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Update weak hash.

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This commit is contained in:
Nikolai Kochetov 2020-03-23 23:45:12 +03:00
parent ad56da8961
commit ee907da84d
1 changed files with 38 additions and 6 deletions
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44
dbms/src/Common/HashTable/Hash.h
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@ -89,12 +89,44 @@ inline UInt32 updateWeakHash32(const DB::UInt8 * pos, size_t size, DB::UInt32 up
{
if (size < 8)
{
auto value = unalignedLoad<DB::UInt64>(pos);
/// 8 bytes were loaded to UInt64 value, but string size is less then 8 bytes.
/// We need to zero excessive bytes to remove the garbage.
/// But instead we move bits, so that we had zeros at left.
/// It helps to have different hash for strings like 'a' and 'a\0'
value <<= UInt8(8 * (8 - size));
DB::UInt64 value = 0;
auto * value_ptr = reinterpret_cast<unsigned char *>(&value);
typedef __attribute__((__aligned__(1))) uint16_t uint16_unaligned_t;
typedef __attribute__((__aligned__(1))) uint32_t uint32_unaligned_t;
/// Adopted code from FastMemcpy.h (memcpy_tiny)
switch (size)
{
case 0:
break;
case 1:
value_ptr[0] = pos[0];
break;
case 3:
value_ptr[2] = pos[2];
case 2:
*reinterpret_cast<uint16_t *>(value_ptr) = *reinterpret_cast<const uint16_unaligned_t *>(pos);
break;
case 5:
value_ptr[4] = pos[4];
case 4:
*reinterpret_cast<uint32_t *>(value_ptr) = *reinterpret_cast<const uint32_unaligned_t *>(pos);
break;
case 6:
*reinterpret_cast<uint32_t *>(value_ptr) = *reinterpret_cast<const uint32_unaligned_t *>(pos);
*reinterpret_cast<uint16_t *>(value_ptr + 4) = *reinterpret_cast<const uint16_unaligned_t *>(pos + 4);
break;
case 7:
*reinterpret_cast<uint32_t *>(value_ptr) = *reinterpret_cast<const uint32_unaligned_t *>(pos);
*reinterpret_cast<uint32_unaligned_t *>(value_ptr + 3) =
*reinterpret_cast<const uint32_unaligned_t *>(pos + 3);
break;
default:
__builtin_unreachable();
}
value |= size;
return intHashCRC32(value, updated_value);
}