Capturing configuration information in the headers

:local: true

The Problem

libc++ supports building the library with a number of different configuration options. In order to support persistent configurations and reduce arbitrary preprocessor logic in the headers, libc++ has a mechanism to capture configuration options in the installed headers so they can be used in the rest of the code.

Design Goals

• The solution should be simple, consistent and robust to avoid subtle bugs.
• Developers should test the code the same way it will be deployed -- in other words, the headers used to run tests should be the same that we install in order to avoid bugs creeping up.
• It should allow different targets or flavors of the library to use a different configuration without having to duplicate all the libc++ headers.

The Solution

When you first configure libc++ using CMake, a __config_site file is generated to capture the various configuration options you selected. The __config header used by all other headers includes this __config_site header first in order to get the correct configuration.

The __config_site header is hence the only place where persistent configuration is stored in the library. That header essentially reflects how the vendor configured the library. As we evolve the library, we can lift configuration options into that header in order to reduce arbitrary hardcoded choices elsewhere in the code. For example, instead of assuming that a specific platform doesn't provide some functionality, we can create a generic macro to guard it and vendors can define the macro when configuring the library on that platform. This makes the “carve off” reusable in other circumstances instead of tying it tightly to a single platform.

Furthermore, the Clang driver now looks for headers in a target-specific directory for libc++. By installing the __config_site header (and only that header) to this target-specific directory, it is possible to share the libc++ headers for multiple targets, and only duplicate the persistent information located in the __config_site header. For example:

include/c++/v1/
  vector
  map
  etc...

include/<targetA>/c++/v1/
  __config_site

include/<targetB>/c++/v1/
  __config_site

When compiling for targetA, Clang will use the __config_site inside include/<targetA>/c++/v1/, and the corresponding __config_site for targetB.