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Commit07f0f6a

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Speed up tail processing when hashing aligned C strings
After encountering the NUL terminator, the word-at-a-time loop exitsand we must hash the remaining bytes. Previously we calculated theterminator's position and re-loaded the remaining bytes from the inputstring. We already have all the data we need in a register, so let'sjust mask off the bytes we need and hash them immediately. The mask canbe cheaply computed without knowing the terminator's position. We stillneed that position for the length calculation, but the CPU can nowdo that in parallel with other work, shortening the dependency chain.Ants Aasma and John NaylorDiscussion:https://postgr.es/m/CANwKhkP7pCiW_5fAswLhs71-JKGEz1c1%2BPC0a_w1fwY4iGMqUA%40mail.gmail.com
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‎src/include/common/hashfn_unstable.h

Lines changed: 34 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -219,8 +219,9 @@ static inline size_t
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fasthash_accum_cstring_aligned(fasthash_state*hs,constchar*str)
220220
{
221221
constchar*conststart=str;
222-
size_tremainder;
222+
uint64chunk;
223223
uint64zero_byte_low;
224+
uint64mask;
224225

225226
Assert(PointerIsAligned(start,uint64));
226227

@@ -239,7 +240,7 @@ fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
239240
*/
240241
for (;;)
241242
{
242-
uint64chunk=*(uint64*)str;
243+
chunk=*(uint64*)str;
243244

244245
#ifdefWORDS_BIGENDIAN
245246
zero_byte_low=haszero64(pg_bswap64(chunk));
@@ -254,14 +255,37 @@ fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
254255
str+=FH_SIZEOF_ACCUM;
255256
}
256257

257-
/*
258-
* The byte corresponding to the NUL will be 0x80, so the rightmost bit
259-
* position will be in the range 7, 15, ..., 63. Turn this into byte
260-
* position by dividing by 8.
261-
*/
262-
remainder=pg_rightmost_one_pos64(zero_byte_low) /BITS_PER_BYTE;
263-
fasthash_accum(hs,str,remainder);
264-
str+=remainder;
258+
if (zero_byte_low&0xFF)
259+
{
260+
/*
261+
* The next byte in the input is the NUL terminator, so we have
262+
* nothing to do.
263+
*/
264+
}
265+
else
266+
{
267+
/*
268+
* Create a mask for the remaining bytes so we can combine them into
269+
* the hash. The mask also covers the NUL terminator, but that's
270+
* harmless. The mask could contain 0x80 in bytes corresponding to the
271+
* input past the terminator, but only where the input byte is zero or
272+
* one, so also harmless.
273+
*/
274+
mask=zero_byte_low | (zero_byte_low-1);
275+
#ifdefWORDS_BIGENDIAN
276+
/* need to mask the upper bytes */
277+
mask=pg_bswap64(mask);
278+
#endif
279+
hs->accum=chunk&mask;
280+
fasthash_combine(hs);
281+
282+
/*
283+
* The byte corresponding to the NUL will be 0x80, so the rightmost
284+
* bit position will be in the range 15, 23, ..., 63. Turn this into
285+
* byte position by dividing by 8.
286+
*/
287+
str+=pg_rightmost_one_pos64(zero_byte_low) /BITS_PER_BYTE;
288+
}
265289

266290
returnstr-start;
267291
}

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