llvm/docs/WritingAnLLVMBackend.html

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  <title>Writing an LLVM backend</title>
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<div class="doc_title">
  Writing an LLVM backend
</div>

<ol>
  <li><a href="#intro">Introduction</a>
  <li><a href="#backends">Writing a backend</a>
  <ol>
    <li><a href="#machine">Machine backends</a>
    <ol>
      <li><a href="#machineTOC">Outline</a></li>
      <li><a href="#machineDetails">Implementation details</a></li>
    </ol></li>  
    <li><a href="#lang">Language backends</a></li>
  </ol></li>
  <li><a href="#related">Related reading material</a>
</ol>

<div class="doc_author">    
  <p>Written by <a href="http://misha.brukman.net">Misha Brukman</a></p>
</div>

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<div class="doc_section">
  <a name="intro">Introduction</a>
</div>
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<div class="doc_text">

<p>This document describes techniques for writing backends for LLVM which
convert the LLVM representation to machine assembly code or other languages.</p>

</div>

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<div class="doc_section">
  <a name="backends">Writing a backend</a>
</div>
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<!-- ======================================================================= -->
<div class="doc_subsection">
  <a name="machine">Machine backends</a>
</div>
    
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<div class="doc_subsubsection">
  <a name="machineTOC">Outline</a>
</div>

<div class="doc_text">

<p>In general, you want to follow the format of X86 or PowerPC (in
<tt>lib/Target</tt>).</p>

<p>To create a static compiler (one that emits text assembly), you need to
implement the following:</p>

<ul>
<li>Describe the register set
  <ul>
  <li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
      the register set and register classes</li>
  <li>Implement a subclass of <tt><a
      href="CodeGenerator.html#mregisterinfo">MRegisterInfo</a></tt></li>
  </ul></li>
<li>Describe the instruction set
  <ul>
  <li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
      the instruction set</li>
  <li>Implement a subclass of <tt><a
      href="CodeGenerator.html#targetinstrinfo">TargetInstrInfo</a></tt></li>
  </ul></li>
<li>Describe the target machine
  <ul>
  <li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
      the target that describes the pointer size and references the instruction
      set</li>
  <li>Implement a subclass of <tt><a
      href="CodeGenerator.html#targetmachine">TargetMachine</a></tt>, which
      configures <tt><a href="CodeGenerator.html#targetdata">TargetData</a></tt>
      correctly</li>
  <li>Register your new target using the <tt>RegisterTarget</tt>
  template:<br><br>
<div class="doc_code"><pre>
RegisterTarget&lt;<em>MyTargetMachine</em>&gt; M("short_name", "  Target name");
</pre></div>
      <br>Here, <em>MyTargetMachine</em> is the name of your implemented
      subclass of <tt><a
      href="CodeGenerator.html#targetmachine">TargetMachine</a></tt>,
      <em>short_name</em> is the option that will be active following
      <tt>-march=</tt> to select a target in llc and lli, and the last string
      is the description of your target to appear in <tt>-help</tt>
      listing.</li>
  </ul></li>
<li>Implement the assembly printer for the architecture.  Usually, if you have
described the instruction set with the assembly printer generator in mind, that
step can be almost automated.</li>
</ul>

<p>You also need to write an instruction selector for your platform.  The
recommended method is the <a
href="CodeGenerator.html#instselect">pattern-matching instruction selector</a>,
examples of which you can see in other targets:
<tt>lib/Target/*/*ISelPattern.cpp</tt>.  The former method for writing
instruction selectors (<b>not</b> recommended for new targets) is evident in
<tt>lib/Target/*/*ISelSimple.cpp</tt>, which are <tt>InstVisitor</tt>-based
translators, generating code for an LLVM instruction at a time.  Creating an
instruction selector is perhaps the most time-consuming part of creating a
back-end.</p>

<p>To create a JIT for your platform:</p>

<ul>
<li>Create a subclass of <tt><a
    href="CodeGenerator.html#targetjitinfo">TargetJITInfo</a></tt></li>
<li>Create a machine code emitter that will be used to emit binary code
    directly into memory, given <tt>MachineInstr</tt>s</li>
</ul>

<p>Note that <tt>lib/target/Skeleton</tt> is a clean skeleton for a new target,
so you might want to start with that and adapt it for your target, and if you
are wondering how things are done, peek in the X86 or PowerPC target.</p>

<p>The Skeleton target is non-functional but provides the basic building blocks
you will need for your endeavor.</p>

</div>

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<div class="doc_subsubsection">
  <a name="machineDetails">Implementation details</a>
</div>

<div class="doc_text">

<ul>

<li><p><b>TableGen register info description</b> - describe a class which
will store the register's number in the binary encoding of the instruction
(e.g., for JIT purposes).</p>

<p>You also need to define register classes to contain these registers, such as
the integer register class and floating-point register class, so that you can
allocate virtual registers to instructions from these sets, and let the
target-independent register allocator automatically choose the actual
architected registers.</p>

<div class="doc_code">
<pre>
// class Register is defined in Target.td
<b>class</b> <em>Target</em>Reg&lt;string name&gt; : Register&lt;name&gt; {
  <b>let</b> Namespace = "<em>Target</em>";
}

<b>class</b> IntReg&lt;<b>bits</b>&lt;5&gt; num, string name&gt; : <em>Target</em>Reg&lt;name&gt; {
  <b>field</b> <b>bits</b>&lt;5&gt; Num = num;
}

<b>def</b> R0 : IntReg&lt;0, "%R0"&gt;;
...

// class RegisterClass is defined in Target.td
<b>def</b> IReg : RegisterClass&lt;i64, 64, [R0, ... ]&gt;;
</pre>
</div>
</li>

<li><p><b>TableGen instruction info description</b> - break up instructions into
classes, usually that's already done by the manufacturer (see instruction
manual).  Define a class for each instruction category.  Define each opcode as a
subclass of the category, with appropriate parameters such as the fixed binary
encoding of opcodes and extended opcodes, and map the register bits to the bits
of the instruction which they are encoded in (for the JIT).  Also specify how
the instruction should be printed so it can use the automatic assembly printer,
e.g.:</p>

<div class="doc_code">
<pre>
// class Instruction is defined in Target.td
<b>class</b> Form&lt;<b>bits</b>&lt;6&gt; opcode, <b>dag</b> OL, <b>string</b> asmstr&gt; : Instruction {
  <b>field</b> <b>bits</b>&lt;42&gt; Inst;

  <b>let</b> Namespace = "<em>Target</em>";
  <b>let</b> Inst{0-6} = opcode;
  <b>let</b> OperandList = OL;
  <b>let</b> AsmString = asmstr;
}

<b>def</b> ADD : Form&lt;42, (ops IReg:$rD, IReg:$rA, IReg:$rB), "add $rD, $rA, $rB"&gt;;
</pre>
</div>
</li>

</ul>

</div>

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<div class="doc_subsection">
  <a name="lang">Language backends</a>
</div>

<div class="doc_text">

<p>For now, just take a look at <tt>lib/Target/CBackend</tt> for an example of
how the C backend is written.</p>

</div>

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<div class="doc_section">
  <a name="related">Related reading material</a>
</div>
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<div class="doc_text">

<ul>
<li><a href="CodeGenerator.html">Code generator</a> -
    describes some of the classes in code generation at a high level, but
    it is not (yet) complete</li>
<li><a href="TableGenFundamentals.html">TableGen fundamentals</a> -
    describes how to use TableGen to describe your target information
    succinctly</li>
<li><a href="HowToSubmitABug.html#codegen">Debugging code generation with
    bugpoint</a> - shows bugpoint usage scenarios to simplify backend
    development</li>
</ul>

</div>

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