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preval is a gem that hooks into the Ruby compilation process and runs optimizations before it gets loaded by the virtual machine.

Certain optimizations that are common in compilers are not immediately possible with Ruby on account of Ruby's flexibility. For example, most compilers will run through a process called constant folding to eliminate the need to perform extraneous operations at runtime (e.g., 5 + 2 in the source can be replaced with 7). However, because Ruby allows you to override the Integer#+ method, it's possible that 5 + 2 would not evaluate to 7. preval assumes that most developers will not override the Integer#+ method, and performs optimizations under that assumption.

Users must opt in to each of preval's optimizations, as there's no real way of telling whether or not it is 100% safe for any codebase. The more optimizations are allowed to run, the more time and memory savings later. Users can also define their own optimizations by subclassing the Preval::Visitor class and using the existing Preval::Node APIs to replace and update the Ruby AST as necessary.


Add this line to your application's Gemfile:

gem 'preval'

And then execute:

$ bundle

Or install it yourself as:

$ gem install preval


If you're using the bootsnap gem, preval will automatically hook into its compilation step. Otherwise, you'll need to manually call Preval.process(source) with your own iseq loader (you can check out yomikomu for an example).

Each optimization is generally named for the function it performs, and can be enabled through the enable! method on the visitor class. If you do not explicitly call enable! on any optimizations, nothing will change with your source. You can also call Preval.enable_all! which will enable every built-in visitor. Be especially careful when doing this.



  • constant expressions with their evaluation (e.g., 5 + 2 becomes 7)
  • arithmetic identities with their evaluation (e.g., a * 1 becomes a)

Unsafe if:

  • you overload any of the Integer operator methods



  • def foo; @foo; end with attr_reader :foo
  • def foo=(value); @foo = value; end with attr_writer :foo

Unsafe if:

  • you overload the attr_reader method
  • you overload the attr_writer method
  • you have custom complex method_added logic



  • .gsub('...', '...') with .tr('...', '...') if the arguments are strings and are both of length 1
  • .map { ... }.flatten(1) with .flat_map { ... }
  • .reverse.each with .reverse_each
  • .shuffle.first with .sample
  • .sort.first with .min
  • .sort.last with .max

Unsafe if:

  • any of the effected methods are overwritten or custom (gsub and tr for the first one, map, flatten, and flat_map for the second, etc.)



  • for ... in ... end loops with ... each do ... end loops
  • while true ... end loops with loop do ... end loops
  • while false ... end loops with nothing
  • until false ... end loops with loop do ... end loops
  • until true ... end loops with nothing

Unsafe if:

  • the object over which you're iterating with a for loop has a custom each method that doesn't do what you'd expect it to do


After checking out the repo, run bin/setup to install dependencies. Then, run rake test to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at


The gem is available as open source under the terms of the MIT License.


Automatically optimizes your Ruby code



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