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Use "interned"
Strings when possible to avoid uneccessary heap allo…
…cation. Fixes ohler55#275 - Background info: https://en.wikipedia.org/wiki/String_interning - Available to C extensions since MRI Ruby 3.0: - https://bugs.ruby-lang.org/issues/13381 - https://bugs.ruby-lang.org/issues/16029 This change makes `Ox` use "interned" (frozen and deduplicated) `String`s anywhere possible, the same effect as calling `String#-@` except without the heap churn of `RVALUE` allocation: https://ruby-doc.org/core/String.html#method-i-2B-40 - Uses the `HAVE_<funcname>` preprocessor macros to detect this functionality and avoid breaking older Rubies (confirmed with at least MRI 2.7.2). - I explicitly use interned `String`s anywhere an allocated `String` seemed entirely internal to `Ox`, e.g. in `sax_value_as_time` when allocating the `String` argument to `ox_time_class`. - Adds a new user-facing option `intern_strings` to control the behavior of `String` return values from user-facing functions, e.g. from `sax_as_s`. - Results in a ~25% reduction in startup GC pressure in my library! :D Inspired by similar changes to `json` and `msgpack`: - flori/json#451 - msgpack/msgpack-ruby#196 My goal is `Object` allocation reduction in the `Ox::Sax` handler of my MIME Type file-identification library caused by the large number of duplicate `String`s in the `shared-mime-info`-format XML it uses as a data source. I was already calling `#-@` (or using the `-some_str_variable` syntax) everywhere possible in my handler, but that only freezes and deduplicates an already-allocated `String`: irb(main):068:0> oid = proc { puts "#{_1} is object_id #{_1.object_id}" } => #<Proc:0x0000564d53eb6850 (irb):66> irb(main):069:0> lol = 'lol'.tap(&oid).-@.tap(&oid) lol is object_id 19800 lol is object_id 19740 => "lol" irb(main):070:0> -lol.tap(&oid).-@.tap(&oid) lol is object_id 19740 lol is object_id 19740 => "lol" That avoids duplicate retained `String`s but still causes my library to take a big GC hit right at startup when it loads its data. All the stats below were collected using SamSaffron's `memory_profiler`: https://github.com/SamSaffron/memory_profiler First, a sanity-check to make sure I didn't break MRI < 3.0, using MRI 2.7 + latest official `Ox` from RubyGems (2.14.5): [okeeblow@emi#CHECKING-YOU-OUT] ruby -v ruby 2.7.2p137 (2020-10-01 revision 5445e04352) [x86_64-linux] [okeeblow@emi#CHECKING-YOU-OUT] ./bin/are-we-unallocated-yet|grep Total Total allocated: 20561351 bytes (401372 objects) Total retained: 1680971 bytes (22102 objects) versus same MRI 2.7 but with my patched `Ox`: [okeeblow@emi#CHECKING-YOU-OUT] gem install ../../ox/ox-2.14.5.gem [okeeblow@emi#CHECKING-YOU-OUT] ./bin/are-we-unallocated-yet|grep Total Total allocated: 20561111 bytes (401366 objects) Total retained: 1680971 bytes (22102 objects) No difference between the two, showing that this patch is a no-op for pre-3.0. Sorry I did not test older versions of MRI or other Rubies like J/Truffle/etc since I don't have them on my system. Now the real gainz can be seen when comparing MRI 3.0 with unpatched `Ox`: [okeeblow@emi#CHECKING-YOU-OUT] bundle install Fetching gem metadata from https://rubygems.org/...... … Installing ox 2.14.5 with native extensions Using checking-you-out 0.7.0 from source at `.` Bundle complete! 11 Gemfile dependencies, 19 gems now installed. [okeeblow@emi#CHECKING-YOU-OUT] ./bin/are-we-unallocated-yet|grep Total Total allocated: 20081080 bytes (390133 objects) Total retained: 1713209 bytes (22095 objects) against same MRI 3.0 with my patched `Ox`: [okeeblow@emi#CHECKING-YOU-OUT] gem install ../../ox/ox-2.14.5.gem Building native extensions. This could take a while... Successfully installed ox-2.14.5 Parsing documentation for ox-2.14.5 unknown encoding name ""UTF-8"" for README.md, skipping Installing ri documentation for ox-2.14.5 Done installing documentation for ox after 0 seconds 1 gem installed [okeeblow@emi#CHECKING-YOU-OUT] ./bin/are-we-unallocated-yet|grep Total Total allocated: 17298804 bytes (322860 objects) Total retained: 1713202 bytes (22073 objects) against same MRI 3.0, my patched `Ox`, and `Ox.parse_sax(intern_strings: true)`: [okeeblow@emi#CHECKING-YOU-OUT] ./bin/are-we-unallocated-yet|grep Total Total allocated: 16414938 bytes (301382 objects) Total retained: 1713226 bytes (22073 objects) This shows that just having Ruby 3.0 available results in a huge win, and opting into immutable `String`s from `Value#as_s` helps me even more! Unit tests pass: [okeeblow@emi#test] ruby tests.rb Loaded suite tests Started ................. 16 tests, 40 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed ..... 18 tests, 42 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed ............................................................................................................................... 151 tests, 249 assertions, 0 failures, 0 errors, 0 pendings, 0 omissions, 0 notifications 100% passed Apologies for possible indentation issues in this patch. There seem to be lots of existing lines with a mix of tabs and spaces — nbd, it happens — so I tried to match the surrounding lines everywhere I made an addition to make sure things line up and look okay in `git diff`. I only use `Ox` for Sax parsing, not for marshalling, so please scrutinize my changes in `gen_load.c` and friends extra hard in case I omitted any `intern_strings` option checks that could result in an unwary user getting an immutable `String` where there wasn't one before. The one change I had to make to a `String#force_encoding`-using test case is an example of what I want to avoid surprising anyone with.
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