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Commita365d9e

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Speed up tail processing when hashing aligned C strings, take two
After encountering the NUL terminator, the word-at-a-time loop exitsand we must hash the remaining bytes. Previously we calculatedthe terminator's position and re-loaded the remaining bytes fromthe input string. This was slower than the unaligned case for veryshort strings. We already have all the data we need in a register,so let's just mask off the bytes we need and hash them immediately.In addition to endianness issues, the previous attempt upset valgrindin the way it computed the mask. Whether by accident or by wisdom,the author's proposed method passes locally with valgrind 3.22.Ants Aasma, with cosmetic adjustments by meDiscussion:https://postgr.es/m/CANwKhkP7pCiW_5fAswLhs71-JKGEz1c1%2BPC0a_w1fwY4iGMqUA%40mail.gmail.com
1 parent0c25fee commita365d9e

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1 file changed

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1 file changed

+36
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‎src/include/common/hashfn_unstable.h

Lines changed: 36 additions & 10 deletions
Original file line numberDiff line numberDiff line change
@@ -219,6 +219,13 @@ fasthash_accum(fasthash_state *hs, const char *k, size_t len)
219219
#definehaszero64(v) \
220220
(((v) - 0x0101010101010101) & ~(v) & 0x8080808080808080)
221221

222+
/* get first byte in memory order */
223+
#ifdefWORDS_BIGENDIAN
224+
#definefirstbyte64(v) ((v) >> 56)
225+
#else
226+
#definefirstbyte64(v) ((v) & 0xFF)
227+
#endif
228+
222229
/*
223230
* all-purpose workhorse for fasthash_accum_cstring
224231
*/
@@ -255,7 +262,7 @@ static inline size_t
255262
fasthash_accum_cstring_aligned(fasthash_state*hs,constchar*str)
256263
{
257264
constchar*conststart=str;
258-
size_tremainder;
265+
uint64chunk;
259266
uint64zero_byte_low;
260267

261268
Assert(PointerIsAligned(start,uint64));
@@ -275,7 +282,7 @@ fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
275282
*/
276283
for (;;)
277284
{
278-
uint64chunk=*(uint64*)str;
285+
chunk=*(uint64*)str;
279286

280287
#ifdefWORDS_BIGENDIAN
281288
zero_byte_low=haszero64(pg_bswap64(chunk));
@@ -290,14 +297,33 @@ fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
290297
str+=FH_SIZEOF_ACCUM;
291298
}
292299

293-
/*
294-
* The byte corresponding to the NUL will be 0x80, so the rightmost bit
295-
* position will be in the range 7, 15, ..., 63. Turn this into byte
296-
* position by dividing by 8.
297-
*/
298-
remainder=pg_rightmost_one_pos64(zero_byte_low) /BITS_PER_BYTE;
299-
fasthash_accum(hs,str,remainder);
300-
str+=remainder;
300+
if (firstbyte64(chunk)!=0)
301+
{
302+
size_tremainder;
303+
uint64mask;
304+
305+
/*
306+
* The byte corresponding to the NUL will be 0x80, so the rightmost
307+
* bit position will be in the range 15, 23, ..., 63. Turn this into
308+
* byte position by dividing by 8.
309+
*/
310+
remainder=pg_rightmost_one_pos64(zero_byte_low) /BITS_PER_BYTE;
311+
312+
/*
313+
* Create a mask for the remaining bytes so we can combine them into
314+
* the hash. This must have the same result as mixing the remaining
315+
* bytes with fasthash_accum().
316+
*/
317+
#ifdefWORDS_BIGENDIAN
318+
mask= ~UINT64CONST(0) <<BITS_PER_BYTE* (FH_SIZEOF_ACCUM-remainder);
319+
#else
320+
mask= ~UINT64CONST(0) >>BITS_PER_BYTE* (FH_SIZEOF_ACCUM-remainder);
321+
#endif
322+
hs->accum=chunk&mask;
323+
fasthash_combine(hs);
324+
325+
str+=remainder;
326+
}
301327

302328
returnstr-start;
303329
}

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