1 // SPDX-License-Identifier: BSD-3-Clause
16  *  Base-2 logarithm log2(n)
18  * Y = (u) computes the base-2 logarithm of
22  * +------------------+-----------------+--------+--------+
24  * +----+-----+-------+----+----+-------+--------+--------+
26  * +----+-----+-------+----+----+-------+--------+--------+
28  * +------------------+-----------------+--------+--------+
61 	uint64_t v;  in base2_logarithm()  local
71 	/*  Skipping for-loop */  in base2_logarithm()
75 	/*  byte of V, and shift them out to the left. Continue with the */  in base2_logarithm()
76 	/*  shifted V for as many bytes as it has. */  in base2_logarithm()
78 	/*  one-based index. */  in base2_logarithm()
79 	/*  Index into the number-of-leading-zeros lookup table.  This lookup */  in base2_logarithm()
83 	/*  Left-shift out all the leading zeros in the high byte. */  in base2_logarithm()
84 	v = (uint64_t)u << shift_factor;  in base2_logarithm()
85 	/*  Update the total number of left-shifts */  in base2_logarithm()
87 	shift_factor = iv[v >> 24];  in base2_logarithm()
88 	/*  Left-shift out all the leading zeros in the high byte. */  in base2_logarithm()
89 	v <<= shift_factor;  in base2_logarithm()
90 	/*  Update the total number of left-shifts */  in base2_logarithm()
92 	shift_factor = iv[v >> 24];  in base2_logarithm()
93 	/*  Left-shift out all the leading zeros in the high byte. */  in base2_logarithm()
94 	v <<= shift_factor;  in base2_logarithm()
95 	/*  Update the total number of left-shifts */  in base2_logarithm()
97 	shift_factor = iv[v >> 24];  in base2_logarithm()
98 	/*  Left-shift out all the leading zeros in the high byte. */  in base2_logarithm()
99 	/*  Update the total number of left-shifts */  in base2_logarithm()
101 	/*  The input has been left-shifted so the most-significant-bit is a 1. */  in base2_logarithm()
104 	x = (uint32_t)v << shift_factor;  in base2_logarithm()
105 	/*  Let Q = int(u).  Then u = Q*2^(-u_fraction_length), */  in base2_logarithm()
106 	/*  and x = Q*2^num_left_shifts * 2^(1-word_length).  Therefore, */  in base2_logarithm()
110 	slice_temp = (v << shift_factor) >> 24;  in base2_logarithm()
114 	a_i = BASE2LOG_WORDLENGTH - num_left_shifts;  in base2_logarithm()
116 	l1_i = iv1[(BASE2LOG_WRAP_SCHAR_BITS & ((x >> 24) + 129)) - 1];  in base2_logarithm()
118 	l2_i = iv1[(BASE2LOG_WRAP_SCHAR_BITS & (slice_temp + 130)) - 1];  in base2_logarithm()
119 	l3_i = iv1[(slice_temp - 127) - 1];  in base2_logarithm()
120 	return s_v + (((x & BASE2LOG_UPPERBYTES) * (int64_t)(l2_i - l3_i)) >> 24);  in base2_logarithm()