[Kaleidoscope][BuildingAJIT] Add a stub Chapter 2 doc.

llvm-svn: 270809

llvm/docs/tutorial/BuildingAJIT2.rst

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+====================================================================
+Building a JIT: Adding Optimizations - An introduction to ORC Layers
+====================================================================
+
+.. contents::
+   :local:
+
+**This tutorial is under active development. It is incomplete and details may
+change frequently.** Nonetheless we invite you to try it out as it stands, and
+we welcome any feedback.
+
+Chapter 2 Introduction
+======================
+
+Welcome to Chapter 2 of the "Building an ORC-based JIT in LLVM" tutorial. This
+chapter shows you how to add IR optimization support to the KaleidoscopeJIT
+class that was introduced in `Chapter 1 <BuildingAJIT1.html>`_ by adding a
+new *ORC Layer* -- IRTransformLayer.
+
+**To be done:**
+
+**(1) Briefly describe FunctionPassManager and the optimizeModule
+method (reference the Kaleidoscope language tutorial chapter 4 for more detail
+about IR optimization - it's covered in detail there, here it just provides a
+motivation for learning about layers).**
+
+**(2) Describe IRTransformLayer, show how it is used to call our optimizeModule
+method.**
+
+**(3) Describe the ORC Layer concept using IRTransformLayer as an example.**
+
+Full Code Listing
+=================
+
+Here is the complete code listing for our running example with an
+IRTransformLayer added to enable optimization. To build this example, use:
+
+.. code-block:: bash
+
+    # Compile
+    clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core orc native` -O3 -o toy
+    # Run
+    ./toy
+
+Here is the code:
+
+.. literalinclude:: ../../examples/Kaleidoscope/BuildingAJIT/Chapter2/KaleidoscopeJIT.h
+   :language: c++
+
+`Next: Adding Per-function Lazy Compilation <BuildingAJIT3.html>`_