Index: llvm/trunk/CMakeLists.txt
===================================================================
--- llvm/trunk/CMakeLists.txt
+++ llvm/trunk/CMakeLists.txt
@@ -93,6 +93,31 @@
   endif()
 endif()
 
+# Side-by-side subprojects layout: automatically set the
+# LLVM_EXTERNAL_${project}_SOURCE_DIR using LLVM_ALL_PROJECTS
+# This allows an easy way of setting up a build directory for llvm and another
+# one for llvm+clang+... using the same sources.
+set(LLVM_ALL_PROJECTS "clang;libcxx;libcxxabi;lldb;compiler-rt;lld;polly")
+set(LLVM_ENABLE_PROJECTS "" CACHE STRING
+	"Semicolon-separated list of projects to build (${LLVM_ALL_PROJECTS}), or \"all\".")
+if( LLVM_ENABLE_PROJECTS STREQUAL "all" )
+  set( LLVM_ENABLE_PROJECTS ${LLVM_ALL_PROJECTS})
+endif()
+foreach(proj ${LLVM_ENABLE_PROJECTS})
+  set(PROJ_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
+  if(NOT EXISTS "${PROJ_DIR}" OR NOT IS_DIRECTORY "${PROJ_DIR}")
+    message(FATAL_ERROR "LLVM_ENABLE_PROJECTS requests ${proj} but directory not found: ${PROJ_DIR}")
+  endif()
+  string(TOUPPER "${proj}" upper_proj)
+  set(LLVM_EXTERNAL_${upper_proj}_SOURCE_DIR   "${CMAKE_CURRENT_SOURCE_DIR}/../${proj}")
+  # There is a widely spread opinion that clang-tools-extra should be merged
+  # into clang. The following simulates it by always enabling clang-tools-extra
+  # when enabling clang.
+  if (proj STREQUAL "clang")
+    set(LLVM_EXTERNAL_CLANG_TOOLS_EXTRA_SOURCE_DIR "${CMAKE_CURRENT_SOURCE_DIR}/../clang-tools-extra")
+  endif()
+endforeach()
+
 # The following only works with the Ninja generator in CMake >= 3.0.
 set(LLVM_PARALLEL_COMPILE_JOBS "" CACHE STRING
   "Define the maximum number of concurrent compilation jobs.")
Index: llvm/trunk/docs/CMake.rst
===================================================================
--- llvm/trunk/docs/CMake.rst
+++ llvm/trunk/docs/CMake.rst
@@ -336,6 +336,14 @@
   will not be used.  If the variable for an external project does not point
   to a valid path, then that project will not be built.
 
+**LLVM_ENABLE_PROJECTS**:STRING
+  Semicolon-separated list of projects to build, or *all* for building all
+  (clang, libcxx, libcxxabi, lldb, compiler-rt, lld, polly) projects.
+  This flag assumes that projects are checked out side-by-side and not nested,
+  i.e. clang needs to be in parallel of llvm instead of nested in `llvm/tools`.
+  This feature allows to have one build for only LLVM and another for clang+llvm
+  using the same source checkout.
+
 **LLVM_EXTERNAL_PROJECTS**:STRING
   Semicolon-separated list of additional external projects to build as part of
   llvm. For each project LLVM_EXTERNAL_<NAME>_SOURCE_DIR have to be specified