154 lines
4.3 KiB
C
154 lines
4.3 KiB
C
/* integer_length - find most significant bit in an 'unsigned int'.
|
|
Copyright (C) 2011-2021 Free Software Foundation, Inc.
|
|
|
|
This file is free software: you can redistribute it and/or modify
|
|
it under the terms of the GNU Lesser General Public License as
|
|
published by the Free Software Foundation; either version 2.1 of the
|
|
License, or (at your option) any later version.
|
|
|
|
This file is distributed in the hope that it will be useful,
|
|
but WITHOUT ANY WARRANTY; without even the implied warranty of
|
|
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
|
|
GNU Lesser General Public License for more details.
|
|
|
|
You should have received a copy of the GNU Lesser General Public License
|
|
along with this program. If not, see <https://www.gnu.org/licenses/>. */
|
|
|
|
/* Written by Bruno Haible <bruno@clisp.org>, 2011. */
|
|
|
|
#include <config.h>
|
|
|
|
/* Specification. */
|
|
#include "integer_length.h"
|
|
|
|
#include <limits.h>
|
|
|
|
#include "float+.h"
|
|
|
|
#if defined _MSC_VER && !(__clang_major__ >= 4)
|
|
# include <intrin.h>
|
|
#endif
|
|
|
|
#define NBITS (sizeof (unsigned int) * CHAR_BIT)
|
|
|
|
int
|
|
integer_length (unsigned int x)
|
|
{
|
|
#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__clang_major__ >= 4)
|
|
if (x == 0)
|
|
return 0;
|
|
else
|
|
return NBITS - __builtin_clz (x);
|
|
#elif defined _MSC_VER
|
|
/* _BitScanReverse
|
|
<https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanreverse-bitscanreverse64> */
|
|
unsigned long bit;
|
|
if (_BitScanReverse (&bit, x))
|
|
return bit + 1;
|
|
else
|
|
return 0;
|
|
#else
|
|
# if defined DBL_EXPBIT0_WORD && defined DBL_EXPBIT0_BIT
|
|
if (NBITS <= DBL_MANT_BIT)
|
|
{
|
|
/* Use 'double' operations.
|
|
Assumes an IEEE 754 'double' implementation. */
|
|
# define DBL_EXP_MASK ((DBL_MAX_EXP - DBL_MIN_EXP) | 7)
|
|
# define DBL_EXP_BIAS (DBL_EXP_MASK / 2 - 1)
|
|
# define NWORDS \
|
|
((sizeof (double) + sizeof (unsigned int) - 1) / sizeof (unsigned int))
|
|
typedef union { double value; unsigned int word[NWORDS]; }
|
|
memory_double;
|
|
|
|
if (x == 0)
|
|
return 0;
|
|
else
|
|
{
|
|
memory_double m;
|
|
unsigned int exponent;
|
|
|
|
if (1)
|
|
{
|
|
/* Use a single integer to floating-point conversion. */
|
|
m.value = x;
|
|
}
|
|
else
|
|
{
|
|
/* Use a single floating-point subtraction. */
|
|
/* 2^(DBL_MANT_DIG-1). */
|
|
static const double TWO_DBL_MANT_DIG =
|
|
/* Assume DBL_MANT_DIG <= 5 * 31.
|
|
Use the identity
|
|
n = floor(n/5) + floor((n+1)/5) + ... + floor((n+4)/5). */
|
|
(double) (1U << ((DBL_MANT_DIG - 1) / 5))
|
|
* (double) (1U << ((DBL_MANT_DIG - 1 + 1) / 5))
|
|
* (double) (1U << ((DBL_MANT_DIG - 1 + 2) / 5))
|
|
* (double) (1U << ((DBL_MANT_DIG - 1 + 3) / 5))
|
|
* (double) (1U << ((DBL_MANT_DIG - 1 + 4) / 5));
|
|
|
|
/* Construct 2^(DBL_MANT_DIG-1) + x by hand. */
|
|
m.word[DBL_EXPBIT0_WORD] =
|
|
(DBL_MANT_DIG + DBL_EXP_BIAS) << DBL_EXPBIT0_BIT;
|
|
m.word[1 - DBL_EXPBIT0_WORD] = x;
|
|
|
|
/* Subtract 2^(DBL_MANT_DIG-1). */
|
|
m.value = m.value - TWO_DBL_MANT_DIG;
|
|
}
|
|
|
|
exponent =
|
|
(m.word[DBL_EXPBIT0_WORD] >> DBL_EXPBIT0_BIT) & DBL_EXP_MASK;
|
|
return exponent - DBL_EXP_BIAS;
|
|
}
|
|
}
|
|
else
|
|
# endif
|
|
if (NBITS == 32)
|
|
{
|
|
/* 6 comparisons. */
|
|
if (x != 0)
|
|
{
|
|
int result = 1;
|
|
if (x >= 0x10000)
|
|
{
|
|
x = x >> 16;
|
|
result += 16;
|
|
}
|
|
if (x >= 0x100)
|
|
{
|
|
x = x >> 8;
|
|
result += 8;
|
|
}
|
|
if (x >= 0x10)
|
|
{
|
|
x = x >> 4;
|
|
result += 4;
|
|
}
|
|
if (x >= 0x4)
|
|
{
|
|
x = x >> 2;
|
|
result += 2;
|
|
}
|
|
if (x >= 0x2)
|
|
{
|
|
x = x >> 1;
|
|
result += 1;
|
|
}
|
|
return result;
|
|
}
|
|
else
|
|
return 0;
|
|
}
|
|
else
|
|
{
|
|
/* Naive loop.
|
|
Works for any value of NBITS. */
|
|
int j;
|
|
|
|
for (j = NBITS - 1; j >= 0; j--)
|
|
if (x & (1U << j))
|
|
return j + 1;
|
|
return 0;
|
|
}
|
|
#endif
|
|
}
|