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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<h1>
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Extending LLVM: Adding instructions, intrinsics, types, etc.
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</h1>
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<ol>
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<li><a href="#introduction">Introduction and Warning</a></li>
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<li><a href="#intrinsic">Adding a new intrinsic function</a></li>
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<li><a href="#instruction">Adding a new instruction</a></li>
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<li><a href="#sdnode">Adding a new SelectionDAG node</a></li>
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<li><a href="#type">Adding a new type</a>
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<ol>
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<li><a href="#fund_type">Adding a new fundamental type</a></li>
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<li><a href="#derived_type">Adding a new derived type</a></li>
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</ol></li>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="http://misha.brukman.net">Misha Brukman</a>,
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Brad Jones, Nate Begeman,
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and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="introduction">Introduction and Warning</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>During the course of using LLVM, you may wish to customize it for your
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research project or for experimentation. At this point, you may realize that
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you need to add something to LLVM, whether it be a new fundamental type, a new
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intrinsic function, or a whole new instruction.</p>
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<p>When you come to this realization, stop and think. Do you really need to
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extend LLVM? Is it a new fundamental capability that LLVM does not support at
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its current incarnation or can it be synthesized from already pre-existing LLVM
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elements? If you are not sure, ask on the <a
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href="http://mail.cs.uiuc.edu/mailman/listinfo/llvmdev">LLVM-dev</a> list. The
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reason is that extending LLVM will get involved as you need to update all the
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different passes that you intend to use with your extension, and there are
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<em>many</em> LLVM analyses and transformations, so it may be quite a bit of
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work.</p>
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<p>Adding an <a href="#intrinsic">intrinsic function</a> is far easier than
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adding an instruction, and is transparent to optimization passes. If your added
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functionality can be expressed as a
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function call, an intrinsic function is the method of choice for LLVM
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extension.</p>
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<p>Before you invest a significant amount of effort into a non-trivial
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extension, <span class="doc_warning">ask on the list</span> if what you are
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looking to do can be done with already-existing infrastructure, or if maybe
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someone else is already working on it. You will save yourself a lot of time and
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effort by doing so.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="intrinsic">Adding a new intrinsic function</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>Adding a new intrinsic function to LLVM is much easier than adding a new
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instruction. Almost all extensions to LLVM should start as an intrinsic
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function and then be turned into an instruction if warranted.</p>
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<ol>
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<li><tt>llvm/docs/LangRef.html</tt>:
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Document the intrinsic. Decide whether it is code generator specific and
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what the restrictions are. Talk to other people about it so that you are
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sure it's a good idea.</li>
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<li><tt>llvm/include/llvm/Intrinsics*.td</tt>:
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Add an entry for your intrinsic. Describe its memory access characteristics
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for optimization (this controls whether it will be DCE'd, CSE'd, etc). Note
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that any intrinsic using the <tt>llvm_int_ty</tt> type for an argument will
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be deemed by <tt>tblgen</tt> as overloaded and the corresponding suffix
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will be required on the intrinsic's name.</li>
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<li><tt>llvm/lib/Analysis/ConstantFolding.cpp</tt>: If it is possible to
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constant fold your intrinsic, add support to it in the
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<tt>canConstantFoldCallTo</tt> and <tt>ConstantFoldCall</tt> functions.</li>
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<li><tt>llvm/test/Regression/*</tt>: Add test cases for your test cases to the
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test suite</li>
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</ol>
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<p>Once the intrinsic has been added to the system, you must add code generator
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support for it. Generally you must do the following steps:</p>
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<dl>
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<dt>Add support to the C backend in <tt>lib/Target/CBackend/</tt></dt>
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<dd>Depending on the intrinsic, there are a few ways to implement this. For
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most intrinsics, it makes sense to add code to lower your intrinsic in
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<tt>LowerIntrinsicCall</tt> in <tt>lib/CodeGen/IntrinsicLowering.cpp</tt>.
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Second, if it makes sense to lower the intrinsic to an expanded sequence of
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C code in all cases, just emit the expansion in <tt>visitCallInst</tt> in
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<tt>Writer.cpp</tt>. If the intrinsic has some way to express it with GCC
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(or any other compiler) extensions, it can be conditionally supported based
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on the compiler compiling the CBE output (see <tt>llvm.prefetch</tt> for an
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example). Third, if the intrinsic really has no way to be lowered, just
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have the code generator emit code that prints an error message and calls
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abort if executed.</dd>
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<dt>Add support to the .td file for the target(s) of your choice in
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<tt>lib/Target/*/*.td</tt>.</dt>
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<dd>This is usually a matter of adding a pattern to the .td file that matches
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the intrinsic, though it may obviously require adding the instructions you
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want to generate as well. There are lots of examples in the PowerPC and X86
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backend to follow.</dd>
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</dl>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="sdnode">Adding a new SelectionDAG node</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p>As with intrinsics, adding a new SelectionDAG node to LLVM is much easier
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than adding a new instruction. New nodes are often added to help represent
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instructions common to many targets. These nodes often map to an LLVM
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instruction (add, sub) or intrinsic (byteswap, population count). In other
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cases, new nodes have been added to allow many targets to perform a common task
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(converting between floating point and integer representation) or capture more
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complicated behavior in a single node (rotate).</p>
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<ol>
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<li><tt>include/llvm/CodeGen/ISDOpcodes.h</tt>:
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Add an enum value for the new SelectionDAG node.</li>
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<li><tt>lib/CodeGen/SelectionDAG/SelectionDAG.cpp</tt>:
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Add code to print the node to <tt>getOperationName</tt>. If your new node
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can be evaluated at compile time when given constant arguments (such as an
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add of a constant with another constant), find the <tt>getNode</tt> method
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that takes the appropriate number of arguments, and add a case for your node
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to the switch statement that performs constant folding for nodes that take
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the same number of arguments as your new node.</li>
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<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
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Add code to <a href="CodeGenerator.html#selectiondag_legalize">legalize,
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promote, and expand</a> the node as necessary. At a minimum, you will need
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to add a case statement for your node in <tt>LegalizeOp</tt> which calls
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LegalizeOp on the node's operands, and returns a new node if any of the
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operands changed as a result of being legalized. It is likely that not all
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targets supported by the SelectionDAG framework will natively support the
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new node. In this case, you must also add code in your node's case
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statement in <tt>LegalizeOp</tt> to Expand your node into simpler, legal
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operations. The case for <tt>ISD::UREM</tt> for expanding a remainder into
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a divide, multiply, and a subtract is a good example.</li>
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<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
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If targets may support the new node being added only at certain sizes, you
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will also need to add code to your node's case statement in
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<tt>LegalizeOp</tt> to Promote your node's operands to a larger size, and
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perform the correct operation. You will also need to add code to
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<tt>PromoteOp</tt> to do this as well. For a good example, see
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<tt>ISD::BSWAP</tt>,
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which promotes its operand to a wider size, performs the byteswap, and then
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shifts the correct bytes right to emulate the narrower byteswap in the
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wider type.</li>
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<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
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Add a case for your node in <tt>ExpandOp</tt> to teach the legalizer how to
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perform the action represented by the new node on a value that has been
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split into high and low halves. This case will be used to support your
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node with a 64 bit operand on a 32 bit target.</li>
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<li><tt>lib/CodeGen/SelectionDAG/DAGCombiner.cpp</tt>:
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If your node can be combined with itself, or other existing nodes in a
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peephole-like fashion, add a visit function for it, and call that function
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from <tt></tt>. There are several good examples for simple combines you
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can do; <tt>visitFABS</tt> and <tt>visitSRL</tt> are good starting places.
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</li>
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<li><tt>lib/Target/PowerPC/PPCISelLowering.cpp</tt>:
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Each target has an implementation of the <tt>TargetLowering</tt> class,
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usually in its own file (although some targets include it in the same
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file as the DAGToDAGISel). The default behavior for a target is to
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assume that your new node is legal for all types that are legal for
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that target. If this target does not natively support your node, then
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tell the target to either Promote it (if it is supported at a larger
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type) or Expand it. This will cause the code you wrote in
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<tt>LegalizeOp</tt> above to decompose your new node into other legal
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nodes for this target.</li>
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<li><tt>lib/Target/TargetSelectionDAG.td</tt>:
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Most current targets supported by LLVM generate code using the DAGToDAG
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method, where SelectionDAG nodes are pattern matched to target-specific
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nodes, which represent individual instructions. In order for the targets
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to match an instruction to your new node, you must add a def for that node
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to the list in this file, with the appropriate type constraints. Look at
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<tt>add</tt>, <tt>bswap</tt>, and <tt>fadd</tt> for examples.</li>
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<li><tt>lib/Target/PowerPC/PPCInstrInfo.td</tt>:
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Each target has a tablegen file that describes the target's instruction
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set. For targets that use the DAGToDAG instruction selection framework,
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add a pattern for your new node that uses one or more target nodes.
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Documentation for this is a bit sparse right now, but there are several
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decent examples. See the patterns for <tt>rotl</tt> in
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<tt>PPCInstrInfo.td</tt>.</li>
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<li>TODO: document complex patterns.</li>
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<li><tt>llvm/test/Regression/CodeGen/*</tt>: Add test cases for your new node
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to the test suite. <tt>llvm/test/Regression/CodeGen/X86/bswap.ll</tt> is
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a good example.</li>
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</ol>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="instruction">Adding a new instruction</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p><span class="doc_warning">WARNING: adding instructions changes the bitcode
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format, and it will take some effort to maintain compatibility with
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the previous version.</span> Only add an instruction if it is absolutely
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necessary.</p>
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<ol>
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<li><tt>llvm/include/llvm/Instruction.def</tt>:
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add a number for your instruction and an enum name</li>
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<li><tt>llvm/include/llvm/Instructions.h</tt>:
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add a definition for the class that will represent your instruction</li>
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<li><tt>llvm/include/llvm/Support/InstVisitor.h</tt>:
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add a prototype for a visitor to your new instruction type</li>
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<li><tt>llvm/lib/AsmParser/Lexer.l</tt>:
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add a new token to parse your instruction from assembly text file</li>
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<li><tt>llvm/lib/AsmParser/llvmAsmParser.y</tt>:
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add the grammar on how your instruction can be read and what it will
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construct as a result</li>
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<li><tt>llvm/lib/Bitcode/Reader/Reader.cpp</tt>:
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add a case for your instruction and how it will be parsed from bitcode</li>
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<li><tt>llvm/lib/VMCore/Instruction.cpp</tt>:
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add a case for how your instruction will be printed out to assembly</li>
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<li><tt>llvm/lib/VMCore/Instructions.cpp</tt>:
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implement the class you defined in
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<tt>llvm/include/llvm/Instructions.h</tt></li>
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<li>Test your instruction</li>
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<li><tt>llvm/lib/Target/*</tt>:
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Add support for your instruction to code generators, or add a lowering
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pass.</li>
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<li><tt>llvm/test/Regression/*</tt>: add your test cases to the test suite.</li>
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</ol>
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<p>Also, you need to implement (or modify) any analyses or passes that you want
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to understand this new instruction.</p>
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</div>
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<!-- *********************************************************************** -->
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<h2>
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<a name="type">Adding a new type</a>
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</h2>
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<!-- *********************************************************************** -->
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<div>
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<p><span class="doc_warning">WARNING: adding new types changes the bitcode
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format, and will break compatibility with currently-existing LLVM
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installations.</span> Only add new types if it is absolutely necessary.</p>
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<!-- ======================================================================= -->
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<h3>
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<a name="fund_type">Adding a fundamental type</a>
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</h3>
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<div>
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<ol>
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<li><tt>llvm/include/llvm/Type.h</tt>:
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add enum for the new type; add static <tt>Type*</tt> for this type</li>
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<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
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add mapping from <tt>TypeID</tt> => <tt>Type*</tt>;
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initialize the static <tt>Type*</tt></li>
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<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
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add ability to parse in the type from text assembly</li>
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<li><tt>llvm/lib/AsmReader/llvmAsmParser.y</tt>:
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add a token for that type</li>
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</ol>
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</div>
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<!-- ======================================================================= -->
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<h3>
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<a name="derived_type">Adding a derived type</a>
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</h3>
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<div>
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<ol>
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<li><tt>llvm/include/llvm/Type.h</tt>:
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add enum for the new type; add a forward declaration of the type
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also</li>
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<li><tt>llvm/include/llvm/DerivedTypes.h</tt>:
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add new class to represent new class in the hierarchy; add forward
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declaration to the TypeMap value type</li>
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<li><tt>llvm/lib/VMCore/Type.cpp</tt>:
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add support for derived type to:
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<div class="doc_code">
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<pre>
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std::string getTypeDescription(const Type &Ty,
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std::vector<const Type*> &TypeStack)
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bool TypesEqual(const Type *Ty, const Type *Ty2,
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std::map<const Type*, const Type*> & EqTypes)
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</pre>
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</div>
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add necessary member functions for type, and factory methods</li>
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<li><tt>llvm/lib/AsmReader/Lexer.l</tt>:
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add ability to parse in the type from text assembly</li>
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<li><tt>llvm/lib/BitCode/Writer/Writer.cpp</tt>:
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modify <tt>void BitcodeWriter::outputType(const Type *T)</tt> to serialize
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your type</li>
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<li><tt>llvm/lib/BitCode/Reader/Reader.cpp</tt>:
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modify <tt>const Type *BitcodeReader::ParseType()</tt> to read your data
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type</li>
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<li><tt>llvm/lib/VMCore/AsmWriter.cpp</tt>:
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modify
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<div class="doc_code">
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<pre>
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void calcTypeName(const Type *Ty,
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std::vector<const Type*> &TypeStack,
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std::map<const Type*,std::string> &TypeNames,
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std::string & Result)
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</pre>
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</div>
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to output the new derived type
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</li>
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</ol>
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</div>
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</div>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="http://llvm.org/">The LLVM Compiler Infrastructure</a>
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<br>
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Last modified: $Date$
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</address>
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