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Add some user facing documentation on ThinLTO and how to use it.



Index: docs/index.rst
--- docs/index.rst
+++ docs/index.rst
@@ -37,6 +37,7 @@
+   ThinLTO
Index: docs/ThinLTO.rst
--- /dev/null
+++ docs/ThinLTO.rst
@@ -0,0 +1,146 @@
+.. contents::
+   :local:
+*ThinLTO* compilation is a new type of LTO that is both scalable and
+incremental. *LTO* (Link Time Optimization) achieves better
+runtime performance through whole-program analysis and cross-module
+optimization. However, regular LTO implements this by merging all
+input into a single monolithic module, which is not scalable
+in time or memory, and also prevents fast incremental compiles.
+In ThinLTO mode, as with regular LTO, clang emits LLVM bitcode after the
+compile phase. The ThinLTO bitcode is augmented with a compact summary
+of the module. During the link step, only the summaries are read and
+merged into a combined summary index, which includes an index of function
+locations for later cross-module function importing. Fast and efficient
+whole-program analysis is then performed on the combined summary index.
+However, all transformations, including function importing, occur
+later when the modules are optimized in fully parallel backends.
+By default, linkers_ that support ThinLTO are set up to launch
+the ThinLTO backends in threads. So the usage model is not affected
+as the distinction between the fast serial thin link step and the backends
+is transparent to the user.
+For more information on the ThinLTO design and current performance,
+see the LLVM blog post `ThinLTO: Scalable and Incremental LTO
+While tuning is still in progress, results in the blog post show that
+ThinLTO already performs well compared to LTO, in many cases matching
+the performance improvement.
+Current Status
+.. _compiler:
+The 3.9 release of clang includes ThinLTO support. However, ThinLTO
+is under active development, and new features, improvements and bugfixes
+are being added for the next release. For the latest ThinLTO support,
+`build a recent version of clang and LLVM
+.. _linkers:
+.. _linker:
+ThinLTO is currently supported for the following linkers:
+- **gold (via the gold-plugin)**:
+  Similar to regular LTO, this requires using
+  a `gold linker configured with plugins enabled
+  <http://llvm.org/docs/GoldPlugin.html>`_.
+- **ld64**:
+  Starting with `Xcode 8 <https://developer.apple.com/xcode/>`_.
+Additionally, support is being added to the *lld* linker.
+To utilize ThinLTO, simply add the -flto=thin option to compile and link. E.g.
+.. code-block:: console
+  % clang -flto=thin -O2 file1.c file2.c -c
+  % clang -flto=thin -O2 file1.o file2.o -o a.out
+As mentioned earlier, by default the linkers will launch the ThinLTO backend
+threads in parallel, passing the resulting native object files back to the
+linker for the final native link.  As such, the usage model the same as
+Controlling Backend Parallelism
+.. _parallelism:
+By default, the ThinLTO link step will launch up to
+``std::thread::hardware_concurrency`` number of threads in parallel.
+For machines with hyper-threading, this is the total number of
+virtual cores. For some applications and machine configurations this
+may be too aggressive, in which case the amount of parallelism can
+be reduced to ``N`` via:
+- gold:
+  ``-Wl,-plugin-opt,jobs=N``
+- ld64:
+  ``-Wl,-mllvm,-threads=N``
+.. _incremental:
+ThinLTO supports fast incremental builds through the use of a cache,
+which currently must be enabled through a linker option.
+- gold (as of LLVM r279883):
+  ``-Wl,-plugin-opt,cache-dir=/path/to/cache``
+- ld64 (support in clang 3.9 and Xcode 8):
+  ``-Wl,-cache_path_lto,/path/to/cache``
+Clang Bootstrap
+To bootstrap clang/LLVM with ThinLTO, follow these steps:
+1. The host compiler_ must be a version of clang that supports ThinLTO.
+#. The host linker_ must support ThinLTO (and in the case of gold, must be
+   `configured with plugins enabled <http://llvm.org/docs/GoldPlugin.html>`_.
+#. Use the following additional `CMake variables
+   <http://llvm.org/docs/CMake.html#options-and-variables>`_
+   when configuring the bootstrap compiler build:
+  * ``-DLLVM_ENABLE_LTO=Thin``
+    (since the ThinLTO link invokes parallel backend jobs)
+  * ``-DCMAKE_C_COMPILER=/path/to/host/clang``
+  * ``-DCMAKE_CXX_COMPILER=/path/to/host/clang++``
+  * ``-DCMAKE_RANLIB=/path/to/host/llvm-ranlib``
+  * ``-DCMAKE_AR=/path/to/host/llvm-nm``
+#. To use additional linker arguments for controlling the backend
+   parallelism_ or enabling incremental_ builds of the bootstrap compiler,
+   after configuring the build, modify the resulting CMakeCache.txt file in the
+   build directory. Specify any additional linker options after
+   ``CMAKE_EXE_LINKER_FLAGS:STRING=``. Note the configure may fail if
+   linker plugin options are instead specified directly in the previous step.
+More Information
+* From LLVM project blog:
+  `ThinLTO: Scalable and Incremental LTO
+  <http://blog.llvm.org/2016/06/thinlto-scalable-and-incremental-lto.html>`_
Index: docs/CommandGuide/clang.rst
--- docs/CommandGuide/clang.rst
+++ docs/CommandGuide/clang.rst
@@ -328,13 +328,19 @@
   model can be overridden with the tls_model attribute. The compiler will try
   to choose a more efficient model if possible.
-.. option:: -flto, -emit-llvm
+.. option:: -flto[=full,thin], -emit-llvm
   Generate output files in LLVM formats, suitable for link time optimization.
   When used with :option:`-S` this generates LLVM intermediate language
   assembly files, otherwise this generates LLVM bitcode format object files
   (which may be passed to the linker depending on the stage selection options).
+  The default for :option:`-flto` is :option:`-flto=full`, in which the
+  LLVM bitcode is suitable for regular Link Time Optimization (LTO), where
+  the linker merges all such modules into a single combined module for
+  optimization. With :option:`-flto=thin`, :doc:`ThinLTO <../ThinLTO>`
+  compilation is invoked instead.
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