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 <h1>
   Extending LLVM: Adding instructions, intrinsics, types, etc.
 </h1>

 <ol>
   <li><a href="#introduction">Introduction and Warning</a></li>
   <li><a href="#intrinsic">Adding a new intrinsic function</a></li>
   <li><a href="#instruction">Adding a new instruction</a></li>
   <li><a href="#sdnode">Adding a new SelectionDAG node</a></li>
   <li><a href="#type">Adding a new type</a>
   <ol>
     <li><a href="#fund_type">Adding a new fundamental type</a></li>
     <li><a href="#derived_type">Adding a new derived type</a></li>
   </ol></li>
 </ol>

 <div class="doc_author">
   <p>Written by <a href="http://misha.brukman.net">Misha Brukman</a>,
   Brad Jones, Nate Begeman,
   and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
 </div>

 <!-- *********************************************************************** -->
 <h2>
   <a name="introduction">Introduction and Warning</a>
 </h2>
 <!-- *********************************************************************** -->

 <div>

 <p>During the course of using LLVM, you may wish to customize it for your
 research project or for experimentation. At this point, you may realize that
 you need to add something to LLVM, whether it be a new fundamental type, a new
 intrinsic function, or a whole new instruction.</p>

 <p>When you come to this realization, stop and think. Do you really need to
 extend LLVM? Is it a new fundamental capability that LLVM does not support at
 its current incarnation or can it be synthesized from already pre-existing LLVM
 elements? If you are not sure, ask on the <a
 href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a> list. The
 reason is that extending LLVM will get involved as you need to update all the
 different passes that you intend to use with your extension, and there are
 <em>many</em> LLVM analyses and transformations, so it may be quite a bit of
 work.</p>

 <p>Adding an <a href="#intrinsic">intrinsic function</a> is far easier than
 adding an instruction, and is transparent to optimization passes.  If your added
 functionality can be expressed as a
 function call, an intrinsic function is the method of choice for LLVM
 extension.</p>

 <p>Before you invest a significant amount of effort into a non-trivial
 extension, <span class="doc_warning">ask on the list</span> if what you are
 looking to do can be done with already-existing infrastructure, or if maybe
 someone else is already working on it. You will save yourself a lot of time and
 effort by doing so.</p>

 </div>

 <!-- *********************************************************************** -->
 <h2>
   <a name="intrinsic">Adding a new intrinsic function</a>
 </h2>
 <!-- *********************************************************************** -->

 <div>

 <p>Adding a new intrinsic function to LLVM is much easier than adding a new
 instruction.  Almost all extensions to LLVM should start as an intrinsic
 function and then be turned into an instruction if warranted.</p>

 <ol>
 <li><tt>llvm/docs/LangRef.html</tt>:
     Document the intrinsic.  Decide whether it is code generator specific and
     what the restrictions are.  Talk to other people about it so that you are
     sure it's a good idea.</li>

 <li><tt>llvm/include/llvm/Intrinsics*.td</tt>:
     Add an entry for your intrinsic.  Describe its memory access characteristics
     for optimization (this controls whether it will be DCE'd, CSE'd, etc). Note
     that any intrinsic using the <tt>llvm_int_ty</tt> type for an argument will
     be deemed by <tt>tblgen</tt> as overloaded and the corresponding suffix
     will be required on the intrinsic's name.</li>

 <li><tt>llvm/lib/Analysis/ConstantFolding.cpp</tt>: If it is possible to
     constant fold your intrinsic, add support to it in the
     <tt>canConstantFoldCallTo</tt> and <tt>ConstantFoldCall</tt> functions.</li>

 <li><tt>llvm/test/Regression/*</tt>: Add test cases for your test cases to the
     test suite</li>
 </ol>

 <p>Once the intrinsic has been added to the system, you must add code generator
 support for it.  Generally you must do the following steps:</p>

 <dl>
 <dt>Add support to the C backend in <tt>lib/Target/CBackend/</tt></dt>

 <dd>Depending on the intrinsic, there are a few ways to implement this.  For
     most intrinsics, it makes sense to add code to lower your intrinsic in
     <tt>LowerIntrinsicCall</tt> in <tt>lib/CodeGen/IntrinsicLowering.cpp</tt>.
     Second, if it makes sense to lower the intrinsic to an expanded sequence of
     C code in all cases, just emit the expansion in <tt>visitCallInst</tt> in
     <tt>Writer.cpp</tt>.  If the intrinsic has some way to express it with GCC
     (or any other compiler) extensions, it can be conditionally supported based
     on the compiler compiling the CBE output (see <tt>llvm.prefetch</tt> for an
     example).  Third, if the intrinsic really has no way to be lowered, just
     have the code generator emit code that prints an error message and calls
     abort if executed.</dd>

 <dt>Add support to the .td file for the target(s) of your choice in
    <tt>lib/Target/*/*.td</tt>.</dt>

 <dd>This is usually a matter of adding a pattern to the .td file that matches
     the intrinsic, though it may obviously require adding the instructions you
     want to generate as well.  There are lots of examples in the PowerPC and X86
     backend to follow.</dd>
 </dl>

 </div>

 <!-- *********************************************************************** -->
 <h2>
   <a name="sdnode">Adding a new SelectionDAG node</a>
 </h2>
 <!-- *********************************************************************** -->

 <div>

 <p>As with intrinsics, adding a new SelectionDAG node to LLVM is much easier
 than adding a new instruction.  New nodes are often added to help represent
 instructions common to many targets.  These nodes often map to an LLVM
 instruction (add, sub) or intrinsic (byteswap, population count).  In other
 cases, new nodes have been added to allow many targets to perform a common task
 (converting between floating point and integer representation) or capture more
 complicated behavior in a single node (rotate).</p>

 <ol>
 <li><tt>include/llvm/CodeGen/ISDOpcodes.h</tt>:
     Add an enum value for the new SelectionDAG node.</li>
 <li><tt>lib/CodeGen/SelectionDAG/SelectionDAG.cpp</tt>:
     Add code to print the node to <tt>getOperationName</tt>.  If your new node
     can be evaluated at compile time when given constant arguments (such as an
     add of a constant with another constant), find the <tt>getNode</tt> method
     that takes the appropriate number of arguments, and add a case for your node
     to the switch statement that performs constant folding for nodes that take
     the same number of arguments as your new node.</li>
 <li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
     Add code to <a href="CodeGenerator.html#selectiondag_legalize">legalize,
     promote, and expand</a> the node as necessary.  At a minimum, you will need
     to add a case statement for your node in <tt>LegalizeOp</tt> which calls
     LegalizeOp on the node's operands, and returns a new node if any of the
     operands changed as a result of being legalized.  It is likely that not all
     targets supported by the SelectionDAG framework will natively support the
     new node.  In this case, you must also add code in your node's case
     statement in <tt>LegalizeOp</tt> to Expand your node into simpler, legal
     operations.  The case for <tt>ISD::UREM</tt> for expanding a remainder into
     a divide, multiply, and a subtract is a good example.</li>
 <li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
     If targets may support the new node being added only at certain sizes, you
     will also need to add code to your node's case statement in
     <tt>LegalizeOp</tt> to Promote your node's operands to a larger size, and
     perform the correct operation.  You will also need to add code to
     <tt>PromoteOp</tt> to do this as well.  For a good example, see
     <tt>ISD::BSWAP</tt>,
     which promotes its operand to a wider size, performs the byteswap, and then
     shifts the correct bytes right to emulate the narrower byteswap in the
     wider type.</li>
 <li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
     Add a case for your node in <tt>ExpandOp</tt> to teach the legalizer how to
     perform the action represented by the new node on a value that has been
     split into high and low halves.  This case will be used to support your
     node with a 64 bit operand on a 32 bit target.</li>
 <li><tt>lib/CodeGen/SelectionDAG/DAGCombiner.cpp</tt>:
     If your node can be combined with itself, or other existing nodes in a
     peephole-like fashion, add a visit function for it, and call that function
     from <tt></tt>.  There are several good examples for simple combines you
     can do; <tt>visitFABS</tt> and <tt>visitSRL</tt> are good starting places.
     </li>
 <li><tt>lib/Target/PowerPC/PPCISelLowering.cpp</tt>:
     Each target has an implementation of the <tt>TargetLowering</tt> class,
     usually in its own file (although some targets include it in the same
     file as the DAGToDAGISel).  The default behavior for a target is to
     assume that your new node is legal for all types that are legal for
     that target.  If this target does not natively support your node, then
     tell the target to either Promote it (if it is supported at a larger
     type) or Expand it.  This will cause the code you wrote in
     <tt>LegalizeOp</tt> above to decompose your new node into other legal
     nodes for this target.</li>
 <li><tt>lib/Target/TargetSelectionDAG.td</tt>:
     Most current targets supported by LLVM generate code using the DAGToDAG
     method, where SelectionDAG nodes are pattern matched to target-specific
     nodes, which represent individual instructions.  In order for the targets
     to match an instruction to your new node, you must add a def for that node
     to the list in this file, with the appropriate type constraints. Look at
     <tt>add</tt>, <tt>bswap</tt>, and <tt>fadd</tt> for examples.</li>
 <li><tt>lib/Target/PowerPC/PPCInstrInfo.td</tt>:
     Each target has a tablegen file that describes the target's instruction
     set.  For targets that use the DAGToDAG instruction selection framework,
     add a pattern for your new node that uses one or more target nodes.
     Documentation for this is a bit sparse right now, but there are several
     decent examples.  See the patterns for <tt>rotl</tt> in
     <tt>PPCInstrInfo.td</tt>.</li>
 <li>TODO: document complex patterns.</li>
 <li><tt>llvm/test/Regression/CodeGen/*</tt>: Add test cases for your new node
     to the test suite.  <tt>llvm/test/Regression/CodeGen/X86/bswap.ll</tt> is
     a good example.</li>
 </ol>

 </div>

 <!-- *********************************************************************** -->
 <h2>
   <a name="instruction">Adding a new instruction</a>
 </h2>
 <!-- *********************************************************************** -->

 <div>

 <p><span class="doc_warning">WARNING: adding instructions changes the bitcode
 format, and it will take some effort to maintain compatibility with
 the previous version.</span> Only add an instruction if it is absolutely
 necessary.</p>

 <ol>

 <li><tt>llvm/include/llvm/Instruction.def</tt>:
     add a number for your instruction and an enum name</li>

 <li><tt>llvm/include/llvm/Instructions.h</tt>:
     add a definition for the class that will represent your instruction</li>

 <li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
     add a prototype for a visitor to your new instruction type</li>

 <li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
     add a new token to parse your instruction from assembly text file</li>

 <li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
     add the grammar on how your instruction can be read and what it will
     construct as a result</li>

 <li><tt>llvm/lib/Bitcode/Reader/Reader.cpp</tt>:
     add a case for your instruction and how it will be parsed from bitcode</li>

 <li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
     add a case for how your instruction will be printed out to assembly</li>

 <li><tt>llvm/lib/VMCore/Instructions.cpp</tt>:
     implement the class you defined in
     <tt>llvm/include/llvm/Instructions.h</tt></li>

 <li>Test your instruction</li>

 <li><tt>llvm/lib/Target/*</tt>:
     Add support for your instruction to code generators, or add a lowering
     pass.</li>

 <li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>

 </ol>

 <p>Also, you need to implement (or modify) any analyses or passes that you want
 to understand this new instruction.</p>

 </div>


 <!-- *********************************************************************** -->
 <h2>
   <a name="type">Adding a new type</a>
 </h2>
 <!-- *********************************************************************** -->

 <div>

 <p><span class="doc_warning">WARNING: adding new types changes the bitcode
 format, and will break compatibility with currently-existing LLVM
 installations.</span> Only add new types if it is absolutely necessary.</p>

 <!-- ======================================================================= -->
 <h3>
   <a name="fund_type">Adding a fundamental type</a>
 </h3>

 <div>

 <ol>

 <li><tt>llvm/include/llvm/Type.h</tt>:
     add enum for the new type; add static <tt>Type*</tt> for this type</li>

 <li><tt>llvm/lib/VMCore/Type.cpp</tt>:
     add mapping from <tt>TypeID</tt> =&gt; <tt>Type*</tt>;
     initialize the static <tt>Type*</tt></li>

 <li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
     add ability to parse in the type from text assembly</li>

 <li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
     add a token for that type</li>

 </ol>

 </div>

 <!-- ======================================================================= -->
 <h3>
   <a name="derived_type">Adding a derived type</a>
 </h3>

 <div>

 <ol>
 <li><tt>llvm/include/llvm/Type.h</tt>:
     add enum for the new type; add a forward declaration of the type
     also</li>

 <li><tt>llvm/include/llvm/DerivedTypes.h</tt>:
     add new class to represent new class in the hierarchy; add forward
     declaration to the TypeMap value type</li>

 <li><tt>llvm/lib/VMCore/Type.cpp</tt>:
     add support for derived type to:
 <div class="doc_code">
 <pre>
 std::string getTypeDescription(const Type &amp;Ty,
   std::vector&lt;const Type*&gt; &amp;TypeStack)
 bool TypesEqual(const Type *Ty, const Type *Ty2,
   std::map&lt;const Type*, const Type*&gt; &amp; EqTypes)
 </pre>
 </div>
     add necessary member functions for type, and factory methods</li>

 <li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
     add ability to parse in the type from text assembly</li>

 <li><tt>llvm/lib/BitCode/Writer/Writer.cpp</tt>:
     modify <tt>void BitcodeWriter::outputType(const Type *T)</tt> to serialize
     your type</li>

 <li><tt>llvm/lib/BitCode/Reader/Reader.cpp</tt>:
     modify <tt>const Type *BitcodeReader::ParseType()</tt> to read your data
     type</li>

 <li><tt>llvm/lib/VMCore/AsmWriter.cpp</tt>:
     modify
 <div class="doc_code">
 <pre>
 void calcTypeName(const Type *Ty,
                   std::vector&lt;const Type*&gt; &amp;TypeStack,
                   std::map&lt;const Type*,std::string&gt; &amp;TypeNames,
                   std::string &amp; Result)
 </pre>
 </div>
     to output the new derived type
 </li>


 </ol>

 </div>

 </div>

 <!-- *********************************************************************** -->

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	"http://www.w3.org/TR/html4/strict.dtd">
	<html>
	<head>
	<title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
	<link rel="stylesheet" href="llvm.css" type="text/css">
	</head>

	<body>

	<h1>
	Extending LLVM: Adding instructions, intrinsics, types, etc.
	</h1>

	<ol>
	<li><a href="#introduction">Introduction and Warning</a></li>
	<li><a href="#intrinsic">Adding a new intrinsic function</a></li>
	<li><a href="#instruction">Adding a new instruction</a></li>
	<li><a href="#sdnode">Adding a new SelectionDAG node</a></li>
	<li><a href="#type">Adding a new type</a>
	<ol>
	<li><a href="#fund_type">Adding a new fundamental type</a></li>
	<li><a href="#derived_type">Adding a new derived type</a></li>
	</ol></li>
	</ol>

	<div class="doc_author">
	<p>Written by <a href="http://misha.brukman.net">Misha Brukman</a>,
	Brad Jones, Nate Begeman,
	and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
	</div>

	<!-- *********************************************************************** -->
	<h2>
	<a name="introduction">Introduction and Warning</a>
	</h2>
	<!-- *********************************************************************** -->

	<div>

	<p>During the course of using LLVM, you may wish to customize it for your
	research project or for experimentation. At this point, you may realize that
	you need to add something to LLVM, whether it be a new fundamental type, a new
	intrinsic function, or a whole new instruction.</p>

	<p>When you come to this realization, stop and think. Do you really need to
	extend LLVM? Is it a new fundamental capability that LLVM does not support at
	its current incarnation or can it be synthesized from already pre-existing LLVM
	elements? If you are not sure, ask on the <a
	href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a> list. The
	reason is that extending LLVM will get involved as you need to update all the
	different passes that you intend to use with your extension, and there are
	<em>many</em> LLVM analyses and transformations, so it may be quite a bit of
	work.</p>

	<p>Adding an <a href="#intrinsic">intrinsic function</a> is far easier than
	adding an instruction, and is transparent to optimization passes. If your added
	functionality can be expressed as a
	function call, an intrinsic function is the method of choice for LLVM
	extension.</p>

	<p>Before you invest a significant amount of effort into a non-trivial
	extension, <span class="doc_warning">ask on the list</span> if what you are
	looking to do can be done with already-existing infrastructure, or if maybe
	someone else is already working on it. You will save yourself a lot of time and
	effort by doing so.</p>

	</div>

	<!-- *********************************************************************** -->
	<h2>
	<a name="intrinsic">Adding a new intrinsic function</a>
	</h2>
	<!-- *********************************************************************** -->

	<div>

	<p>Adding a new intrinsic function to LLVM is much easier than adding a new
	instruction. Almost all extensions to LLVM should start as an intrinsic
	function and then be turned into an instruction if warranted.</p>

	<ol>
	<li><tt>llvm/docs/LangRef.html</tt>:
	Document the intrinsic. Decide whether it is code generator specific and
	what the restrictions are. Talk to other people about it so that you are
	sure it's a good idea.</li>

	<li><tt>llvm/include/llvm/Intrinsics*.td</tt>:
	Add an entry for your intrinsic. Describe its memory access characteristics
	for optimization (this controls whether it will be DCE'd, CSE'd, etc). Note
	that any intrinsic using the <tt>llvm_int_ty</tt> type for an argument will
	be deemed by <tt>tblgen</tt> as overloaded and the corresponding suffix
	will be required on the intrinsic's name.</li>

	<li><tt>llvm/lib/Analysis/ConstantFolding.cpp</tt>: If it is possible to
	constant fold your intrinsic, add support to it in the
	<tt>canConstantFoldCallTo</tt> and <tt>ConstantFoldCall</tt> functions.</li>

	<li><tt>llvm/test/Regression/*</tt>: Add test cases for your test cases to the
	test suite</li>
	</ol>

	<p>Once the intrinsic has been added to the system, you must add code generator
	support for it. Generally you must do the following steps:</p>

	<dl>
	<dt>Add support to the C backend in <tt>lib/Target/CBackend/</tt></dt>

	<dd>Depending on the intrinsic, there are a few ways to implement this. For
	most intrinsics, it makes sense to add code to lower your intrinsic in
	<tt>LowerIntrinsicCall</tt> in <tt>lib/CodeGen/IntrinsicLowering.cpp</tt>.
	Second, if it makes sense to lower the intrinsic to an expanded sequence of
	C code in all cases, just emit the expansion in <tt>visitCallInst</tt> in
	<tt>Writer.cpp</tt>. If the intrinsic has some way to express it with GCC
	(or any other compiler) extensions, it can be conditionally supported based
	on the compiler compiling the CBE output (see <tt>llvm.prefetch</tt> for an
	example). Third, if the intrinsic really has no way to be lowered, just
	have the code generator emit code that prints an error message and calls
	abort if executed.</dd>

	<dt>Add support to the .td file for the target(s) of your choice in
	<tt>lib/Target//.td</tt>.</dt>

	<dd>This is usually a matter of adding a pattern to the .td file that matches
	the intrinsic, though it may obviously require adding the instructions you
	want to generate as well. There are lots of examples in the PowerPC and X86
	backend to follow.</dd>
	</dl>

	</div>

	<!-- *********************************************************************** -->
	<h2>
	<a name="sdnode">Adding a new SelectionDAG node</a>
	</h2>
	<!-- *********************************************************************** -->

	<div>

	<p>As with intrinsics, adding a new SelectionDAG node to LLVM is much easier
	than adding a new instruction. New nodes are often added to help represent
	instructions common to many targets. These nodes often map to an LLVM
	instruction (add, sub) or intrinsic (byteswap, population count). In other
	cases, new nodes have been added to allow many targets to perform a common task
	(converting between floating point and integer representation) or capture more
	complicated behavior in a single node (rotate).</p>

	<ol>
	<li><tt>include/llvm/CodeGen/ISDOpcodes.h</tt>:
	Add an enum value for the new SelectionDAG node.</li>
	<li><tt>lib/CodeGen/SelectionDAG/SelectionDAG.cpp</tt>:
	Add code to print the node to <tt>getOperationName</tt>. If your new node
	can be evaluated at compile time when given constant arguments (such as an
	add of a constant with another constant), find the <tt>getNode</tt> method
	that takes the appropriate number of arguments, and add a case for your node
	to the switch statement that performs constant folding for nodes that take
	the same number of arguments as your new node.</li>
	<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
	Add code to <a href="CodeGenerator.html#selectiondag_legalize">legalize,
	promote, and expand</a> the node as necessary. At a minimum, you will need
	to add a case statement for your node in <tt>LegalizeOp</tt> which calls
	LegalizeOp on the node's operands, and returns a new node if any of the
	operands changed as a result of being legalized. It is likely that not all
	targets supported by the SelectionDAG framework will natively support the
	new node. In this case, you must also add code in your node's case
	statement in <tt>LegalizeOp</tt> to Expand your node into simpler, legal
	operations. The case for <tt>ISD::UREM</tt> for expanding a remainder into
	a divide, multiply, and a subtract is a good example.</li>
	<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
	If targets may support the new node being added only at certain sizes, you
	will also need to add code to your node's case statement in
	<tt>LegalizeOp</tt> to Promote your node's operands to a larger size, and
	perform the correct operation. You will also need to add code to
	<tt>PromoteOp</tt> to do this as well. For a good example, see
	<tt>ISD::BSWAP</tt>,
	which promotes its operand to a wider size, performs the byteswap, and then
	shifts the correct bytes right to emulate the narrower byteswap in the
	wider type.</li>
	<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
	Add a case for your node in <tt>ExpandOp</tt> to teach the legalizer how to
	perform the action represented by the new node on a value that has been
	split into high and low halves. This case will be used to support your
	node with a 64 bit operand on a 32 bit target.</li>
	<li><tt>lib/CodeGen/SelectionDAG/DAGCombiner.cpp</tt>:
	If your node can be combined with itself, or other existing nodes in a
	peephole-like fashion, add a visit function for it, and call that function
	from <tt></tt>. There are several good examples for simple combines you
	can do; <tt>visitFABS</tt> and <tt>visitSRL</tt> are good starting places.
	</li>
	<li><tt>lib/Target/PowerPC/PPCISelLowering.cpp</tt>:
	Each target has an implementation of the <tt>TargetLowering</tt> class,
	usually in its own file (although some targets include it in the same
	file as the DAGToDAGISel). The default behavior for a target is to
	assume that your new node is legal for all types that are legal for
	that target. If this target does not natively support your node, then
	tell the target to either Promote it (if it is supported at a larger
	type) or Expand it. This will cause the code you wrote in
	<tt>LegalizeOp</tt> above to decompose your new node into other legal
	nodes for this target.</li>
	<li><tt>lib/Target/TargetSelectionDAG.td</tt>:
	Most current targets supported by LLVM generate code using the DAGToDAG
	method, where SelectionDAG nodes are pattern matched to target-specific
	nodes, which represent individual instructions. In order for the targets
	to match an instruction to your new node, you must add a def for that node
	to the list in this file, with the appropriate type constraints. Look at
	<tt>add</tt>, <tt>bswap</tt>, and <tt>fadd</tt> for examples.</li>
	<li><tt>lib/Target/PowerPC/PPCInstrInfo.td</tt>:
	Each target has a tablegen file that describes the target's instruction
	set. For targets that use the DAGToDAG instruction selection framework,
	add a pattern for your new node that uses one or more target nodes.
	Documentation for this is a bit sparse right now, but there are several
	decent examples. See the patterns for <tt>rotl</tt> in
	<tt>PPCInstrInfo.td</tt>.</li>
	<li>TODO: document complex patterns.</li>
	<li><tt>llvm/test/Regression/CodeGen/*</tt>: Add test cases for your new node
	to the test suite. <tt>llvm/test/Regression/CodeGen/X86/bswap.ll</tt> is
	a good example.</li>
	</ol>

	</div>

	<!-- *********************************************************************** -->
	<h2>
	<a name="instruction">Adding a new instruction</a>
	</h2>
	<!-- *********************************************************************** -->

	<div>

	<p><span class="doc_warning">WARNING: adding instructions changes the bitcode
	format, and it will take some effort to maintain compatibility with
	the previous version.</span> Only add an instruction if it is absolutely
	necessary.</p>

	<ol>

	<li><tt>llvm/include/llvm/Instruction.def</tt>:
	add a number for your instruction and an enum name</li>

	<li><tt>llvm/include/llvm/Instructions.h</tt>:
	add a definition for the class that will represent your instruction</li>

	<li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
	add a prototype for a visitor to your new instruction type</li>

	<li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
	add a new token to parse your instruction from assembly text file</li>

	<li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
	add the grammar on how your instruction can be read and what it will
	construct as a result</li>

	<li><tt>llvm/lib/Bitcode/Reader/Reader.cpp</tt>:
	add a case for your instruction and how it will be parsed from bitcode</li>

	<li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
	add a case for how your instruction will be printed out to assembly</li>

	<li><tt>llvm/lib/VMCore/Instructions.cpp</tt>:
	implement the class you defined in
	<tt>llvm/include/llvm/Instructions.h</tt></li>

	<li>Test your instruction</li>

	<li><tt>llvm/lib/Target/*</tt>:
	Add support for your instruction to code generators, or add a lowering
	pass.</li>

	<li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>

	</ol>

	<p>Also, you need to implement (or modify) any analyses or passes that you want
	to understand this new instruction.</p>

	</div>


	<!-- *********************************************************************** -->
	<h2>
	<a name="type">Adding a new type</a>
	</h2>
	<!-- *********************************************************************** -->

	<div>

	<p><span class="doc_warning">WARNING: adding new types changes the bitcode
	format, and will break compatibility with currently-existing LLVM
	installations.</span> Only add new types if it is absolutely necessary.</p>

	<!-- ======================================================================= -->
	<h3>
	<a name="fund_type">Adding a fundamental type</a>
	</h3>

	<div>

	<ol>

	<li><tt>llvm/include/llvm/Type.h</tt>:
	add enum for the new type; add static <tt>Type*</tt> for this type</li>

	<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
	add mapping from <tt>TypeID</tt> => <tt>Type*</tt>;
	initialize the static <tt>Type*</tt></li>

	<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
	add ability to parse in the type from text assembly</li>

	<li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
	add a token for that type</li>

	</ol>

	</div>

	<!-- ======================================================================= -->
	<h3>
	<a name="derived_type">Adding a derived type</a>
	</h3>

	<div>

	<ol>
	<li><tt>llvm/include/llvm/Type.h</tt>:
	add enum for the new type; add a forward declaration of the type
	also</li>

	<li><tt>llvm/include/llvm/DerivedTypes.h</tt>:
	add new class to represent new class in the hierarchy; add forward
	declaration to the TypeMap value type</li>

	<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
	add support for derived type to:
	<div class="doc_code">
	<pre>
	std::string getTypeDescription(const Type &Ty,
	std::vector<const Type*> &TypeStack)
	bool TypesEqual(const Type Ty, const Type Ty2,
	std::map<const Type, const Type> & EqTypes)
	</pre>
	</div>
	add necessary member functions for type, and factory methods</li>

	<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
	add ability to parse in the type from text assembly</li>

	<li><tt>llvm/lib/BitCode/Writer/Writer.cpp</tt>:
	modify <tt>void BitcodeWriter::outputType(const Type *T)</tt> to serialize
	your type</li>

	<li><tt>llvm/lib/BitCode/Reader/Reader.cpp</tt>:
	modify <tt>const Type *BitcodeReader::ParseType()</tt> to read your data
	type</li>

	<li><tt>llvm/lib/VMCore/AsmWriter.cpp</tt>:
	modify
	<div class="doc_code">
	<pre>
	void calcTypeName(const Type *Ty,
	std::vector<const Type*> &TypeStack,
	std::map<const Type*,std::string> &TypeNames,
	std::string & Result)
	</pre>
	</div>
	to output the new derived type
	</li>


	</ol>

	</div>

	</div>

	<!-- *********************************************************************** -->

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