xref: /freebsd/lib/libc/stdbit/stdc_leading_ones.c (revision 6296500a85c8474e3ff3fe2f8e4a9d56dd0acd64) 
1 /*
2  * Copyright (c) 2025 Robert Clausecker <fuz@FreeBSD.org>
3  *
4  * SPDX-License-Identifier: BSD-2-Clause
5  */
6 
7 #include <assert.h>
8 #include <limits.h>
9 #include <stdbit.h>
10 
11 /* Avoid triggering undefined behavior if x == 0. */
12 static_assert(UCHAR_WIDTH < UINT_WIDTH,
13     "stdc_leading_ones_uc needs UCHAR_WIDTH < UINT_WIDTH");
14 
15 unsigned int
stdc_leading_ones_uc(unsigned char x)16 stdc_leading_ones_uc(unsigned char x)
17 {
18 	const int offset = UINT_WIDTH - UCHAR_WIDTH;
19 
20 	return (__builtin_clz(~(x << offset)));
21 }
22 
23 /* Avoid triggering undefined behavior if x == 0. */
24 static_assert(USHRT_WIDTH < UINT_WIDTH,
25     "stdc_leading_ones_us needs USHRT_WIDTH < UINT_WIDTH");
26 
27 unsigned int
stdc_leading_ones_us(unsigned short x)28 stdc_leading_ones_us(unsigned short x)
29 {
30 	const int offset = UINT_WIDTH - USHRT_WIDTH;
31 
32 	return (__builtin_clz(~(x << offset)));
33 }
34 
35 unsigned int
stdc_leading_ones_ui(unsigned int x)36 stdc_leading_ones_ui(unsigned int x)
37 {
38 	if (x == ~0U)
39 		return (UINT_WIDTH);
40 
41 	return (__builtin_clz(~x));
42 }
43 
44 unsigned int
stdc_leading_ones_ul(unsigned long x)45 stdc_leading_ones_ul(unsigned long x)
46 {
47 	if (x == ~0UL)
48 		return (ULONG_WIDTH);
49 
50 	return (__builtin_clzl(~x));
51 }
52 
53 unsigned int
stdc_leading_ones_ull(unsigned long long x)54 stdc_leading_ones_ull(unsigned long long x)
55 {
56 	if (x == ~0ULL)
57 		return (ULLONG_WIDTH);
58 
59 	return (__builtin_clzll(~x));
60 }
61