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Add a couple very minor tweaks

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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@25359 91177308-0d34-0410-b5e6-96231b3b80d8
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Chris Lattner 2006-01-16 16:31:40 +00:00
parent 76bb6ae211
commit b5f6e2520e
1 changed files with 11 additions and 7 deletions
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docs/ExtendingLLVM.html
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@ -26,7 +26,8 @@
<div class="doc_author"> <div class="doc_author">
<p>Written by <a href="http://misha.brukman.net">Misha Brukman</a>, <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> Brad Jones, Nate Begeman,
and <a href="http://nondot.org/sabre">Chris Lattner</a></p>
</div> </div>
<!-- *********************************************************************** --> <!-- *********************************************************************** -->
@ -123,7 +124,8 @@ 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 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 <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 (or any other compiler) extensions, it can be conditionally supported based on
the compiler compiling the CBE output (see llvm.prefetch for an example). 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 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. generator emit code that prints an error message and calls abort if executed.
</dd> </dd>
@ -136,13 +138,14 @@ generator emit code that prints an error message and calls abort if executed.
code, you will likely need to add support for your intrinsic there as well. code, you will likely need to add support for your intrinsic there as well.
This is usually accomplished by adding a new node, and then teaching the This is usually accomplished by adding a new node, and then teaching the
SelectionDAG code how to handle that node. To do this, follow the steps in SelectionDAG code how to handle that node. To do this, follow the steps in
the next section, Adding a new SelectionDAG node.</dd> the <a href="#sdnode">Adding a new SelectionDAG node</a> section.</dd>
<dl> <dl>
<dt>Once you have added the new node, add code to <dt>Once you have added the new node, add code to
<tt>SelectionDAG/SelectionDAGISel.cpp</tt> to recognize the intrinsic. In most <tt>SelectionDAG/SelectionDAGISel.cpp</tt> to recognize the intrinsic. In most
cases, the intrinsic will just be turned into the node you just added. For an cases, the intrinsic will just be turned into the node you just added. For an
example of this, see how <tt>visitIntrinsicCall</tt> handles Intrinsic::ctpop example of this, see how <tt>visitIntrinsicCall</tt> handles
<tt>Intrinsic::ctpop_*</tt>.
</dt> </dt>
</div> </div>
@ -182,14 +185,15 @@ complicated behavior in a single node (rotate).</p>
targets supported by the SelectionDAG framework will natively support the 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 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 statement in <tt>LegalizeOp</tt> to Expand your node into simpler, legal
operations. The case for ISD::UREM for expanding a remainder into a operations. The case for <tt>ISD::UREM</tt> for expanding a remainder into
multiply and a subtract is a good example.</li> a divide, multiply, and a subtract is a good example.</li>
<li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>: <li><tt>lib/CodeGen/SelectionDAG/LegalizeDAG.cpp</tt>:
If targets may support the new node being added only at certain sizes, you 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 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 <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 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 ISD::BSWAP, <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 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 shifts the correct bytes right to emulate the narrower byteswap in the
wider type.</li> wider type.</li>