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Teach fasthash_accum to use platform endianness for bytewise loads 

This function previously used a mix of word-wise loads and bytewise
loads. The bytewise loads happened to be little-endian regardless of
platform. This in itself is not a problem. However, a future commit
will require the same result whether A) the input is loaded as a
word with the relevent bytes masked-off, or B) the input is loaded
one byte at a time.

While at it, improve debuggability of the internal hash state.

Discussion: https://postgr.es/m/CANWCAZZpuV1mES1mtSpAq8tWJewbrv4gEz6R_k4gzNG8GZ5gag%40mail.gmail.com
This commit is contained in:
John Naylor 2024-04-06 16:59:28 +07:00
parent 98f320eb2e
commit 0c25fee359
1 changed files with 39 additions and 5 deletions
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44
src/include/common/hashfn_unstable.h
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@ -131,9 +131,6 @@ fasthash_combine(fasthash_state *hs)
{
hs->hash ^= fasthash_mix(hs->accum, 0);
hs->hash *= 0x880355f21e6d1965;
/* reset hash state for next input */
hs->accum = 0;
}
/* accumulate up to 8 bytes of input and combine it into the hash */
@ -142,9 +139,44 @@ fasthash_accum(fasthash_state *hs, const char *k, size_t len)
{
uint32 lower_four;
Assert(hs->accum == 0);
Assert(len <= FH_SIZEOF_ACCUM);
hs->accum = 0;
/*
* For consistency, bytewise loads must match the platform's endianness.
*/
#ifdef WORDS_BIGENDIAN
switch (len)
{
case 8:
memcpy(&hs->accum, k, 8);
break;
case 7:
hs->accum |= (uint64) k[6] << 8;
/* FALLTHROUGH */
case 6:
hs->accum |= (uint64) k[5] << 16;
/* FALLTHROUGH */
case 5:
hs->accum |= (uint64) k[4] << 24;
/* FALLTHROUGH */
case 4:
memcpy(&lower_four, k, sizeof(lower_four));
hs->accum |= (uint64) lower_four << 32;
break;
case 3:
hs->accum |= (uint64) k[2] << 40;
/* FALLTHROUGH */
case 2:
hs->accum |= (uint64) k[1] << 48;
/* FALLTHROUGH */
case 1:
hs->accum |= (uint64) k[0] << 56;
break;
case 0:
return;
}
#else
switch (len)
{
case 8:
@ -175,6 +207,7 @@ fasthash_accum(fasthash_state *hs, const char *k, size_t len)
case 0:
return;
}
#endif
fasthash_combine(hs);
}
@ -288,7 +321,8 @@ fasthash_accum_cstring(fasthash_state *hs, const char *str)
if (PointerIsAligned(str, uint64))
{
len = fasthash_accum_cstring_aligned(hs, str);
Assert(hs_check.hash == hs->hash && len_check == len);
Assert(len_check == len);
Assert(hs_check.hash == hs->hash);
return len;
}
#endif /* SIZEOF_VOID_P */