Basic support for profiling

.gitignore

@@ -65,3 +65,7 @@ TAGS
 
 # For bundle binstubs
 bin/*
+!bin/rubocop-profile
+
+# For stackprof or others
+tmp/*

CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## master (unreleased)
 
+### New features
+
+* [#8242](https://github.com/rubocop-hq/rubocop/pull/8242): Internal profiling available with `bin/rubocop-profile` and rake tasks. ([@marcandre][])
+
 ## 0.87.1 (2020-07-07)
 
 ### Bug fixes

Gemfile

@@ -14,6 +14,7 @@ gem 'rubocop-rspec', '~> 1.39.0'
 # Stop upgrading SimpleCov until the following issue will be resolved.
 # https://github.com/codeclimate/test-reporter/issues/418
 gem 'simplecov', '~> 0.10', '< 0.18'
+gem 'stackprof', platform: :mri
 gem 'test-queue'
 gem 'yard', '~> 0.9'
 

bin/rubocop-profile

@@ -0,0 +1,16 @@
+#!/usr/bin/env ruby
+# frozen_string_literal: true
+
+require 'stackprof'
+
+StackProf.start
+start = Process.clock_gettime(Process::CLOCK_MONOTONIC)
+begin
+  load "#{__dir__}/../exe/rubocop"
+ensure
+  delta = Process.clock_gettime(Process::CLOCK_MONOTONIC) - start
+  StackProf.stop
+  Dir.mkdir('tmp') unless File.exist?('tmp')
+  StackProf.results('tmp/stackprof.dump')
+  puts "Finished in #{delta.round(1)} seconds"
+end

tasks/prof.rake

@@ -0,0 +1,40 @@
+# frozen_string_literal: true
+
+namespace :prof do
+  DUMP_PATH = 'tmp/stackprof.dump'
+
+  desc 'Run RuboCop on itself with profiling on'
+  task :run, [:path] do |_task, args|
+    path = args.fetch(:path, '.')
+    cmd = "bin/rubocop-profile #{path}"
+    system cmd
+  end
+
+  task :run_if_needed, [:path] do
+    Rake::Task[:run].run unless File.exist?(DUMP_PATH)
+  end
+
+  desc 'List the slowest cops'
+  task slow_cops: :run_if_needed do
+    method = 'RuboCop::Cop::Commissioner#trigger_responding_cops'
+    cmd = "stackprof #{DUMP_PATH} --text --method '#{method}'"
+    puts cmd
+    output = `#{cmd}`
+    _header, list, _code = *output
+      .lines
+      .grep_v(/RuboCop::Cop::Commissioner/) # ignore internal calls
+      .slice_when { |line| line.match?(/callees.*:|code:/) }
+    puts list.first(40)
+  end
+
+  desc 'Check a particular method by walking through the callstack'
+  task :walk, [:method] => :run_if_needed do |_task, args|
+    method = args.fetch(:method) do
+      warn 'usage: bundle exec rake walk[Class#method]'
+      exit!
+    end
+    cmd = "stackprof #{DUMP_PATH} --walk --method '#{method}'"
+    puts cmd
+    system cmd
+  end
+end