Document the usage of BOOTSTRAP_XXX with stage2 builds
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@349299 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/docs/AdvancedBuilds.rst b/docs/AdvancedBuilds.rst
index d2a2ef5..8fb1eb5 100644
--- a/docs/AdvancedBuilds.rst
+++ b/docs/AdvancedBuilds.rst
@@ -51,6 +51,15 @@
$ cmake -G Ninja -DCLANG_ENABLE_BOOTSTRAP=On -DCLANG_BOOTSTRAP_PASSTHROUGH="CMAKE_INSTALL_PREFIX;CMAKE_VERBOSE_MAKEFILE" <path to source>
$ ninja stage2
+CMake options starting by BOOTSTRAP_ will be passed only to the stage2 build.
+This gives the opportunity to use Clang specific build flags.
+For example, the following CMake call will enabled '-fno-addrsig' only during
+the stage2 build for C and C++.
+
+.. code-block:: console
+
+ $ cmake [..] -DBOOTSTRAP_CMAKE_CXX_FLAGS='-fno-addrsig' -DBOOTSTRAP_CMAKE_C_FLAGS='-fno-addrsig' [..]
+
The clang build system refers to builds as stages. A stage1 build is a standard
build using the compiler installed on the host, and a stage2 build is built
using the stage1 compiler. This nomenclature holds up to more stages too. In
