Tribblix: manual page: stdc_first_leading

STDC_FIRST_LEADING_ONE(9F) Kernel Functions for Drivers

NAME

stdc_first_leading_one, stdc_first_leading_one_uc,
stdc_first_leading_one_us, stdc_first_leading_one_ui,
stdc_first_leading_one_ul, stdc_first_leading_one_ull - find index of
most significant one bit

SYNOPSIS

#include <sys/stdbit.h>

unsigned int
stdc_first_leading_one(generic_value_type value);

unsigned int
stdc_first_leading_one_uc(unsigned char value);

unsigned int
stdc_first_leading_one_us(unsigned short value);

unsigned int
stdc_first_leading_one_ui(unsigned int value);

unsigned int
stdc_first_leading_one_ul(unsigned long value);

unsigned int
stdc_first_leading_one_ull(unsigned long long value);

DESCRIPTION

The stdc_first_leading_one() family of functions returns the 1s-based
index of the first one bit in value starting at the most significant
bit. If there is no one bit in value then zero is returned.

The stdc_first_leading_one() function is generic and will operate on
all 8, 16, 32, and 64-bit unsigned integers; however, it is only
available in C23. The other functions all operate on a specific
integer type, but otherwise behave the same and are available
regardless of the C language version.

The way that the index is constructed is not necessarily intuitive.
The C standard counts the most significant index starting with the most
significant bit as index value 0. Consider the 16-bit value 0x952b.
Generally we would consider the value `b' as bits 0 to 3 while the
value `9' as bits 12 to 15. Bit 15 is actually most significant index
0. Bit 14, most significant index 1. Bit 0, most significant index
15. This example, 0x952b, would return the value 1 (when using the
generic or unsigned short form) as the function is defined to return
this particular index plus one. Zero is reserved for when there is no
leading zero bit at all.

Note that if a non-zero unsigned integer is promoted, it will always be
filled with leading zeros which will cause the returned value to
increase as the first one bit is further away from the most significant
bit.

While this is similar in function to the ddi_fls(9F) functions which
find the last set value and identify the same bits, the ddi_fls(9F)
functions determine the index starting from the least significant bit,
instead of the most significant bit.

CONTEXT

These functions may be called from user, kernel, or interrupt context.

RETURN VALUES

The functions in the stdc_first_leading_one() family always return the
most significant index of the first leading one bit in value, plus one.
Otherwise, if there are no one bits in value, 0 will be returned.
These functions cannot fail.

INTERFACE STABILITY