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 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

source code of compiler-rt/lib/builtins/clzsi2.c