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  <title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
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<div class="doc_title">
  Extending LLVM: Adding instructions, intrinsics, types, etc.
</div>

<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="#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, and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
</div>

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<div class="doc_section">
  <a name="introduction">Introduction and Warning</a>
</div>
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<div class="doc_text">

<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 easier than adding
an instruction, and is transparent to optimization passes which treat it as an
unanalyzable function.  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>

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<div class="doc_section">
  <a name="intrinsic">Adding a new intrinsic function</a>
</div>
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<div class="doc_text">

<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.h</tt>:
    add an enum in the <tt>llvm::Intrinsic</tt> namespace</li>

<li><tt>llvm/lib/VMCore/Verifier.cpp</tt>:
    Add code to check the invariants of the intrinsic are respected.</li>

<li><tt>llvm/lib/VMCore/Function.cpp (<tt>Function::getIntrinsicID()</tt>)</tt>:
    Identify the new intrinsic function, returning the enum for the intrinsic
    that you added.</li>

<li><tt>llvm/lib/Analysis/BasicAliasAnalysis.cpp</tt>: If the new intrinsic does
    not access memory or does not write to memory, add it to the relevant list
    of functions.</li>

<li><tt>llvm/lib/Transforms/Utils/Local.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>Test your intrinsic</li>

<li><tt>llvm/test/Regression/*</tt>: add 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.  First,
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>.  Second, 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 llvm.prefetch 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 a enum value for the SelectionDAG node in
<tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt></dt>

<dd>Also, add code to <tt>lib/CodeGen/SelectionDAG/SelectionDAG.cpp</tt> (and
<tt>SelectionDAGPrinter.cpp</tt>) to print the node.</dd>

<dt>Add code to <tt>SelectionDAG/SelectionDAGISel.cpp</tt> to recognize the
intrinsic.</dt>

<dd>Presumably the intrinsic should be recognized and turned into the node you
added above.</dd>

<dt>Add code to <tt>SelectionDAG/LegalizeDAG.cpp</tt> to <a
href="CodeGenerator.html#selectiondag_legalize">legalize, promote, and
expand</a> the node as necessary.</dt>

<dd>If the intrinsic can be expanded to primitive operations, legalize can break
the node down into other elementary operations that are be supported.</dd>

<dt>Add target-specific support to specific code generators.</dt>

<dd>Extend the code generators you are interested in to recognize and support
the node, emitting the code you want.</dd>
</dl>

<p>
Unfortunately, the process of extending the code generator to support a new node
is not extremely well documented.  As such, it is often helpful to look at other
intrinsics (e.g. <tt>llvm.ctpop</tt>) to see how they are recognized and turned
into a node by <tt>SelectionDAGISel.cpp</tt>, legalized by
<tt>LegalizeDAG.cpp</tt>, then finally emitted by the various code generators.
</p>

</div>

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<div class="doc_section">
  <a name="instruction">Adding a new instruction</a>
</div>
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<div class="doc_text">

<p><span class="doc_warning">WARNING: adding instructions changes the bytecode
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/Bytecode/Reader/Reader.cpp</tt>:
    add a case for your instruction and how it will be parsed from bytecode</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>


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<div class="doc_section">
  <a name="type">Adding a new type</a>
</div>
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<div class="doc_text">

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

</div>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="fund_type">Adding a fundamental type</a>
</div>

<div class="doc_text">

<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>

<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="derived_type">Adding a derived type</a>
</div>

<div class="doc_text">

<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/DerivedType.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/ByteCode/Writer/Writer.cpp</tt>:
    modify <tt>void BytecodeWriter::outputType(const Type *T)</tt> to serialize
    your type</li>

<li><tt>llvm/lib/ByteCode/Reader/Reader.cpp</tt>:
    modify <tt>const Type *BytecodeReader::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>

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