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Commitf0705bb

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Add functions to calculate the next power of 2
There are many areas in the code where we need to determine the nexthighest power of 2 of a given number. We tend to always do that in anad-hoc way each time, generally with some tight for loop which performs abitshift left once per loop and goes until it finds a number above thegiven number.Here we add two generic functions which make use of the existingpg_leftmost_one_pos* functions which, when available, will allow us tocalculate the next power of 2 without any looping.Here we don't add any code which uses these new functions. That will bedone in follow-up commits.Author: David Fetter, with some minor adjustments by meReviewed-by: John Naylor, Jesse ZhangDiscussion:https://postgr.es/m/20200114173553.GE32763%40fetter.org
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‎src/include/port/pg_bitutils.h

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@@ -129,6 +129,78 @@ pg_rightmost_one_pos64(uint64 word)
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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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staticinlineuint32
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pg_nextpower2_32(uint32num)
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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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returnnum;/* 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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staticinlineuint64
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pg_nextpower2_64(uint64num)
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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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returnnum;/* 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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staticinlineuint32
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pg_ceil_log2_32(uint32num)
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{
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if (num<2)
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return0;
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else
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returnpg_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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staticinlineuint64
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pg_ceil_log2_64(uint64num)
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{
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if (num<2)
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return0;
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else
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returnpg_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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externint(*pg_popcount32) (uint32word);
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externint(*pg_popcount64) (uint64word);

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