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
19COMPILER_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