Use SipHash-1-3 for string and integer hashing #2149
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Hashtable bucket keys are now represented using a union:
union zend_bucket_key {
zend_string *str;
zend_ulong num;
}
The bucket->h value is now a pure hash and no longer doubles as
being the integer key value.
The top bit of bucket->h (HT_IS_STR_BIT) determines whether the
key is a string or and integer. We can do this, because the top
bit will always be 1 after hashtable mask application, so its
value does not influence bucket distribution quality.
The zend_hash_integer() function is a bit of a misnomer -- introducing
integer hashing is not as simple as replacing this function.
For testing using a Murmur64 finalizer.
Using dummy key for testing. After changing hash implementation run to avoid full rebuild: touch Zend/zend_hash.c Zend/zend_string.c ext/oci8/oci8.c \ ext/opcache/zend_accelerator_hash.c sapi/phpdbg/phpdbg_bp.c \ sapi/phpdbg/phpdbg_cmd.c Conflicts: Zend/zend_string.h
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Oh, this is intriguing. |
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Closing this for now as I don't consider it feasible in the current form. Would require at least some kind of adaptive hash switch for integer keys. |
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Hash switch? |
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@TazeTSchnitzel Dynamically switching from djb to siphash based on insert collision threshold, for example. (Other ideas on how to make this work efficiently welcome.) |
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Ah, because of the performance hit? That makes sense. |
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Rebase of an old experiment for SipHash-2-4. Nowadays SipHash-1-3 is considered sufficient for hash tables, so we try that.
Micro-benchmark results for array operations x86-64 (first column with siphash, second column baseline):
As such, we see about 120% regression for operations on non-packed integer hashes and about 50% on non-cached string hashes.