docs/tutorial/LangImpl4.rst - platform/external/llvm - Git at Google

 ==============================================
 Kaleidoscope: Adding JIT and Optimizer Support
 ==============================================

 .. contents::
    :local:

 Chapter 4 Introduction
 ======================

 Welcome to Chapter 4 of the "`Implementing a language with
 LLVM <index.html>`_" tutorial. Chapters 1-3 described the implementation
 of a simple language and added support for generating LLVM IR. This
 chapter describes two new techniques: adding optimizer support to your
 language, and adding JIT compiler support. These additions will
 demonstrate how to get nice, efficient code for the Kaleidoscope
 language.

 Trivial Constant Folding
 ========================

 Our demonstration for Chapter 3 is elegant and easy to extend.
 Unfortunately, it does not produce wonderful code. The IRBuilder,
 however, does give us obvious optimizations when compiling simple code:

 ::

     ready> def test(x) 1+2+x;
     Read function definition:
     define double @test(double %x) {
     entry:
             %addtmp = fadd double 3.000000e+00, %x
             ret double %addtmp
     }

 This code is not a literal transcription of the AST built by parsing the
 input. That would be:

 ::

     ready> def test(x) 1+2+x;
     Read function definition:
     define double @test(double %x) {
     entry:
             %addtmp = fadd double 2.000000e+00, 1.000000e+00
             %addtmp1 = fadd double %addtmp, %x
             ret double %addtmp1
     }

 Constant folding, as seen above, in particular, is a very common and
 very important optimization: so much so that many language implementors
 implement constant folding support in their AST representation.

 With LLVM, you don't need this support in the AST. Since all calls to
 build LLVM IR go through the LLVM IR builder, the builder itself checked
 to see if there was a constant folding opportunity when you call it. If
 so, it just does the constant fold and return the constant instead of
 creating an instruction.

 Well, that was easy :). In practice, we recommend always using
 ``IRBuilder`` when generating code like this. It has no "syntactic
 overhead" for its use (you don't have to uglify your compiler with
 constant checks everywhere) and it can dramatically reduce the amount of
 LLVM IR that is generated in some cases (particular for languages with a
 macro preprocessor or that use a lot of constants).

 On the other hand, the ``IRBuilder`` is limited by the fact that it does
 all of its analysis inline with the code as it is built. If you take a
 slightly more complex example:

 ::

     ready> def test(x) (1+2+x)*(x+(1+2));
     ready> Read function definition:
     define double @test(double %x) {
     entry:
             %addtmp = fadd double 3.000000e+00, %x
             %addtmp1 = fadd double %x, 3.000000e+00
             %multmp = fmul double %addtmp, %addtmp1
             ret double %multmp
     }

 In this case, the LHS and RHS of the multiplication are the same value.
 We'd really like to see this generate "``tmp = x+3; result = tmp*tmp;``"
 instead of computing "``x+3``" twice.

 Unfortunately, no amount of local analysis will be able to detect and
 correct this. This requires two transformations: reassociation of
 expressions (to make the add's lexically identical) and Common
 Subexpression Elimination (CSE) to delete the redundant add instruction.
 Fortunately, LLVM provides a broad range of optimizations that you can
 use, in the form of "passes".

 LLVM Optimization Passes
 ========================

 LLVM provides many optimization passes, which do many different sorts of
 things and have different tradeoffs. Unlike other systems, LLVM doesn't
 hold to the mistaken notion that one set of optimizations is right for
 all languages and for all situations. LLVM allows a compiler implementor
 to make complete decisions about what optimizations to use, in which
 order, and in what situation.

 As a concrete example, LLVM supports both "whole module" passes, which
 look across as large of body of code as they can (often a whole file,
 but if run at link time, this can be a substantial portion of the whole
 program). It also supports and includes "per-function" passes which just
 operate on a single function at a time, without looking at other
 functions. For more information on passes and how they are run, see the
 `How to Write a Pass <../WritingAnLLVMPass.html>`_ document and the
 `List of LLVM Passes <../Passes.html>`_.

 For Kaleidoscope, we are currently generating functions on the fly, one
 at a time, as the user types them in. We aren't shooting for the
 ultimate optimization experience in this setting, but we also want to
 catch the easy and quick stuff where possible. As such, we will choose
 to run a few per-function optimizations as the user types the function
 in. If we wanted to make a "static Kaleidoscope compiler", we would use
 exactly the code we have now, except that we would defer running the
 optimizer until the entire file has been parsed.

 In order to get per-function optimizations going, we need to set up a
 `FunctionPassManager <../WritingAnLLVMPass.html#passmanager>`_ to hold
 and organize the LLVM optimizations that we want to run. Once we have
 that, we can add a set of optimizations to run. The code looks like
 this:

 .. code-block:: c++

       FunctionPassManager OurFPM(TheModule);

       // Set up the optimizer pipeline.  Start with registering info about how the
       // target lays out data structures.
       OurFPM.add(new DataLayout(*TheExecutionEngine->getDataLayout()));
       // Provide basic AliasAnalysis support for GVN.
       OurFPM.add(createBasicAliasAnalysisPass());
       // Do simple "peephole" optimizations and bit-twiddling optzns.
       OurFPM.add(createInstructionCombiningPass());
       // Reassociate expressions.
       OurFPM.add(createReassociatePass());
       // Eliminate Common SubExpressions.
       OurFPM.add(createGVNPass());
       // Simplify the control flow graph (deleting unreachable blocks, etc).
       OurFPM.add(createCFGSimplificationPass());

       OurFPM.doInitialization();

       // Set the global so the code gen can use this.
       TheFPM = &OurFPM;

       // Run the main "interpreter loop" now.
       MainLoop();

 This code defines a ``FunctionPassManager``, "``OurFPM``". It requires a
 pointer to the ``Module`` to construct itself. Once it is set up, we use
 a series of "add" calls to add a bunch of LLVM passes. The first pass is
 basically boilerplate, it adds a pass so that later optimizations know
 how the data structures in the program are laid out. The
 "``TheExecutionEngine``" variable is related to the JIT, which we will
 get to in the next section.

 In this case, we choose to add 4 optimization passes. The passes we
 chose here are a pretty standard set of "cleanup" optimizations that are
 useful for a wide variety of code. I won't delve into what they do but,
 believe me, they are a good starting place :).

 Once the PassManager is set up, we need to make use of it. We do this by
 running it after our newly created function is constructed (in
 ``FunctionAST::Codegen``), but before it is returned to the client:

 .. code-block:: c++

       if (Value *RetVal = Body->Codegen()) {
         // Finish off the function.
         Builder.CreateRet(RetVal);

         // Validate the generated code, checking for consistency.
         verifyFunction(*TheFunction);

         // Optimize the function.
         TheFPM->run(*TheFunction);

         return TheFunction;
       }

 As you can see, this is pretty straightforward. The
 ``FunctionPassManager`` optimizes and updates the LLVM Function\* in
 place, improving (hopefully) its body. With this in place, we can try
 our test above again:

 ::

     ready> def test(x) (1+2+x)*(x+(1+2));
     ready> Read function definition:
     define double @test(double %x) {
     entry:
             %addtmp = fadd double %x, 3.000000e+00
             %multmp = fmul double %addtmp, %addtmp
             ret double %multmp
     }

 As expected, we now get our nicely optimized code, saving a floating
 point add instruction from every execution of this function.

 LLVM provides a wide variety of optimizations that can be used in
 certain circumstances. Some `documentation about the various
 passes <../Passes.html>`_ is available, but it isn't very complete.
 Another good source of ideas can come from looking at the passes that
 ``Clang`` runs to get started. The "``opt``" tool allows you to
 experiment with passes from the command line, so you can see if they do
 anything.

 Now that we have reasonable code coming out of our front-end, lets talk
 about executing it!

 Adding a JIT Compiler
 =====================

 Code that is available in LLVM IR can have a wide variety of tools
 applied to it. For example, you can run optimizations on it (as we did
 above), you can dump it out in textual or binary forms, you can compile
 the code to an assembly file (.s) for some target, or you can JIT
 compile it. The nice thing about the LLVM IR representation is that it
 is the "common currency" between many different parts of the compiler.

 In this section, we'll add JIT compiler support to our interpreter. The
 basic idea that we want for Kaleidoscope is to have the user enter
 function bodies as they do now, but immediately evaluate the top-level
 expressions they type in. For example, if they type in "1 + 2;", we
 should evaluate and print out 3. If they define a function, they should
 be able to call it from the command line.

 In order to do this, we first declare and initialize the JIT. This is
 done by adding a global variable and a call in ``main``:

 .. code-block:: c++

     static ExecutionEngine *TheExecutionEngine;
     ...
     int main() {
       ..
       // Create the JIT.  This takes ownership of the module.
       TheExecutionEngine = EngineBuilder(TheModule).create();
       ..
     }

 This creates an abstract "Execution Engine" which can be either a JIT
 compiler or the LLVM interpreter. LLVM will automatically pick a JIT
 compiler for you if one is available for your platform, otherwise it
 will fall back to the interpreter.

 Once the ``ExecutionEngine`` is created, the JIT is ready to be used.
 There are a variety of APIs that are useful, but the simplest one is the
 "``getPointerToFunction(F)``" method. This method JIT compiles the
 specified LLVM Function and returns a function pointer to the generated
 machine code. In our case, this means that we can change the code that
 parses a top-level expression to look like this:

 .. code-block:: c++

     static void HandleTopLevelExpression() {
       // Evaluate a top-level expression into an anonymous function.
       if (FunctionAST *F = ParseTopLevelExpr()) {
         if (Function *LF = F->Codegen()) {
           LF->dump();  // Dump the function for exposition purposes.

           // JIT the function, returning a function pointer.
           void *FPtr = TheExecutionEngine->getPointerToFunction(LF);

           // Cast it to the right type (takes no arguments, returns a double) so we
           // can call it as a native function.
           double (*FP)() = (double (*)())(intptr_t)FPtr;
           fprintf(stderr, "Evaluated to %f\n", FP());
         }

 Recall that we compile top-level expressions into a self-contained LLVM
 function that takes no arguments and returns the computed double.
 Because the LLVM JIT compiler matches the native platform ABI, this
 means that you can just cast the result pointer to a function pointer of
 that type and call it directly. This means, there is no difference
 between JIT compiled code and native machine code that is statically
 linked into your application.

 With just these two changes, lets see how Kaleidoscope works now!

 ::

     ready> 4+5;
     Read top-level expression:
     define double @0() {
     entry:
       ret double 9.000000e+00
     }

     Evaluated to 9.000000

 Well this looks like it is basically working. The dump of the function
 shows the "no argument function that always returns double" that we
 synthesize for each top-level expression that is typed in. This
 demonstrates very basic functionality, but can we do more?

 ::

     ready> def testfunc(x y) x + y*2;
     Read function definition:
     define double @testfunc(double %x, double %y) {
     entry:
       %multmp = fmul double %y, 2.000000e+00
       %addtmp = fadd double %multmp, %x
       ret double %addtmp
     }

     ready> testfunc(4, 10);
     Read top-level expression:
     define double @1() {
     entry:
       %calltmp = call double @testfunc(double 4.000000e+00, double 1.000000e+01)
       ret double %calltmp
     }

     Evaluated to 24.000000

 This illustrates that we can now call user code, but there is something
 a bit subtle going on here. Note that we only invoke the JIT on the
 anonymous functions that *call testfunc*, but we never invoked it on
 *testfunc* itself. What actually happened here is that the JIT scanned
 for all non-JIT'd functions transitively called from the anonymous
 function and compiled all of them before returning from
 ``getPointerToFunction()``.

 The JIT provides a number of other more advanced interfaces for things
 like freeing allocated machine code, rejit'ing functions to update them,
 etc. However, even with this simple code, we get some surprisingly
 powerful capabilities - check this out (I removed the dump of the
 anonymous functions, you should get the idea by now :) :

 ::

     ready> extern sin(x);
     Read extern:
     declare double @sin(double)

     ready> extern cos(x);
     Read extern:
     declare double @cos(double)

     ready> sin(1.0);
     Read top-level expression:
     define double @2() {
     entry:
       ret double 0x3FEAED548F090CEE
     }

     Evaluated to 0.841471

     ready> def foo(x) sin(x)*sin(x) + cos(x)*cos(x);
     Read function definition:
     define double @foo(double %x) {
     entry:
       %calltmp = call double @sin(double %x)
       %multmp = fmul double %calltmp, %calltmp
       %calltmp2 = call double @cos(double %x)
       %multmp4 = fmul double %calltmp2, %calltmp2
       %addtmp = fadd double %multmp, %multmp4
       ret double %addtmp
     }

     ready> foo(4.0);
     Read top-level expression:
     define double @3() {
     entry:
       %calltmp = call double @foo(double 4.000000e+00)
       ret double %calltmp
     }

     Evaluated to 1.000000

 Whoa, how does the JIT know about sin and cos? The answer is
 surprisingly simple: in this example, the JIT started execution of a
 function and got to a function call. It realized that the function was
 not yet JIT compiled and invoked the standard set of routines to resolve
 the function. In this case, there is no body defined for the function,
 so the JIT ended up calling "``dlsym("sin")``" on the Kaleidoscope
 process itself. Since "``sin``" is defined within the JIT's address
 space, it simply patches up calls in the module to call the libm version
 of ``sin`` directly.

 The LLVM JIT provides a number of interfaces (look in the
 ``ExecutionEngine.h`` file) for controlling how unknown functions get
 resolved. It allows you to establish explicit mappings between IR
 objects and addresses (useful for LLVM global variables that you want to
 map to static tables, for example), allows you to dynamically decide on
 the fly based on the function name, and even allows you to have the JIT
 compile functions lazily the first time they're called.

 One interesting application of this is that we can now extend the
 language by writing arbitrary C++ code to implement operations. For
 example, if we add:

 .. code-block:: c++

     /// putchard - putchar that takes a double and returns 0.
     extern "C"
     double putchard(double X) {
       putchar((char)X);
       return 0;
     }

 Now we can produce simple output to the console by using things like:
 "``extern putchard(x); putchard(120);``", which prints a lowercase 'x'
 on the console (120 is the ASCII code for 'x'). Similar code could be
 used to implement file I/O, console input, and many other capabilities
 in Kaleidoscope.

 This completes the JIT and optimizer chapter of the Kaleidoscope
 tutorial. At this point, we can compile a non-Turing-complete
 programming language, optimize and JIT compile it in a user-driven way.
 Next up we'll look into `extending the language with control flow
 constructs <LangImpl5.html>`_, tackling some interesting LLVM IR issues
 along the way.

 Full Code Listing
 =================

 Here is the complete code listing for our running example, enhanced with
 the LLVM JIT and optimizer. To build this example, use:

 .. code-block:: bash

     # Compile
     clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core jit native` -O3 -o toy
     # Run
     ./toy

 If you are compiling this on Linux, make sure to add the "-rdynamic"
 option as well. This makes sure that the external functions are resolved
 properly at runtime.

 Here is the code:

 .. literalinclude:: ../../examples/Kaleidoscope/Chapter4/toy.cpp
    :language: c++

 `Next: Extending the language: control flow <LangImpl5.html>`_
	==============================================
	Kaleidoscope: Adding JIT and Optimizer Support
	==============================================

	.. contents::
	:local:

	Chapter 4 Introduction
	======================

	Welcome to Chapter 4 of the "`Implementing a language with
	LLVM <index.html>`_" tutorial. Chapters 1-3 described the implementation
	of a simple language and added support for generating LLVM IR. This
	chapter describes two new techniques: adding optimizer support to your
	language, and adding JIT compiler support. These additions will
	demonstrate how to get nice, efficient code for the Kaleidoscope
	language.

	Trivial Constant Folding
	========================

	Our demonstration for Chapter 3 is elegant and easy to extend.
	Unfortunately, it does not produce wonderful code. The IRBuilder,
	however, does give us obvious optimizations when compiling simple code:

	::

	ready> def test(x) 1+2+x;
	Read function definition:
	define double @test(double %x) {
	entry:
	%addtmp = fadd double 3.000000e+00, %x
	ret double %addtmp
	}

	This code is not a literal transcription of the AST built by parsing the
	input. That would be:

	::

	ready> def test(x) 1+2+x;
	Read function definition:
	define double @test(double %x) {
	entry:
	%addtmp = fadd double 2.000000e+00, 1.000000e+00
	%addtmp1 = fadd double %addtmp, %x
	ret double %addtmp1
	}

	Constant folding, as seen above, in particular, is a very common and
	very important optimization: so much so that many language implementors
	implement constant folding support in their AST representation.

	With LLVM, you don't need this support in the AST. Since all calls to
	build LLVM IR go through the LLVM IR builder, the builder itself checked
	to see if there was a constant folding opportunity when you call it. If
	so, it just does the constant fold and return the constant instead of
	creating an instruction.

	Well, that was easy :). In practice, we recommend always using
	``IRBuilder`` when generating code like this. It has no "syntactic
	overhead" for its use (you don't have to uglify your compiler with
	constant checks everywhere) and it can dramatically reduce the amount of
	LLVM IR that is generated in some cases (particular for languages with a
	macro preprocessor or that use a lot of constants).

	On the other hand, the ``IRBuilder`` is limited by the fact that it does
	all of its analysis inline with the code as it is built. If you take a
	slightly more complex example:

	::

	ready> def test(x) (1+2+x)*(x+(1+2));
	ready> Read function definition:
	define double @test(double %x) {
	entry:
	%addtmp = fadd double 3.000000e+00, %x
	%addtmp1 = fadd double %x, 3.000000e+00
	%multmp = fmul double %addtmp, %addtmp1
	ret double %multmp
	}

	In this case, the LHS and RHS of the multiplication are the same value.
	We'd really like to see this generate "``tmp = x+3; result = tmp*tmp;``"
	instead of computing "``x+3``" twice.

	Unfortunately, no amount of local analysis will be able to detect and
	correct this. This requires two transformations: reassociation of
	expressions (to make the add's lexically identical) and Common
	Subexpression Elimination (CSE) to delete the redundant add instruction.
	Fortunately, LLVM provides a broad range of optimizations that you can
	use, in the form of "passes".

	LLVM Optimization Passes
	========================

	LLVM provides many optimization passes, which do many different sorts of
	things and have different tradeoffs. Unlike other systems, LLVM doesn't
	hold to the mistaken notion that one set of optimizations is right for
	all languages and for all situations. LLVM allows a compiler implementor
	to make complete decisions about what optimizations to use, in which
	order, and in what situation.

	As a concrete example, LLVM supports both "whole module" passes, which
	look across as large of body of code as they can (often a whole file,
	but if run at link time, this can be a substantial portion of the whole
	program). It also supports and includes "per-function" passes which just
	operate on a single function at a time, without looking at other
	functions. For more information on passes and how they are run, see the
	`How to Write a Pass <../WritingAnLLVMPass.html>`_ document and the
	`List of LLVM Passes <../Passes.html>`_.

	For Kaleidoscope, we are currently generating functions on the fly, one
	at a time, as the user types them in. We aren't shooting for the
	ultimate optimization experience in this setting, but we also want to
	catch the easy and quick stuff where possible. As such, we will choose
	to run a few per-function optimizations as the user types the function
	in. If we wanted to make a "static Kaleidoscope compiler", we would use
	exactly the code we have now, except that we would defer running the
	optimizer until the entire file has been parsed.

	In order to get per-function optimizations going, we need to set up a
	`FunctionPassManager <../WritingAnLLVMPass.html#passmanager>`_ to hold
	and organize the LLVM optimizations that we want to run. Once we have
	that, we can add a set of optimizations to run. The code looks like
	this:

	.. code-block:: c++

	FunctionPassManager OurFPM(TheModule);

	// Set up the optimizer pipeline. Start with registering info about how the
	// target lays out data structures.
	OurFPM.add(new DataLayout(*TheExecutionEngine->getDataLayout()));
	// Provide basic AliasAnalysis support for GVN.
	OurFPM.add(createBasicAliasAnalysisPass());
	// Do simple "peephole" optimizations and bit-twiddling optzns.
	OurFPM.add(createInstructionCombiningPass());
	// Reassociate expressions.
	OurFPM.add(createReassociatePass());
	// Eliminate Common SubExpressions.
	OurFPM.add(createGVNPass());
	// Simplify the control flow graph (deleting unreachable blocks, etc).
	OurFPM.add(createCFGSimplificationPass());

	OurFPM.doInitialization();

	// Set the global so the code gen can use this.
	TheFPM = &OurFPM;

	// Run the main "interpreter loop" now.
	MainLoop();

	This code defines a ``FunctionPassManager``, "``OurFPM``". It requires a
	pointer to the ``Module`` to construct itself. Once it is set up, we use
	a series of "add" calls to add a bunch of LLVM passes. The first pass is
	basically boilerplate, it adds a pass so that later optimizations know
	how the data structures in the program are laid out. The
	"``TheExecutionEngine``" variable is related to the JIT, which we will
	get to in the next section.

	In this case, we choose to add 4 optimization passes. The passes we
	chose here are a pretty standard set of "cleanup" optimizations that are
	useful for a wide variety of code. I won't delve into what they do but,
	believe me, they are a good starting place :).

	Once the PassManager is set up, we need to make use of it. We do this by
	running it after our newly created function is constructed (in
	``FunctionAST::Codegen``), but before it is returned to the client:

	.. code-block:: c++

	if (Value *RetVal = Body->Codegen()) {
	// Finish off the function.
	Builder.CreateRet(RetVal);

	// Validate the generated code, checking for consistency.
	verifyFunction(*TheFunction);

	// Optimize the function.
	TheFPM->run(*TheFunction);

	return TheFunction;
	}

	As you can see, this is pretty straightforward. The
	``FunctionPassManager`` optimizes and updates the LLVM Function\* in
	place, improving (hopefully) its body. With this in place, we can try
	our test above again:

	::

	ready> def test(x) (1+2+x)*(x+(1+2));
	ready> Read function definition:
	define double @test(double %x) {
	entry:
	%addtmp = fadd double %x, 3.000000e+00
	%multmp = fmul double %addtmp, %addtmp
	ret double %multmp
	}

	As expected, we now get our nicely optimized code, saving a floating
	point add instruction from every execution of this function.

	LLVM provides a wide variety of optimizations that can be used in
	certain circumstances. Some `documentation about the various
	passes <../Passes.html>`_ is available, but it isn't very complete.
	Another good source of ideas can come from looking at the passes that
	``Clang`` runs to get started. The "``opt``" tool allows you to
	experiment with passes from the command line, so you can see if they do
	anything.

	Now that we have reasonable code coming out of our front-end, lets talk
	about executing it!

	Adding a JIT Compiler
	=====================

	Code that is available in LLVM IR can have a wide variety of tools
	applied to it. For example, you can run optimizations on it (as we did
	above), you can dump it out in textual or binary forms, you can compile
	the code to an assembly file (.s) for some target, or you can JIT
	compile it. The nice thing about the LLVM IR representation is that it
	is the "common currency" between many different parts of the compiler.

	In this section, we'll add JIT compiler support to our interpreter. The
	basic idea that we want for Kaleidoscope is to have the user enter
	function bodies as they do now, but immediately evaluate the top-level
	expressions they type in. For example, if they type in "1 + 2;", we
	should evaluate and print out 3. If they define a function, they should
	be able to call it from the command line.

	In order to do this, we first declare and initialize the JIT. This is
	done by adding a global variable and a call in ``main``:

	.. code-block:: c++

	static ExecutionEngine *TheExecutionEngine;
	...
	int main() {
	..
	// Create the JIT. This takes ownership of the module.
	TheExecutionEngine = EngineBuilder(TheModule).create();
	..
	}

	This creates an abstract "Execution Engine" which can be either a JIT
	compiler or the LLVM interpreter. LLVM will automatically pick a JIT
	compiler for you if one is available for your platform, otherwise it
	will fall back to the interpreter.

	Once the ``ExecutionEngine`` is created, the JIT is ready to be used.
	There are a variety of APIs that are useful, but the simplest one is the
	"``getPointerToFunction(F)``" method. This method JIT compiles the
	specified LLVM Function and returns a function pointer to the generated
	machine code. In our case, this means that we can change the code that
	parses a top-level expression to look like this:

	.. code-block:: c++

	static void HandleTopLevelExpression() {
	// Evaluate a top-level expression into an anonymous function.
	if (FunctionAST *F = ParseTopLevelExpr()) {
	if (Function *LF = F->Codegen()) {
	LF->dump(); // Dump the function for exposition purposes.

	// JIT the function, returning a function pointer.
	void *FPtr = TheExecutionEngine->getPointerToFunction(LF);

	// Cast it to the right type (takes no arguments, returns a double) so we
	// can call it as a native function.
	double (FP)() = (double ()())(intptr_t)FPtr;
	fprintf(stderr, "Evaluated to %f\n", FP());
	}

	Recall that we compile top-level expressions into a self-contained LLVM
	function that takes no arguments and returns the computed double.
	Because the LLVM JIT compiler matches the native platform ABI, this
	means that you can just cast the result pointer to a function pointer of
	that type and call it directly. This means, there is no difference
	between JIT compiled code and native machine code that is statically
	linked into your application.

	With just these two changes, lets see how Kaleidoscope works now!

	::

	ready> 4+5;
	Read top-level expression:
	define double @0() {
	entry:
	ret double 9.000000e+00
	}

	Evaluated to 9.000000

	Well this looks like it is basically working. The dump of the function
	shows the "no argument function that always returns double" that we
	synthesize for each top-level expression that is typed in. This
	demonstrates very basic functionality, but can we do more?

	::

	ready> def testfunc(x y) x + y*2;
	Read function definition:
	define double @testfunc(double %x, double %y) {
	entry:
	%multmp = fmul double %y, 2.000000e+00
	%addtmp = fadd double %multmp, %x
	ret double %addtmp
	}

	ready> testfunc(4, 10);
	Read top-level expression:
	define double @1() {
	entry:
	%calltmp = call double @testfunc(double 4.000000e+00, double 1.000000e+01)
	ret double %calltmp
	}

	Evaluated to 24.000000

	This illustrates that we can now call user code, but there is something
	a bit subtle going on here. Note that we only invoke the JIT on the
	anonymous functions that call testfunc, but we never invoked it on
	testfunc itself. What actually happened here is that the JIT scanned
	for all non-JIT'd functions transitively called from the anonymous
	function and compiled all of them before returning from
	``getPointerToFunction()``.

	The JIT provides a number of other more advanced interfaces for things
	like freeing allocated machine code, rejit'ing functions to update them,
	etc. However, even with this simple code, we get some surprisingly
	powerful capabilities - check this out (I removed the dump of the
	anonymous functions, you should get the idea by now :) :

	::

	ready> extern sin(x);
	Read extern:
	declare double @sin(double)

	ready> extern cos(x);
	Read extern:
	declare double @cos(double)

	ready> sin(1.0);
	Read top-level expression:
	define double @2() {
	entry:
	ret double 0x3FEAED548F090CEE
	}

	Evaluated to 0.841471

	ready> def foo(x) sin(x)sin(x) + cos(x)cos(x);
	Read function definition:
	define double @foo(double %x) {
	entry:
	%calltmp = call double @sin(double %x)
	%multmp = fmul double %calltmp, %calltmp
	%calltmp2 = call double @cos(double %x)
	%multmp4 = fmul double %calltmp2, %calltmp2
	%addtmp = fadd double %multmp, %multmp4
	ret double %addtmp
	}

	ready> foo(4.0);
	Read top-level expression:
	define double @3() {
	entry:
	%calltmp = call double @foo(double 4.000000e+00)
	ret double %calltmp
	}

	Evaluated to 1.000000

	Whoa, how does the JIT know about sin and cos? The answer is
	surprisingly simple: in this example, the JIT started execution of a
	function and got to a function call. It realized that the function was
	not yet JIT compiled and invoked the standard set of routines to resolve
	the function. In this case, there is no body defined for the function,
	so the JIT ended up calling "``dlsym("sin")``" on the Kaleidoscope
	process itself. Since "``sin``" is defined within the JIT's address
	space, it simply patches up calls in the module to call the libm version
	of ``sin`` directly.

	The LLVM JIT provides a number of interfaces (look in the
	``ExecutionEngine.h`` file) for controlling how unknown functions get
	resolved. It allows you to establish explicit mappings between IR
	objects and addresses (useful for LLVM global variables that you want to
	map to static tables, for example), allows you to dynamically decide on
	the fly based on the function name, and even allows you to have the JIT
	compile functions lazily the first time they're called.

	One interesting application of this is that we can now extend the
	language by writing arbitrary C++ code to implement operations. For
	example, if we add:

	.. code-block:: c++

	/// putchard - putchar that takes a double and returns 0.
	extern "C"
	double putchard(double X) {
	putchar((char)X);
	return 0;
	}

	Now we can produce simple output to the console by using things like:
	"``extern putchard(x); putchard(120);``", which prints a lowercase 'x'
	on the console (120 is the ASCII code for 'x'). Similar code could be
	used to implement file I/O, console input, and many other capabilities
	in Kaleidoscope.

	This completes the JIT and optimizer chapter of the Kaleidoscope
	tutorial. At this point, we can compile a non-Turing-complete
	programming language, optimize and JIT compile it in a user-driven way.
	Next up we'll look into `extending the language with control flow
	constructs <LangImpl5.html>`_, tackling some interesting LLVM IR issues
	along the way.

	Full Code Listing
	=================

	Here is the complete code listing for our running example, enhanced with
	the LLVM JIT and optimizer. To build this example, use:

	.. code-block:: bash

	# Compile
	clang++ -g toy.cpp `llvm-config --cxxflags --ldflags --system-libs --libs core jit native` -O3 -o toy
	# Run
	./toy

	If you are compiling this on Linux, make sure to add the "-rdynamic"
	option as well. This makes sure that the external functions are resolved
	properly at runtime.

	Here is the code:

	.. literalinclude:: ../../examples/Kaleidoscope/Chapter4/toy.cpp
	:language: c++

	`Next: Extending the language: control flow <LangImpl5.html>`_