Feature #13174: Smaller id_table on 64bit platform

• Status: Open
• Assignee: -
• Target version: -

[funny_falcon (Yura Sokolov)]

Reduce size of id_table by 25% on 64bit platform (19% on smallest table):

• embed collision bit into key
• use array of keys + array of values instead of array of struct item_t.
  Both arrays allocated at once, and pointer to values is calculated.

Additional changes:

• stricter check in next_id_base.
  One bit is needed for collision check on 64bit platform.
  Range is more limited on 32bit platform with ID_SCOPE_SHIFT and RUBY_SPECIAL_SHIFT.
• stricter check for result of next_id_base.
  Rais exception in rb_sym2id, assert in rb_make_internal_id.
• allow to shrink id_table.
  Shrink was disabled for mix_table cause it leads to error in id_table type
  detection. Since mix_table were removed, it is safe to shrink it again.
• improve large table performance by mixin higher bits into step value.

Performance of small tables almost the same.
Mixing higher bits takes 1-2% for small tables on hit, but improves large table
performance.

Report: https://gist.github.com/funny-falcon/7a1af6b3891ae636b12237f25eb20690

https://github.com/funny-falcon/ruby/tree/smaller_id_table
https://github.com/ruby/ruby/compare/trunk...funny-falcon:smaller_id_table.patch