llvm-project

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