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robin_hood.h: Change hash val shifts for info and idx generation
#148
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Instead of generating the `info` hash bits with a mask + shift, we can
just use an unsigned shift from the end as that will bring in zeros.
With that change the `idx` hash bits can start from the begining so we
no longer need to shift out the `info` hash bits.
Saves 2x instructions. Only ALU but this function is in the critical
path.
There does not appear to be any issue with false hits changing the tag
bits used.
False Tag Match Rates:
: New, Cur
Insert Seq Ints:
2^10 : 2.539, 2.832
2^12 : 3.076, 3.003
2^14 : 2.960, 3.497
2^16 : 4.048, 4.306
2^18 : 4.543, 4.732
2^20 : 5.182, 5.553
2^22 : 5.579, 5.557
2^24 : 6.237, 6.017
2^26 : 6.599, 6.432
Insert Rand Ints (seed = 0):
2^10 : 2.637, 4.199
2^12 : 2.979, 3.613
2^14 : 3.662, 3.955
2^16 : 4.309, 4.849
2^18 : 4.626, 4.564
2^20 : 4.849, 5.044
2^22 : 6.050, 5.279
2^24 : 5.664, 6.350
2^26 : 6.640, 6.771
Insert Strings:
All Words(0) : 4.974, 5.231
10k Words(1) : 3.550, 4.000
URLS(2) : 5.548, 5.683
URLS(2) w/ "http://" : 5.466, 5.831
URLS(2) w/ "https://" : 5.569, 5.761
[0]: https://github.com/dwyl/english-words
[1]: https://github.com/first20hours/google-10000-english
[2]: https://www.domcop.com/top-10-million-websites
richardbiely
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Instead of generating the
infohash bits with a mask + shift, we canjust use an unsigned shift from the end as that will bring in zeros.
With that change the
idxhash bits can start from the begining so weno longer need to shift out the
infohash bits.Saves 2x instructions. Only ALU but this function is in the critical
path.
There does not appear to be any issue with false hits changing the tag
bits used.
False Tag Match Rates:
Insert Seq Ints:
2^10 : 2.539, 2.832
2^12 : 3.076, 3.003
2^14 : 2.960, 3.497
2^16 : 4.048, 4.306
2^18 : 4.543, 4.732
2^20 : 5.182, 5.553
2^22 : 5.579, 5.557
2^24 : 6.237, 6.017
2^26 : 6.599, 6.432
Insert Rand Ints (seed = 0):
2^10 : 2.637, 4.199
2^12 : 2.979, 3.613
2^14 : 3.662, 3.955
2^16 : 4.309, 4.849
2^18 : 4.626, 4.564
2^20 : 4.849, 5.044
2^22 : 6.050, 5.279
2^24 : 5.664, 6.350
2^26 : 6.640, 6.771
Insert Strings:
All Words(0) : 4.974, 5.231
10k Words(1) : 3.550, 4.000
URLS(2) : 5.548, 5.683
URLS(2) w/ "http://" : 5.466, 5.831
URLS(2) w/ "https://" : 5.569, 5.761