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 }
54