vcpkg/docs/users/selecting-library-features.md

Selecting library features

Installing a library

We will look at llvm as an example. LLVM is a compiler infrasture. It supports optimizing llvm-ir and generating machine code. You could install it using:

> .\vcpkg install llvm

On Windows, this will install the 32-bit x86 LLVM, since that's the default triplet on Windows. If you are building for 64-bit Windows instead, you can use the following command to change the default triplet:

> .\vcpkg install --triplet x64-windows llvm

We have more documentation on triplets here. Currently we can't choose build type debug or release using command line switches.

With llvm now installed, we can execute:

> # llc takes llvm IR and generates machine code
> .\installed\x86-windows\bin\llc.exe --version # or x86-windows, or replace with the actual triplet

we see:

  Registered Targets:
    x86    - 32-bit X86: Pentium-Pro and above
    x86-64 - 64-bit X86: EM64T and AMD64

Installing additional features

But llvm supports many more targets, from ARM to SPARC to SystemZ. However, clearly our current installation doesn't include ARM as a target; thus, we need to learn how vcpkg allows us to install other LLVM targets. The llvm port allows this via the "target-*" features.

If we do:

.\vcpkg search llvm

We can see:

llvm                 10.0.0#6         The LLVM Compiler Infrastructure
llvm[clang]                           Build C Language Family Front-end.
llvm[clang-tools-extra]               Build Clang tools.
...
llvm[target-all]                      Build with all backends.
llvm[target-amdgpu]                   Build with AMDGPU backend.
llvm[target-arm]                      Build with ARM backend.
...

We can install any of these targets by using the install-feature syntax:

> .\vcpkg install llvm[target-arm] # Installs LLVM with the ARM target

Opting out of default feature

The llvm port includes a few default features that you as a user may not want: for example, the clang feature is default, which means that vcpkg install llvm will also build and install clang. If you are writing a compiler that uses LLVM as a backend, you're likely not interested in installing clang as well, and we can do that by disabling default features with the special core "feature":

> .\vcpkg install llvm[core,default-targets] # removing the default-feature with "core" also removes all of the default targets you get

Further reading

• The Feature Packages specification was the initial design for features.