From 85b4ba2836437ff30ff565005b2500500200611b Mon Sep 17 00:00:00 2001
From: Ryuta Kamizono <kamipo@gmail.com>
Date: Fri, 14 Jun 2019 21:24:54 +0900
Subject: No allocation `Arel::Visitors::ToSql#visit`

Each `visit o, collector` allocates one extra array due to
receiving args by splat array.

https://github.com/rails/rails/blob/2c3332cc4c0fa77dbe2e13e8a792f80fbd8f4ad3/activerecord/lib/arel/visitors/visitor.rb#L27-L29

Currently 1,000 times `User.where(id: 1).to_sql` allocates 13,000
arrays in `visitor.accept`. This avoids receiving args by splat array,
it makes `visitor.accept` no array allocation.

```ruby
ObjectSpace::AllocationTracer.setup(%i{path line type})

pp ObjectSpace::AllocationTracer.trace {
  1_000.times { User.where(id: 1).to_sql }
}.select { |k, _| k[2] == :T_ARRAY && k[0].end_with?("visitor.rb", "to_sql.rb") }
```

Before (2c3332cc4c0fa77dbe2e13e8a792f80fbd8f4ad3):

```
{["~/rails/activerecord/lib/arel/visitors/to_sql.rb",
  18,
  :T_ARRAY]=>[1000, 0, 0, 0, 0, 0],
 ["~/rails/activerecord/lib/active_record/connection_adapters/determine_if_preparable_visitor.rb",
  11,
  :T_ARRAY]=>[1000, 0, 0, 0, 0, 0],
 ["~/rails/activerecord/lib/arel/visitors/visitor.rb",
  12,
  :T_ARRAY]=>[1000, 0, 0, 0, 0, 0],
 ["~/rails/activerecord/lib/arel/visitors/to_sql.rb",
  788,
  :T_ARRAY]=>[3000, 0, 0, 0, 0, 0],
 ["~/rails/activerecord/lib/arel/visitors/to_sql.rb",
  794,
  :T_ARRAY]=>[3000, 0, 0, 0, 0, 0],
 ["~/rails/activerecord/lib/arel/visitors/to_sql.rb",
  156,
  :T_ARRAY]=>[1000, 0, 0, 0, 0, 0],
 ["~/rails/activerecord/lib/arel/visitors/to_sql.rb",
  443,
  :T_ARRAY]=>[1000, 0, 0, 0, 0, 0],
 ["~/rails/activerecord/lib/arel/visitors/to_sql.rb",
  603,
  :T_ARRAY]=>[1000, 0, 0, 0, 0, 0],
 ["~/rails/activerecord/lib/arel/visitors/to_sql.rb",
  611,
  :T_ARRAY]=>[1000, 0, 0, 0, 0, 0]}
```

After (this change):

```
{}
```
---
 activerecord/lib/arel/visitors/depth_first.rb | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

(limited to 'activerecord/lib/arel/visitors/depth_first.rb')

diff --git a/activerecord/lib/arel/visitors/depth_first.rb b/activerecord/lib/arel/visitors/depth_first.rb
index ca11d9c879..98c3f92cf1 100644
--- a/activerecord/lib/arel/visitors/depth_first.rb
+++ b/activerecord/lib/arel/visitors/depth_first.rb
@@ -9,7 +9,7 @@ module Arel # :nodoc: all
       end
 
       private
-        def visit(o)
+        def visit(o, _ = nil)
           super
           @block.call o
         end
-- 
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