[libcxx] Capture configuration information when installing the libc++ headers
Summary:
Hi all,
This patch is a successor to D11963. However it has changed dramatically and I felt it would be best to start a new review thread.
Please read the design documentation added in this patch for a description of how it works.
Reviewers: mclow.lists, danalbert, jroelofs, EricWF
Subscribers: vkalintiris, rnk, ed, espositofulvio, asl, eugenis, cfe-commits
Differential Revision: http://reviews.llvm.org/D13407
llvm-svn: 250235
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: f9f796e79b95a70346a945a589e833b71123e983
diff --git a/docs/DesignDocs/CapturingConfigInfo.rst b/docs/DesignDocs/CapturingConfigInfo.rst
new file mode 100644
index 0000000..0f16ca1
--- /dev/null
+++ b/docs/DesignDocs/CapturingConfigInfo.rst
@@ -0,0 +1,88 @@
+=======================================================
+Capturing configuration information during installation
+=======================================================
+
+.. contents::
+ :local:
+
+The Problem
+===========
+
+Currently the libc++ supports building the library with a number of different
+configuration options. Unfortunately all of that configuration information is
+lost when libc++ is installed. In order to support "persistent"
+configurations libc++ needs a mechanism to capture the configuration options
+in the INSTALLED headers.
+
+
+Design Goals
+============
+
+* The solution should not INSTALL any additional headers. We don't want an extra
+ #include slowing everybody down.
+
+* The solution should not unduly affect libc++ developers. The problem is limited
+ to installed versions of libc++ and the solution should be as well.
+
+* The solution should not modify any existing headers EXCEPT during installation.
+ It makes developers lives harder if they have to regenerate the libc++ headers
+ every time they are modified.
+
+* The solution should not make any of the libc++ headers dependant on
+ files generated by the build system. The headers should be able to compile
+ out of the box without any modification.
+
+* The solution should not have ANY effect on users who don't need special
+ configuration options. The vast majority of users will never need this so it
+ shouldn't cost them.
+
+
+The Solution
+============
+
+When you first configure libc++ using CMake we check to see if we need to
+capture any options. If we haven't been given any "persistent" options then
+we do NOTHING.
+
+Otherwise we create a custom installation rule that modifies the installed __config
+header. The rule first generates a dummy "__config_site" header containing the required
+#defines. The contents of the dummy header are then prependend to the installed
+__config header. By manually prepending the files we avoid the cost of an
+extra #include and we allow the __config header to be ignorant of the extra
+ configuration all together. An example "__config" header generated when
+-DLIBCXX_ENABLE_THREADS=OFF is given to CMake would look something like:
+
+.. code-block:: cpp
+
+ //===----------------------------------------------------------------------===//
+ //
+ // The LLVM Compiler Infrastructure
+ //
+ // This file is dual licensed under the MIT and the University of Illinois Open
+ // Source Licenses. See LICENSE.TXT for details.
+ //
+ //===----------------------------------------------------------------------===//
+
+ #ifndef _LIBCPP_CONFIG_SITE
+ #define _LIBCPP_CONFIG_SITE
+
+ /* #undef _LIBCPP_HAS_NO_GLOBAL_FILESYSTEM_NAMESPACE */
+ /* #undef _LIBCPP_HAS_NO_STDIN */
+ /* #undef _LIBCPP_HAS_NO_STDOUT */
+ #define _LIBCPP_HAS_NO_THREADS
+ /* #undef _LIBCPP_HAS_NO_MONOTONIC_CLOCK */
+ /* #undef _LIBCPP_HAS_NO_THREAD_UNSAFE_C_FUNCTIONS */
+
+ #endif
+ // -*- C++ -*-
+ //===--------------------------- __config ---------------------------------===//
+ //
+ // The LLVM Compiler Infrastructure
+ //
+ // This file is dual licensed under the MIT and the University of Illinois Open
+ // Source Licenses. See LICENSE.TXT for details.
+ //
+ //===----------------------------------------------------------------------===//
+
+ #ifndef _LIBCPP_CONFIG
+ #define _LIBCPP_CONFIG
diff --git a/docs/index.rst b/docs/index.rst
index acd9229..f34979f 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -124,6 +124,12 @@
Design Documents
----------------
+.. toctree::
+ :maxdepth: 1
+
+ DesignDocs/CapturingConfigInfo
+
+
* `<atomic> design <http://libcxx.llvm.org/atomic_design.html>`_
* `<type_traits> design <http://libcxx.llvm.org/type_traits_design.html>`_
* `Status of debug mode <http://libcxx.llvm.org/debug_mode.html>`_