[Kaleidoscope][BuildingAJIT] Add stub chapter text for Building A JIT Chapter 4.

llvm-svn: 271064

llvm/docs/tutorial/BuildingAJIT4.rst

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+===========================================================================
+Building a JIT: Extreme Laziness - Using Compile Callbacks to JIT from ASTs
+===========================================================================
+
+.. 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 4 Introduction
+======================
+
+Welcome to Chapter 4 of the "Building an ORC-based JIT in LLVM" tutorial. This
+chapter introduces the Compile Callbacks and Indirect Stubs APIs and shows how
+they can be used to replace the CompileOnDemand layer from
+`Chapter 3 <BuildingAJIT3.html>`_ with a custom lazy-JITing scheme that JITs
+directly from Kaleidoscope ASTs.
+
+**To be done:**
+
+**(1) Describe the drawbacks of JITing from IR (have to compile to IR first,
+which reduces the benefits of laziness).**
+
+**(2) Describe CompileCallbackManagers and IndirectStubManagers in detail.**
+
+**(3) Run through the implementation of addFunctionAST.**
+
+Full Code Listing
+=================
+
+Here is the complete code listing for our running example that JITs lazily from
+Kaleidoscope ASTS. 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/Chapter4/KaleidoscopeJIT.h
+   :language: c++
+
+`Next: Remote-JITing -- Process-isolation and laziness-at-a-distance <BuildingAJIT5.html>`_