Feature #20684: Add optimized instructions for frozen literal Hash and Array

• Status: Closed
• Assignee: -
• Target version: -

[etienne (Étienne Barrié)]

Context

Methods that take empty arrays or empty hashes as default values allocate a new object each time the method is called without the argument. Often they don't mutate the parameter. To prevent an allocation, in performance critical sections, a constant is defined that holds a frozen hash or array, and the constant is defined as the default value for the parameter.

Here are some examples:
Rails: https://github.com/rails/rails/blob/607d61e884237c223c24c6f47efa0b561dd8b637/activerecord/lib/active_record/relation/query_methods.rb#L159-L160
Roda: https://github.com/jeremyevans/roda/blob/102926a02dcabc9a31674e3cf98f049139c31492/lib/roda/plugins.rb#L9-L10
dry-rb: https://github.com/dry-rb/dry-container/blob/1ee41bb109455d06bf22ebcbd94b050cc4773733/lib/dry/container/mixin.rb#L68C5-L68C15
and many other gems: https://gist.github.com/casperisfine/47f22243d4ad203855256ef5bfae7979

Additionally when defining a frozen literal constant, we're currently inefficient because we store the literal in the bytecode, we dup it just to freeze it again. It doesn't amount to much but would be nice to avoid.

Proposal

Introduce 2 new optimized instructions opt_ary_freeze and opt_hash_freeze that behave like opt_str_freeze for their respective types. If the freeze method hasn't been redefined, they simply push the frozen literal value on the stack. Like for opt_str_freeze, these instructions are added by the peephole optimizer when applicable.

In the specific case of empty array and empty hash, we use a pre-allocated global empty frozen object to avoid retaining a distinct empty object each time.

This will allow code like this: https://github.com/ruby/ruby/blob/566f2eb501d94d4047a9aad4af0d74c6a96f34a9/lib/rubygems/resolver/api_set/gem_parser.rb to be shortened and simplified like this:

diff --git i/lib/rubygems/resolver/api_set/gem_parser.rb w/lib/rubygems/resolver/api_set/gem_parser.rb
index 643b857107..34146fd426 100644
--- i/lib/rubygems/resolver/api_set/gem_parser.rb
+++ w/lib/rubygems/resolver/api_set/gem_parser.rb
@@ -1,15 +1,12 @@
 # frozen_string_literal: true
 
 class Gem::Resolver::APISet::GemParser
-  EMPTY_ARRAY = [].freeze
-  private_constant :EMPTY_ARRAY
-
   def parse(line)
     version_and_platform, rest = line.split(" ", 2)
     version, platform = version_and_platform.split("-", 2)
     dependencies, requirements = rest.split("|", 2).map! {|s| s.split(",") } if rest
-    dependencies = dependencies ? dependencies.map! {|d| parse_dependency(d) } : EMPTY_ARRAY
-    requirements = requirements ? requirements.map! {|d| parse_dependency(d) } : EMPTY_ARRAY
+    dependencies = dependencies ? dependencies.map! {|d| parse_dependency(d) } : [].freeze
+    requirements = requirements ? requirements.map! {|d| parse_dependency(d) } : [].freeze
     [version, platform, dependencies, requirements]
   end

Overall it's a minor optimization but also a very simple patch and makes code nicer.

PR: https://github.com/ruby/ruby/pull/11406

[etienne (Étienne Barrié)]

• Description updated (diff)

[matz (Yukihiro Matsumoto)]

Basically agreed. But the existence (and side effect) of this optimization should be implementation defined, especially regarding object_id.

Matz.

[byroot (Jean Boussier)]

Makes sense, thank you Matz!

[etienne (Étienne Barrié)]

• Status changed from Open to Closed

Applied in changeset git|a99707cd9c6a1d53cf8ebc883dc210219bd67a28.

---

Optimized instruction for Array#freeze

If an Array which is empty or only using literals is frozen, we detect
this as a peephole optimization and change the instructions to be
opt_ary_freeze.

[Feature #20684]

Co-authored-by: Jean Boussier [e-mail]