brew: Linuxbrew doesn't build an optimized LLVM (or potentially other CMake packages)

brew config output

HOMEBREW_VERSION: 3.1.9-64-geeef3f4
ORIGIN: https://github.com/Homebrew/brew
HEAD: eeef3f4fe7b9a5c1f8355d3867edf4aa0c294b00
Last commit: 8 hours ago
Core tap ORIGIN: https://github.com/Homebrew/linuxbrew-core
Core tap HEAD: 7cdfe2454bc0382eb22884439b04bc0b8ff70ebd
Core tap last commit: 3 hours ago
Core tap branch: fix-libcxx
HOMEBREW_PREFIX: /home/chandlerc/.linuxbrew
HOMEBREW_REPOSITORY: /home/chandlerc/.linuxbrew/Homebrew
HOMEBREW_CELLAR: /home/chandlerc/.linuxbrew/Cellar
HOMEBREW_CASK_OPTS: []
HOMEBREW_EDITOR: nvim
HOMEBREW_MAKE_JOBS: 96
Homebrew Ruby: 2.6.3 => /home/chandlerc/.linuxbrew/Homebrew/Library/Homebrew/vendor/portable-ruby/2.6.3_2/bin/ruby
CPU: 96-core 64-bit skylake
Clang: 12.0.0 build (parse error)
Git: 2.32.0 => /bin/git
Curl: 7.74.0 => /usr/bin/curl
Kernel: Linux 5.10.28 x86_64 GNU/Linux
OS: Debian GNU/Linux 11 (bullseye)
Host glibc: 2.31
/usr/bin/gcc: 10.2.1
/usr/bin/ruby: 2.7.3
glibc: N/A
gcc@5: N/A
xorg: N/A

brew doctor output

Please note that these warnings are just used to help the Homebrew maintainers
with debugging if you file an issue. If everything you use Homebrew for is
working fine: please don't worry or file an issue; just ignore this. Thanks!

Warning: "config" scripts exist outside your system or Homebrew directories.
`./configure` scripts often look for *-config scripts to determine if
software packages are installed, and which additional flags to use when
compiling and linking.

Having additional scripts in your path can confuse software installed via
Homebrew if the config script overrides a system or Homebrew-provided
script of the same name. We found the following "config" scripts:
  /home/chandlerc/.cargo/bin/cargo-install-update-config
  /home/chandlerc/.pyenv/shims/python3.7m-config
  /home/chandlerc/.pyenv/shims/python-config
  /home/chandlerc/.pyenv/shims/python3-config
  /home/chandlerc/.pyenv/shims/python3.8-config
  /home/chandlerc/.pyenv/shims/pre-commit-validate-config
  /home/chandlerc/.pyenv/shims/python3.7-config
  /home/chandlerc/.pyenv/shims/python3.6m-config
  /home/chandlerc/.pyenv/shims/python3.9-config
  /home/chandlerc/.pyenv/shims/python3.6-config

Warning: Your Homebrew's prefix is not /home/linuxbrew/.linuxbrew.
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prefix (/home/linuxbrew/.linuxbrew).
You will encounter build failures with some formulae.
Please create pull requests instead of asking for help on Homebrew's GitHub,
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any issues you experience while you are running this
unsupported configuration.


Warning: Some taps are not on the default git origin branch and may not receive
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  git -C $(brew --repo homebrew/core) checkout master

• I ran brew update and am still able to reproduce my issue.
• I have resolved all warnings from brew doctor and that did not fix my problem.

What were you trying to do (and why)?

Installing LLVM (including Clang and libc++) for use in building C++ projects.

What happened (include all command output)?

The installation of LLVM, Clang, and all related tools are completely unoptimized.

Timing any significant build with the installed clang ran about 10x slower than expected.

What did you expect to happen?

The installed tools to be optimized.

This appears to be because CMake’s default flags for release builds are overridden. Removing that fixes the issue. You can see what this looks like here:

https://github.com/chandlerc/brew/commit/578fe416570abe90d09ab036a6aeff68ebb7b5d2

I’m happy to submit that as a pull request, but wanted to first check if this is the correct direction to take. I understand that Homebrew wants to override optimization levels in some cases, but it seems like the shims already provide the tools to do that, and removing these flags from CMake seems likely to leave ~all CMake built packages on Linux unoptimized.

Note that I did check that macOS is unaffected. Also it seems macOS is unaffected with the patch above applied.

Step-by-step reproduction instructions (by running brew commands)

brew install llvm
# compile some code with `clang++`, time it, observe it is slow on linux