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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
221 lines
4.7 KiB
C
221 lines
4.7 KiB
C
/*-------------------------------------------------------------------------
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*
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* pg_bitutils.h
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* Miscellaneous functions for bit-wise operations.
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*
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*
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* Copyright (c) 2019-2020, PostgreSQL Global Development Group
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*
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* src/include/port/pg_bitutils.h
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*
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*-------------------------------------------------------------------------
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*/
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#ifndef PG_BITUTILS_H
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#define PG_BITUTILS_H
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extern PGDLLIMPORT const uint8 pg_leftmost_one_pos[256];
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extern PGDLLIMPORT const uint8 pg_rightmost_one_pos[256];
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extern PGDLLIMPORT const uint8 pg_number_of_ones[256];
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/*
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* pg_leftmost_one_pos32
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* Returns the position of the most significant set bit in "word",
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* measured from the least significant bit. word must not be 0.
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*/
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static inline int
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pg_leftmost_one_pos32(uint32 word)
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{
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#ifdef HAVE__BUILTIN_CLZ
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Assert(word != 0);
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return 31 - __builtin_clz(word);
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#else
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int shift = 32 - 8;
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Assert(word != 0);
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while ((word >> shift) == 0)
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shift -= 8;
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return shift + pg_leftmost_one_pos[(word >> shift) & 255];
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#endif /* HAVE__BUILTIN_CLZ */
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}
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/*
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* pg_leftmost_one_pos64
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* As above, but for a 64-bit word.
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*/
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static inline int
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pg_leftmost_one_pos64(uint64 word)
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{
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#ifdef HAVE__BUILTIN_CLZ
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Assert(word != 0);
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#if defined(HAVE_LONG_INT_64)
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return 63 - __builtin_clzl(word);
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#elif defined(HAVE_LONG_LONG_INT_64)
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return 63 - __builtin_clzll(word);
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#else
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#error must have a working 64-bit integer datatype
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#endif
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#else /* !HAVE__BUILTIN_CLZ */
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int shift = 64 - 8;
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Assert(word != 0);
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while ((word >> shift) == 0)
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shift -= 8;
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return shift + pg_leftmost_one_pos[(word >> shift) & 255];
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#endif /* HAVE__BUILTIN_CLZ */
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}
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/*
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* pg_rightmost_one_pos32
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* Returns the position of the least significant set bit in "word",
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* measured from the least significant bit. word must not be 0.
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*/
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static inline int
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pg_rightmost_one_pos32(uint32 word)
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{
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#ifdef HAVE__BUILTIN_CTZ
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Assert(word != 0);
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return __builtin_ctz(word);
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#else
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int result = 0;
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Assert(word != 0);
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while ((word & 255) == 0)
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{
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word >>= 8;
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result += 8;
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}
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result += pg_rightmost_one_pos[word & 255];
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return result;
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#endif /* HAVE__BUILTIN_CTZ */
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}
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/*
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* pg_rightmost_one_pos64
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* As above, but for a 64-bit word.
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*/
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static inline int
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pg_rightmost_one_pos64(uint64 word)
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{
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#ifdef HAVE__BUILTIN_CTZ
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Assert(word != 0);
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#if defined(HAVE_LONG_INT_64)
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return __builtin_ctzl(word);
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#elif defined(HAVE_LONG_LONG_INT_64)
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return __builtin_ctzll(word);
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#else
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#error must have a working 64-bit integer datatype
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#endif
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#else /* !HAVE__BUILTIN_CTZ */
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int result = 0;
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Assert(word != 0);
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while ((word & 255) == 0)
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{
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word >>= 8;
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result += 8;
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}
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result += pg_rightmost_one_pos[word & 255];
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return result;
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#endif /* HAVE__BUILTIN_CTZ */
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}
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/*
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* pg_nextpower2_32
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* Returns the next highest power of 2 of 'num', or 'num', if it's
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* already a power of 2.
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*
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* 'num' mustn't be 0 or be above PG_UINT32_MAX / 2 + 1.
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*/
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static inline uint32
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pg_nextpower2_32(uint32 num)
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{
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Assert(num > 0 && num <= PG_UINT32_MAX / 2 + 1);
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/*
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* A power 2 number has only 1 bit set. Subtracting 1 from such a number
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* will turn on all previous bits resulting in no common bits being set
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* between num and num-1.
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*/
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if ((num & (num - 1)) == 0)
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return num; /* already power 2 */
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return ((uint32) 1) << (pg_leftmost_one_pos32(num) + 1);
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}
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/*
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* pg_nextpower2_64
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* Returns the next highest power of 2 of 'num', or 'num', if it's
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* already a power of 2.
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*
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* 'num' mustn't be 0 or be above PG_UINT64_MAX / 2 + 1.
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*/
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static inline uint64
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pg_nextpower2_64(uint64 num)
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{
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Assert(num > 0 && num <= PG_UINT64_MAX / 2 + 1);
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/*
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* A power 2 number has only 1 bit set. Subtracting 1 from such a number
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* will turn on all previous bits resulting in no common bits being set
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* between num and num-1.
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*/
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if ((num & (num - 1)) == 0)
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return num; /* already power 2 */
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return ((uint64) 1) << (pg_leftmost_one_pos64(num) + 1);
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}
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/*
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* pg_ceil_log2_32
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* Returns equivalent of ceil(log2(num))
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*/
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static inline uint32
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pg_ceil_log2_32(uint32 num)
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{
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if (num < 2)
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return 0;
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else
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return pg_leftmost_one_pos32(num - 1) + 1;
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}
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/*
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* pg_ceil_log2_64
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* Returns equivalent of ceil(log2(num))
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*/
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static inline uint64
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pg_ceil_log2_64(uint64 num)
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{
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if (num < 2)
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return 0;
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else
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return pg_leftmost_one_pos64(num - 1) + 1;
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}
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/* Count the number of one-bits in a uint32 or uint64 */
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extern int (*pg_popcount32) (uint32 word);
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extern int (*pg_popcount64) (uint64 word);
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/* Count the number of one-bits in a byte array */
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extern uint64 pg_popcount(const char *buf, int bytes);
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/*
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* Rotate the bits of "word" to the right by n bits.
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*/
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static inline uint32
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pg_rotate_right32(uint32 word, int n)
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{
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return (word >> n) | (word << (sizeof(word) * BITS_PER_BYTE - n));
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}
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#endif /* PG_BITUTILS_H */
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