[ruby-core:118485] [Ruby master Bug#20604] Performance regression in C++ extensions due to lack of optimization flags by default since Ruby 2.7

Issue #20604 has been updated by ntkme (Natsuki Natsume).


This can be reproduced with the following benchmark script:

``` ruby
# frozen_string_literal: true

require 'bundler/inline'

gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
  gem 'bootstrap', '~> 5.3'
  gem 'sassc'
end

BOOTSTRAP =3D Gem::Specification.find_by_name('bootstrap').gem_dir + '/asse=
ts/stylesheets/_bootstrap.scss'

require 'benchmark/ips'
require 'sassc'

Benchmark.ips do |x|
  x.time =3D 15

  x.report("CXXFLAGS=3D#{RbConfig::CONFIG["CXXFLAGS"].inspect}") do
    SassC::Engine.new(File.read(BOOTSTRAP), { style: :compressed, filename:=
 BOOTSTRAP }).render
  end

  x.compare!
end
```

Begin with the default on ruby:3.3 docker container with gcc 12.2.0.

Please make sure to `gem uninstall -a sassc` before the test to start clean.

```
ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [aarch64-linux]
Warming up --------------------------------------
         CXXFLAGS=3D""     1.000 i/100ms
Calculating -------------------------------------
         CXXFLAGS=3D""      0.765 (=B1 0.0%) i/s -     12.000 in  15.684859s
```

Now edit `$RUBYARCHDIR/rbconfig.rb` to manually patch the default CXXFLAGS =
as the following:

```
  CONFIG["CXXFLAGS"] =3D "$(optflags)"
```=20

Note: This file is read-only by default, and may need to be force overwritt=
en with depends on your editor.

Now we can run the same test again.

Please make sure to `gem uninstall -a sassc` before the test to start clean.

You can see this is about 7 times faster with `$optflags` on linux gcc.

```
ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [aarch64-linux]
Warming up --------------------------------------
CXXFLAGS=3D"-O3 -fno-fast-math"
                         1.000 i/100ms
Calculating -------------------------------------
CXXFLAGS=3D"-O3 -fno-fast-math"
                          5.483 (=B1 0.0%) i/s -     83.000 in  15.139519s
```

---

I also tested on mac with clang 15 using homebrew ruby 3.3 out of box:

```
ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [arm64-darwin23]
Warming up --------------------------------------
CXXFLAGS=3D"-fdeclspec"
                         1.000 i/100ms
Calculating -------------------------------------
CXXFLAGS=3D"-fdeclspec"
                          0.517 (=B1 0.0%) i/s -      8.000 in  15.475395s
```

It's more than 10 times faster with `$optflags` with clang 15 on mac:.

```
ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [arm64-darwin23]
Warming up --------------------------------------
CXXFLAGS=3D"-fdeclspec -O3 -fno-fast-math"
                         1.000 i/100ms
Calculating -------------------------------------
CXXFLAGS=3D"-fdeclspec -O3 -fno-fast-math"
                          5.601 (=B1 0.0%) i/s -     85.000 in  15.177061s
```

----------------------------------------
Bug #20604: Performance regression in C++ extensions due to lack of optimiz=
ation flags by default since Ruby 2.7
https://bugs.ruby-lang.org/issues/20604#change-109006

* Author: ntkme (Natsuki Natsume)
* Status: Open
* ruby -v: ruby 3.3.3 (2024-06-12 revision f1c7b6f435) [arm64-darwin23]
* Backport: 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
----------------------------------------
I found a significant performance regression in `sassc` gem when comparing =
Ruby 2.6 and later, that the extension is running more than 10x slower.

I have tracked it down to this commit: https://github.com/ruby/ruby/commit/=
733aa2f8b578d03bbcb91d2f496b01e3b990c7e8

This commit removed all default CXXFLAGS from RbConfig that were previously=
 set, including `$optflags`, which has `-O3`.

Many of the C++ extensions like sassc use extconf/mkmf to compile, and they=
 have assumed optflags are already set for CXXFLAGS. Without `-O3` as part =
of the default, the extensions are compiled without any optimizations and t=
hus become extremely slow.

It is difficult for a regular user to find out the reason of the slowness, =
nor to say figuring out how to override CXXFLAGS for a native extension.

I'm not sure what was the reason of the change, but I think we should at le=
ast set `$optflags` in CXXFLAGS by default.



--=20
https://bugs.ruby-lang.org/
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