RE: DT_GNU_HASH latest patches

["djamel anonymous" <djam8193ah at hotmail dot com>]

Hello, i am writing you this time about the first variant.after looking at 
the benchmark results i noted that there have been a reduction in the number 
of l1 cache misses; a reduction in l1 cache misses means a win of 12 cycles 
; the difference between the latency of l2 cache and that of l1 cache 
15-3.on the other hand replacing a division by a binary and & is a win of at 
least 25 cycles, so it think that avoiding tthe division in the common case 
may improve performance.
changing the size of the main hash table is not an ideal solution because it 
is difficult to choose a good power of 2 size and there is no gurantee that 
a power of 2 size will give short chains.this also will introduce a great 
variability in the size nbcukets, they may be oversized which may give a big 
waste of space.on the other hand the solution of a filter before the main 
hash table that is a power of 2 has several advantages:
1-it is small: on average its size is nsymbols bytes, which has more chance 
to fit in the l1 cache than 3*nsymols or 4*nsymbols bytes for tha current 
hash table.
2-because it is small oversizing or undersizeing will not have a big impact 
on the overall space utilization.
3-the benchmark of take3 shows an increase in l2 cache misses because of the 
enlarged size of the hash table; the reduced size of the filter hash table 
may do less l2 cache misses.
4-in the take3 hash table there is a waste of space because the symbol index 
word inside the hash table is useless. according to the charts of yesterday 
there is between 5% and 12% empty buckets inside each hash table.for each 
empty bucket the symbol index will have no role and will be wasted. with the 
filter hash table every added word will participate in reducing the 
probability to go to the second hash table.
when an access is done to the filter hash table and the bit is found to be 
set, there will be an access to the normal hash table.there is still a 
chance to avoid a third memory access if the bucket is found empty(between 
5% and 12 %) so the average number of memeory accesses may not increase too 
much compared to take3.
I think that nbucket must not be a power of two, otherwise every access to 
the normal hash table will access a non empty bucket and third memory access 
to the hash chains will be done.
the following is a function that computes the nearest power of 2 to a given 
integer number.
static inline unsigned pow2_round(unsigned int input)
{
	unsigned log2=32-__builtin_clz(input);
	if ((1<<(log2-2))&(input))
		return 1<<log2;
	else
		return 1<<(log2-1);
};
the corresponding size of the filter hash table would be 
8*pow2_round(nsymbols) bits or pow2_round(nsymbols)/4 32 bits integers.
best regards.

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