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311 lines
11 KiB
HTML
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
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"http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<title>Writing an LLVM backend</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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<div class="doc_title">
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Writing an LLVM backend
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</div>
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<ol>
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<li><a href="#intro">Introduction</a>
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<li><a href="#backends">Writing a backend</a>
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<ol>
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<li><a href="#machine">Machine backends</a>
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<ol>
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<li><a href="#machineTOC">Outline</a></li>
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<li><a href="#machineDetails">Implementation details</a></li>
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</ol></li>
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<li><a href="#lang">Language backends</a></li>
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</ol></li>
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<li><a href="#related">Related reading material</a>
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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></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="intro">Introduction</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This document describes techniques for writing backends for LLVM which
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convert the LLVM representation to machine assembly code or other languages.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="backends">Writing a backend</a>
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</div>
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<!-- *********************************************************************** -->
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="machine">Machine backends</a>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="machineTOC">Outline</a>
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</div>
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<div class="doc_text">
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<p>In general, you want to follow the format of SPARC, X86 or PowerPC (in
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<tt>lib/Target</tt>). SPARC is the simplest backend, and is RISC, so if
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you're working on a RISC target, it is a good one to start with.</p>
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<p>To create a static compiler (one that emits text assembly), you need to
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implement the following:</p>
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<ul>
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<li>Describe the register set.
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<ul>
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<li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
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the register set and register classes</li>
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<li>Implement a subclass of <tt><a
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href="CodeGenerator.html#targetregisterinfo">TargetRegisterInfo</a></tt></li>
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</ul></li>
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<li>Describe the instruction set.
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<ul>
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<li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
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the instruction set</li>
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<li>Implement a subclass of <tt><a
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href="CodeGenerator.html#targetinstrinfo">TargetInstrInfo</a></tt></li>
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</ul></li>
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<li>Describe the target machine.
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<ul>
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<li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
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the target that describes the pointer size and references the instruction
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set</li>
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<li>Implement a subclass of <tt><a
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href="CodeGenerator.html#targetmachine">TargetMachine</a></tt>, which
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configures <tt><a href="CodeGenerator.html#targetdata">TargetData</a></tt>
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correctly</li>
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<li>Register your new target using the <tt>RegisterTarget</tt>
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template:<br><br>
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<div class="doc_code"><pre>
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RegisterTarget<<em>MyTargetMachine</em>> M("short_name", " Target name");
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</pre></div>
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<br>Here, <em>MyTargetMachine</em> is the name of your implemented
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subclass of <tt><a
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href="CodeGenerator.html#targetmachine">TargetMachine</a></tt>,
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<em>short_name</em> is the option that will be active following
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<tt>-march=</tt> to select a target in llc and lli, and the last string
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is the description of your target to appear in <tt>-help</tt>
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listing.</li>
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</ul></li>
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<li>Implement the assembly printer for the architecture.
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<ul>
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<li>Define all of the assembly strings for your target, adding them to the
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instructions in your *InstrInfo.td file.</li>
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<li>Implement the <tt>llvm::AsmPrinter</tt> interface.</li>
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</ul>
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</li>
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<li>Implement an instruction selector for the architecture.
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<ul>
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<li>The recommended method is the <a href="CodeGenerator.html#instselect">
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pattern-matching DAG-to-DAG instruction selector</a> (for example, see
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the PowerPC backend in PPCISelDAGtoDAG.cpp). Parts of instruction
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selector creation can be performed by adding patterns to the instructions
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in your <tt>.td</tt> file.</li>
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</ul>
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</li>
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<li>Optionally, add subtarget support.
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<ul>
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<li>If your target has multiple subtargets (e.g. variants with different
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capabilities), implement the <tt>llvm::TargetSubtarget</tt> interface
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for your architecture. This allows you to add <tt>-mcpu=</tt> and
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<tt>-mattr=</tt> options.</li>
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</ul>
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<li>Optionally, add JIT support.
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<ul>
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<li>Create a subclass of <tt><a
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href="CodeGenerator.html#targetjitinfo">TargetJITInfo</a></tt></li>
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<li>Create a machine code emitter that will be used to emit binary code
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directly into memory, given <tt>MachineInstr</tt>s</li>
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</ul>
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</ul>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsubsection">
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<a name="machineDetails">Implementation details</a>
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</div>
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<div class="doc_text">
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<ul>
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<li><p><b>TableGen register info description</b> - describe a class which
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will store the register's number in the binary encoding of the instruction
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(e.g., for JIT purposes).</p>
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<p>You also need to define register classes to contain these registers, such as
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the integer register class and floating-point register class, so that you can
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allocate virtual registers to instructions from these sets, and let the
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target-independent register allocator automatically choose the actual
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architected registers.</p>
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<div class="doc_code">
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<pre>
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// class Register is defined in Target.td
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<b>class</b> <em>Target</em>Reg<string name> : Register<name> {
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<b>let</b> Namespace = "<em>Target</em>";
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}
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<b>class</b> IntReg<<b>bits</b><5> num, string name> : <em>Target</em>Reg<name> {
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<b>field</b> <b>bits</b><5> Num = num;
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}
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<b>def</b> R0 : IntReg<0, "%R0">;
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...
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// class RegisterClass is defined in Target.td
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<b>def</b> IReg : RegisterClass<i64, 64, [R0, ... ]>;
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</pre>
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</div>
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</li>
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<li><p><b>TableGen instruction info description</b> - break up instructions into
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classes, usually that's already done by the manufacturer (see instruction
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manual). Define a class for each instruction category. Define each opcode as a
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subclass of the category, with appropriate parameters such as the fixed binary
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encoding of opcodes and extended opcodes, and map the register bits to the bits
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of the instruction which they are encoded in (for the JIT). Also specify how
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the instruction should be printed so it can use the automatic assembly printer,
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e.g.:</p>
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<div class="doc_code">
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<pre>
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// class Instruction is defined in Target.td
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<b>class</b> Form<<b>bits</b><6> opcode, <b>dag</b> OL, <b>string</b> asmstr> : Instruction {
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<b>field</b> <b>bits</b><42> Inst;
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<b>let</b> Namespace = "<em>Target</em>";
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<b>let</b> Inst{0-6} = opcode;
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<b>let</b> OperandList = OL;
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<b>let</b> AsmString = asmstr;
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}
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<b>def</b> ADD : Form<42, (ops IReg:$rD, IReg:$rA, IReg:$rB), "add $rD, $rA, $rB">;
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</pre>
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</div>
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</li>
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</ul>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="lang">Language backends</a>
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</div>
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<div class="doc_text">
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<p>For now, just take a look at <tt>lib/Target/CBackend</tt> for an example of
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how the C backend is written.</p>
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</div>
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<!-- ======================================================================= -->
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<div class="doc_subsection">
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<a name="files">Files to create/modify</a>
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</div>
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<div class="doc_text">
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<p>To actually create your backend, you need to create and modify a few files.
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Here, the absolute minimum will be discussed. To actually use LLVM's target
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independent codegenerator, you must <a href="CodeGenerator.html">implement extra
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things</a>.</p>
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<p>First of all, you should create a subdirectory under <tt>lib/Target</tt>,
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which will hold all the files related to your target. Let's assume that our
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target is called, "Dummy", we would create the directory
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<tt>lib/Target/Dummy</tt>.</p>
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<p>In this new directory, you should put a <tt>Makefile</tt>. You can probably
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copy one from another target and modify it. It should at least contain the
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<tt>LEVEL</tt>, <tt>LIBRARYNAME</tt> and <tt>TARGET</tt> variables, and then
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include <tt>$(LEVEL)/Makefile.common</tt>. Be careful to give the library the
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correct name, it must be named <tt>LLVMDummy</tt> (see the MIPS target, for
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example). Alternatively, you can split the library into
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<tt>LLVMDummyCodeGen</tt> and <tt>LLVMDummyAsmPrinter</tt>, the latter of which
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should be implemented in a subdirectory below <tt>lib/Target/Dummy</tt> (see the
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PowerPC target, for example).</p>
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<p>Note that these two naming schemes are hardcoded into llvm-config. Using any
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other naming scheme will confuse llvm-config and produce lots of (seemingly
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unrelated) linker errors when linking <tt>llc</tt>.</p>
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<p>To make your target actually do something, you need to implement a subclass
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of <tt>TargetMachine</tt>. This implementation should typically be in the file
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<tt>lib/Target/DummyTargetMachine.cpp</tt>, but any file in the
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<tt>lib/Target</tt> directory will be built and should work. To use LLVM's <a
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href="CodeGenerator.html">target independent code generator</a>, you should
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create a subclass of <tt>LLVMTargetMachine</tt>. This is what all current
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machine backends do. To create a target from scratch, create a subclass of
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<tt>TargetMachine</tt>. This is what the current language backends do.</p>
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<p>To get LLVM to actually build and link your target, you also need to add it
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to the <tt>TARGETS_TO_BUILD</tt> variable. To do this, you need to modify the
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<tt>configure</tt> script to know about your target when parsing the
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<tt>--enable-targets</tt> option. Search the <tt>configure</tt> script for
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<tt>TARGETS_TO_BUILD</tt>, add your target to the lists there (some creativity
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required) and then reconfigure. Alternatively, you can change
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<tt>autotools/configure.ac</tt> and regenerate <tt>configure</tt> by running
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<tt>./autoconf/AutoRegen.sh</tt>.
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section">
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<a name="related">Related reading material</a>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<ul>
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<li><a href="CodeGenerator.html">Code generator</a> -
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describes some of the classes in code generation at a high level, but
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it is not (yet) complete</li>
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<li><a href="TableGenFundamentals.html">TableGen fundamentals</a> -
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describes how to use TableGen to describe your target information
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succinctly</li>
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<li><a href="HowToSubmitABug.html#codegen">Debugging code generation with
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bugpoint</a> - shows bugpoint usage scenarios to simplify backend
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development</li>
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</ul>
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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://misha.brukman.net">Misha Brukman</a><br>
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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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</body>
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</html>
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