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Extract index from the high hash bits instead of the low bits. #71

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Extract index from the high hash bits instead of the low bits. #71

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Commits on Apr 25, 2019

  1. Extract index from the high hash bits instead of the low bits. 

    Some Hashers, and FxHasher in particular, mix the low bits poorly; so only use the high bits for both hashes.

Instead of introducing bucket_shift next to bucket_mask in the top-level struct, derive them both from capacity_log2. This avoids exacerbating struct size in rust-lang#69.

This change is also a prerequisite for rayon-based parallel collect, which requires that the high bits of the index are always in the same place regardless of table size.

 name                     old ns/iter  new ns/iter  diff ns/iter   diff %  speedup
 find_existing            0            0                       0     NaN%    x NaN
 find_existing_high_bits  84,320       3,442             -80,878  -95.92%  x 24.50
 find_nonexisting         0            0                       0     NaN%    x NaN
 get_remove_insert        25           29                      4   16.00%   x 0.86
 grow_by_insertion        205          209                     4    1.95%   x 0.98
 grow_by_insertion_kb     290          180                  -110  -37.93%   x 1.61
 hashmap_as_queue         25           26                      1    4.00%   x 0.96
 insert_8_char_string     18,038       17,491               -547   -3.03%   x 1.03
 new_drop                 0            0                       0     NaN%    x NaN
 new_insert_drop          45           50                      5   11.11%   x 0.90
    @edre
    edre committed Apr 25, 2019
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  2. Fix serde tests that depended on iteration order.

    @edre
    edre committed Apr 25, 2019
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  3. Make serde tests not dependent on the hash order on all platforms.

    @edre
    edre committed Apr 25, 2019
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