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<title>Extending LLVM: Adding instructions, intrinsics, types, etc.</title>
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Extending LLVM: Adding instructions, intrinsics, types, etc.
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<li><a href="#introduction">Introduction and Warning</a></li>
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<a name="introduction">Introduction and Warning</a>
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<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
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<a name="intrinsic">Adding a new intrinsic function</a>
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<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
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>
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-<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>
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<a name="sdnode">Adding a new SelectionDAG node</a>
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<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
complicated behavior in a single node (rotate).</p>
<ol>
-<li><tt>include/llvm/CodeGen/SelectionDAGNodes.h</tt>:
+<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
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<a name="instruction">Adding a new instruction</a>
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<p><span class="doc_warning">WARNING: adding instructions changes the bitcode
format, and it will take some effort to maintain compatibility with
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<a name="type">Adding a new type</a>
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<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>
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<a name="fund_type">Adding a fundamental type</a>
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<ol>
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<a name="derived_type">Adding a derived type</a>
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<ol>
<li><tt>llvm/include/llvm/Type.h</tt>:
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