12, 16, 20, 24, 28]
  33   // return r + ctz(x)
  34   t = ((x & 0x3) == 0) << 1;
  35   x >>= t;
  36   x &= 3; // x = [0 - 3]
  37   r += t; // r = [0 - 30] and is even
  38   // return r + ctz(x)
  39
  40   //  The branch-less return statement below is equivalent
  41   //  to the following switch statement:
  42   //     switch (x)
  43   //    {
  44   //     case 0:
  45   //         return r + 2;
  46   //     case 2:
  47   //         return r + 1;
  48   //     case 1:
  49   //     case 3:
  50   //         return r;
  51   //     }
  52   return r + ((2 - (x >> 1)) & -((x & 1) == 0));
  53 }