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Revert "Speed up tail processing when hashing aligned C strings, take two"

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This reverts commit a365d9e2e8c1ead27203a4431211098292777d3b.

Older versions of Valgrind raise an error, so go back to the bytewise
loop for the final word in the input.

Reported-by: Anton A. Melnikov <a.melnikov@postgrespro.ru>
Discussion: https://postgr.es/m/a3a959f6-14b8-4819-ac04-eaf2aa2e868d@postgrespro.ru
Backpatch-through: 17
This commit is contained in:
John Naylor 2025-01-29 13:35:43 +07:00
parent 4f071349c0
commit 235328ee4a
1 changed files with 10 additions and 36 deletions
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46
src/include/common/hashfn_unstable.h
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@ -219,13 +219,6 @@ fasthash_accum(fasthash_state *hs, const char *k, size_t len)
#define haszero64(v) \
(((v) - 0x0101010101010101) & ~(v) & 0x8080808080808080)
/* get first byte in memory order */
#ifdef WORDS_BIGENDIAN
#define firstbyte64(v) ((v) >> 56)
#else
#define firstbyte64(v) ((v) & 0xFF)
#endif
/*
* all-purpose workhorse for fasthash_accum_cstring
*/
@ -262,7 +255,7 @@ static inline size_t
fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
{
const char *const start = str;
uint64 chunk;
size_t remainder;
uint64 zero_byte_low;
Assert(PointerIsAligned(start, uint64));
@ -282,7 +275,7 @@ fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
*/
for (;;)
{
chunk = *(uint64 *) str;
uint64 chunk = *(uint64 *) str;
#ifdef WORDS_BIGENDIAN
zero_byte_low = haszero64(pg_bswap64(chunk));
@ -297,33 +290,14 @@ fasthash_accum_cstring_aligned(fasthash_state *hs, const char *str)
str += FH_SIZEOF_ACCUM;
}
if (firstbyte64(chunk) != 0)
{
size_t remainder;
uint64 mask;
/*
* The byte corresponding to the NUL will be 0x80, so the rightmost
* bit position will be in the range 15, 23, ..., 63. Turn this into
* byte position by dividing by 8.
*/
remainder = pg_rightmost_one_pos64(zero_byte_low) / BITS_PER_BYTE;
/*
* Create a mask for the remaining bytes so we can combine them into
* the hash. This must have the same result as mixing the remaining
* bytes with fasthash_accum().
*/
#ifdef WORDS_BIGENDIAN
mask = ~UINT64CONST(0) << BITS_PER_BYTE * (FH_SIZEOF_ACCUM - remainder);
#else
mask = ~UINT64CONST(0) >> BITS_PER_BYTE * (FH_SIZEOF_ACCUM - remainder);
#endif
hs->accum = chunk & mask;
fasthash_combine(hs);
str += remainder;
}
/*
* The byte corresponding to the NUL will be 0x80, so the rightmost bit
* position will be in the range 7, 15, ..., 63. Turn this into byte
* position by dividing by 8.
*/
remainder = pg_rightmost_one_pos64(zero_byte_low) / BITS_PER_BYTE;
fasthash_accum(hs, str, remainder);
str += remainder;
return str - start;
}