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Commit0ce3873

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Micro-optimize AllocSetFreeIndex() by reference to pg_bitutils code.
Use __builtin_clz() where available. Where it isn't, we can still wina little by using the pg_leftmost_one_pos[] lookup table instead ofhaving a private table.Also drop the initial right shift by ALLOC_MINBITS in favor ofsubtracting ALLOC_MINBITS from the leftmost-one-pos result. Thisis a win because the compiler can fold that adjustment into otherconstants it'd have to add anyway, making the shift-removal free.Also, we can explain this coding as an unrolled form ofpg_leftmost_one_pos32(), even though that's a bit ahistoricalsince it long predates pg_bitutils.h.John Naylor, with some cosmetic adjustments by meDiscussion:https://postgr.es/m/CACPNZCuNUGMxjK7WTn_=WZnRbfASDdBxmjsVf2+m9MdmeNw_sg@mail.gmail.com
1 parent27a3b2a commit0ce3873

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-25
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1 file changed

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‎src/backend/utils/mmgr/aset.c

Lines changed: 31 additions & 25 deletions
Original file line numberDiff line numberDiff line change
@@ -46,6 +46,7 @@
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#include"postgres.h"
4848

49+
#include"port/pg_bitutils.h"
4950
#include"utils/memdebug.h"
5051
#include"utils/memutils.h"
5152

@@ -297,18 +298,6 @@ static const MemoryContextMethods AllocSetMethods = {
297298
#endif
298299
};
299300

300-
/*
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* Table for AllocSetFreeIndex
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*/
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#defineLT16(n) n, n, n, n, n, n, n, n, n, n, n, n, n, n, n, n
304-
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staticconstunsignedcharLogTable256[256]=
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{
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0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4,
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LT16(5),LT16(6),LT16(6),LT16(7),LT16(7),LT16(7),LT16(7),
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LT16(8),LT16(8),LT16(8),LT16(8),LT16(8),LT16(8),LT16(8),LT16(8)
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};
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312301
/* ----------
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* Debug macros
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* ----------
@@ -337,24 +326,41 @@ static inline int
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AllocSetFreeIndex(Sizesize)
338327
{
339328
intidx;
340-
unsignedintt,
341-
tsize;
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343330
if (size> (1 <<ALLOC_MINBITS))
344331
{
345-
tsize= (size-1) >>ALLOC_MINBITS;
346-
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/*
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* At this point we need to obtain log2(tsize)+1, ie, the number of
349-
* not-all-zero bits at the right. We used to do this with a
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* shift-and-count loop, but this function is enough of a hotspot to
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* justify micro-optimization effort. The best approach seems to be
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* to use a lookup table. Note that this code assumes that
353-
* ALLOCSET_NUM_FREELISTS <= 17, since we only cope with two bytes of
354-
* the tsize value.
332+
/*----------
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* At this point we must compute ceil(log2(size >> ALLOC_MINBITS)).
334+
* This is the same as
335+
*pg_leftmost_one_pos32((size - 1) >> ALLOC_MINBITS) + 1
336+
* or equivalently
337+
*pg_leftmost_one_pos32(size - 1) - ALLOC_MINBITS + 1
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*
339+
* However, rather than just calling that function, we duplicate the
340+
* logic here, allowing an additional optimization. It's reasonable
341+
* to assume that ALLOC_CHUNK_LIMIT fits in 16 bits, so we can unroll
342+
* the byte-at-a-time loop in pg_leftmost_one_pos32 and just handle
343+
* the last two bytes.
344+
*
345+
* Yes, this function is enough of a hot-spot to make it worth this
346+
* much trouble.
347+
*----------
355348
*/
349+
#ifdefHAVE__BUILTIN_CLZ
350+
idx=31-__builtin_clz((uint32)size-1)-ALLOC_MINBITS+1;
351+
#else
352+
uint32t,
353+
tsize;
354+
355+
/* Statically assert that we only have a 16-bit input value. */
356+
StaticAssertStmt(ALLOC_CHUNK_LIMIT< (1 <<16),
357+
"ALLOC_CHUNK_LIMIT must be less than 64kB");
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tsize=size-1;
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t=tsize >>8;
357-
idx=t ?LogTable256[t]+8 :LogTable256[tsize];
361+
idx=t ?pg_leftmost_one_pos[t]+8 :pg_leftmost_one_pos[tsize];
362+
idx-=ALLOC_MINBITS-1;
363+
#endif
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359365
Assert(idx<ALLOCSET_NUM_FREELISTS);
360366
}

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