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| author | ed <ed@FreeBSD.org> | 2009-06-02 17:52:33 +0000 |
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| committer | ed <ed@FreeBSD.org> | 2009-06-02 17:52:33 +0000 |
| commit | 3277b69d734b9c90b44ebde4ede005717e2c3b2e (patch) | |
| tree | 64ba909838c23261cace781ece27d106134ea451 /docs/ExtendingLLVM.html | |
| download | FreeBSD-src-3277b69d734b9c90b44ebde4ede005717e2c3b2e.zip FreeBSD-src-3277b69d734b9c90b44ebde4ede005717e2c3b2e.tar.gz | |
Import LLVM, at r72732.
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diff --git a/docs/ExtendingLLVM.html b/docs/ExtendingLLVM.html new file mode 100644 index 0000000..8351f1a --- /dev/null +++ b/docs/ExtendingLLVM.html @@ -0,0 +1,391 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" + "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> + +<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="#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> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="introduction">Introduction and Warning</a> +</div> +<!-- *********************************************************************** --> + +<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 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> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="intrinsic">Adding a new intrinsic function</a> +</div> +<!-- *********************************************************************** --> + +<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*.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> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="sdnode">Adding a new SelectionDAG node</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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/SelectionDAGNodes.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> + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="instruction">Adding a new instruction</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + + +<!-- *********************************************************************** --> +<div class="doc_section"> + <a name="type">Adding a new type</a> +</div> +<!-- *********************************************************************** --> + +<div class="doc_text"> + +<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> + +</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> => <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/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> + +<!-- *********************************************************************** --> + +<hr> +<address> + <a href="http://jigsaw.w3.org/css-validator/check/referer"><img + src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> + <a href="http://validator.w3.org/check/referer"><img + src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> + + <a href="http://llvm.org">The LLVM Compiler Infrastructure</a> + <br> + Last modified: $Date: 2008-12-11 19:23:24 +0100 (Thu, 11 Dec 2008) $ +</address> + +</body> +</html> |
