1 //===-- clzsi2.c - Implement __clzsi2 -------------------------------------===//
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 __clzsi2 for the compiler_rt library.
10 //
11 //===----------------------------------------------------------------------===//
12 
13 #include "int_lib.h"
14 
15 // Returns: the number of leading 0-bits
16 
17 // Precondition: a != 0
18 
19 COMPILER_RT_ABI si_int __clzsi2(si_int a) {
20   su_int x = (su_int)a;
21   si_int t = ((x & 0xFFFF0000) == 0) << 4; // if (x is small) t = 16 else 0
22   x >>= 16 - t;                            // x = [0 - 0xFFFF]
23   su_int r = t;                            // r = [0, 16]
24   // return r + clz(x)
25   t = ((x & 0xFF00) == 0) << 3;
26   x >>= 8 - t; // x = [0 - 0xFF]
27   r += t;      // r = [0, 8, 16, 24]
28   // return r + clz(x)
29   t = ((x & 0xF0) == 0) << 2;
30   x >>= 4 - t; // x = [0 - 0xF]
31   r += t;      // r = [0, 4, 8, 12, 16, 20, 24, 28]
32   // return r + clz(x)
33   t = ((x & 0xC) == 0) << 1;
34   x >>= 2 - t; // x = [0 - 3]
35   r += t;      // r = [0 - 30] and is even
36   // return r + clz(x)
37   //     switch (x)
38   //     {
39   //     case 0:
40   //         return r + 2;
41   //     case 1:
42   //         return r + 1;
43   //     case 2:
44   //     case 3:
45   //         return r;
46   //     }
47   return r + ((2 - x) & -((x & 2) == 0));
48 }
49