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Add functions to calculate the next power of 2

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There are many areas in the code where we need to determine the next
highest power of 2 of a given number.  We tend to always do that in an
ad-hoc way each time, generally with some tight for loop which performs a
bitshift left once per loop and goes until it finds a number above the
given number.

Here we add two generic functions which make use of the existing
pg_leftmost_one_pos* functions which, when available, will allow us to
calculate the next power of 2 without any looping.

Here we don't add any code which uses these new functions. That will be
done in follow-up commits.

Author: David Fetter, with some minor adjustments by me
Reviewed-by: John Naylor, Jesse Zhang
Discussion: https://postgr.es/m/20200114173553.GE32763%40fetter.org
This commit is contained in:
David Rowley 2020-04-08 16:22:52 +12:00
parent 7a5d74b7dd
commit f0705bb628
1 changed files with 72 additions and 0 deletions
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72
src/include/port/pg_bitutils.h
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@ -129,6 +129,78 @@ pg_rightmost_one_pos64(uint64 word)
#endif /* HAVE__BUILTIN_CTZ */
}
/*
* pg_nextpower2_32
* Returns the next highest power of 2 of 'num', or 'num', if it's
* already a power of 2.
*
* 'num' mustn't be 0 or be above PG_UINT32_MAX / 2 + 1.
*/
static inline uint32
pg_nextpower2_32(uint32 num)
{
Assert(num > 0 && num <= PG_UINT32_MAX / 2 + 1);
/*
* A power 2 number has only 1 bit set. Subtracting 1 from such a number
* will turn on all previous bits resulting in no common bits being set
* between num and num-1.
*/
if ((num & (num - 1)) == 0)
return num; /* already power 2 */
return ((uint32) 1) << (pg_leftmost_one_pos32(num) + 1);
}
/*
* pg_nextpower2_64
* Returns the next highest power of 2 of 'num', or 'num', if it's
* already a power of 2.
*
* 'num' mustn't be 0 or be above PG_UINT64_MAX / 2 + 1.
*/
static inline uint64
pg_nextpower2_64(uint64 num)
{
Assert(num > 0 && num <= PG_UINT64_MAX / 2 + 1);
/*
* A power 2 number has only 1 bit set. Subtracting 1 from such a number
* will turn on all previous bits resulting in no common bits being set
* between num and num-1.
*/
if ((num & (num - 1)) == 0)
return num; /* already power 2 */
return ((uint64) 1) << (pg_leftmost_one_pos64(num) + 1);
}
/*
* pg_ceil_log2_32
* Returns equivalent of ceil(log2(num))
*/
static inline uint32
pg_ceil_log2_32(uint32 num)
{
if (num < 2)
return 0;
else
return pg_leftmost_one_pos32(num - 1) + 1;
}
/*
* pg_ceil_log2_64
* Returns equivalent of ceil(log2(num))
*/
static inline uint64
pg_ceil_log2_64(uint64 num)
{
if (num < 2)
return 0;
else
return pg_leftmost_one_pos64(num - 1) + 1;
}
/* Count the number of one-bits in a uint32 or uint64 */
extern int (*pg_popcount32) (uint32 word);
extern int (*pg_popcount64) (uint64 word);