Jarek Poplawski a écrit :
Eric Dumazet wrote, On 12/14/2007 12:09 PM:
...
+ /*
+ * Instead of returning hash % ht->cfg.size (implying a divide)
+ * we return the high 32 bits of the (hash * ht->cfg.size) that will
+ * give results between [0 and cfg.size-1] and same hash distribution,
+ * but using a multiply, less expensive than a divide
+ */
+ return ((u64)hash * ht->cfg.size) >> 32;
Are we sure of the same hash distribution? Probably I miss something,
but: if this 'hash' is well distributed on 32 bits, and ht->cfg.size
is smaller than 32 bits, e.g. 256 (8 bits), then this multiplication
moves to the higher 32 of u64 only max. 8 bits of the most significant
byte, and the other three bytes are never used, while division is
always affected by all four bytes...
Not sure what you are saying... but if size=256, then, yes, we want a final
result between 0 and 255, so three bytes are nul.
'size' is the size of hashtable, its not a random 32bits value :)
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