brew install llvm

~ $ brew install llvm
==> Downloading https://mirrors.aliyun.com/homebrew/homebrew-bottles/bottles/libffi-3.3.mojave.bottle.tar.gz
######################################################################## 100.0%
==> Downloading https://mirrors.aliyun.com/homebrew/homebrew-bottles/bottles/llvm-10.0.0_3.mojave.bottle.tar.gz
######################################################################## 100.0%
==> Installing dependencies for llvm: libffi
==> Installing llvm dependency: libffi
==> Pouring libffi-3.3.mojave.bottle.tar.gz
==> Caveats
libffi is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

For compilers to find libffi you may need to set:
  export LDFLAGS="-L/usr/local/opt/libffi/lib"
  export CPPFLAGS="-I/usr/local/opt/libffi/include"

For pkg-config to find libffi you may need to set:
  export PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig"

==> Summary
  /usr/local/Cellar/libffi/3.3: 16 files, 484.8KB
==> Installing llvm
==> Pouring llvm-10.0.0_3.mojave.bottle.tar.gz
==> Caveats
To use the bundled libc++ please add the following LDFLAGS:
  LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"

llvm is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH run:
  echo 'export PATH="/usr/local/opt/llvm/bin:$PATH"' >> /Users/liuhuajian/.bash_profile

For compilers to find llvm you may need to set:
  export LDFLAGS="-L/usr/local/opt/llvm/lib"
  export CPPFLAGS="-I/usr/local/opt/llvm/include"

==> Summary
  /usr/local/Cellar/llvm/10.0.0_3: 7,055 files, 1GB
==> Caveats
==> libffi
libffi is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

For compilers to find libffi you may need to set:
  export LDFLAGS="-L/usr/local/opt/libffi/lib"
  export CPPFLAGS="-I/usr/local/opt/libffi/include"

For pkg-config to find libffi you may need to set:
  export PKG_CONFIG_PATH="/usr/local/opt/libffi/lib/pkgconfig"

==> llvm
To use the bundled libc++ please add the following LDFLAGS:
  LDFLAGS="-L/usr/local/opt/llvm/lib -Wl,-rpath,/usr/local/opt/llvm/lib"

llvm is keg-only, which means it was not symlinked into /usr/local,
because macOS already provides this software and installing another version in
parallel can cause all kinds of trouble.

If you need to have llvm first in your PATH run:
  echo 'export PATH="/usr/local/opt/llvm/bin:$PATH"' >> /Users/liuhuajian/.bash_profile

For compilers to find llvm you may need to set:
  export LDFLAGS="-L/usr/local/opt/llvm/lib"
  export CPPFLAGS="-I/usr/local/opt/llvm/include"