The OR-ing itself is bad for a hash function, and shifting 3 bits
left was not enough to undo the damage done by shifting
(RUBY_SPECIAL_SHIFT+3) bits right. Experimentally, shifting 16-17
bits seemed to work well in preparing the number for murmur hash.
Add a few more benchmarks to based on bm_hash_shift to ensure
we don't hurt performance too much with tweaks.
I'm pretty confident about this change and commiting it now;
especially since we're still using Murmur behind it (but perhaps
we can update to a newer hash from Murmur...)
[ruby-core:72028] [Feature #11405]
target 0: a (ruby 2.3.0dev (2015-12-11 trunk 53027) [x86_64-linux]) at "/home/ew/rrrr/b/ruby"
target 1: b (ruby 2.3.0dev (2015-12-11 master 53027) [x86_64-linux]) at "/home/ew/ruby/b/ruby"
benchmark results:
minimum results in each 5 measurements.
Execution time (sec)
name a b
hash_aref_dsym 0.279 0.276
hash_aref_dsym_long 4.951 4.936
hash_aref_fix 0.281 0.283
hash_aref_flo 0.060 0.060
hash_aref_miss 0.409 0.410
hash_aref_str 0.387 0.385
hash_aref_sym 0.275 0.270
hash_aref_sym_long 0.410 0.411
hash_flatten 0.252 0.237
hash_ident_flo 0.035 0.032
hash_ident_num 0.254 0.251
hash_ident_obj 0.252 0.256
hash_ident_str 0.250 0.252
hash_ident_sym 0.259 0.270
hash_keys 0.267 0.267
hash_shift 0.016 0.015
hash_shift_u16 0.074 0.072
hash_shift_u24 0.071 0.071
hash_shift_u32 0.073 0.072
hash_to_proc 0.008 0.008
hash_values 0.263 0.264
Speedup ratio: compare with the result of `a' (greater is better)
name b
hash_aref_dsym 1.009
hash_aref_dsym_long 1.003
hash_aref_fix 0.993
hash_aref_flo 1.001
hash_aref_miss 0.996
hash_aref_str 1.006
hash_aref_sym 1.017
hash_aref_sym_long 0.998
hash_flatten 1.061
hash_ident_flo 1.072
hash_ident_num 1.012
hash_ident_obj 0.987
hash_ident_str 0.993
hash_ident_sym 0.959
hash_keys 0.997
hash_shift 1.036
hash_shift_u16 1.039
hash_shift_u24 1.001
hash_shift_u32 1.017
hash_to_proc 1.001
hash_values 0.995
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@53037 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
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