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<title>Writing an LLVM backend</title>
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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="#machine">Machine backends</a></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>
</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>Now, for static code generation you also need to write an instruction
selector for your platform: see <tt>lib/Target/*/*ISelSimple.cpp</tt> which
is no longer "simple" but it gives you the idea: you have to be able to create
MachineInstrs for any given LLVM instruction using the <tt>InstVisitor</tt>
pattern, and produce a <tt>MachineFunction</tt> with
<tt>MachineBasicBlock</tt>s full of <tt><a
href="CodeGenerator.html#machineinstr">MachineInstr</a></tt>s for a
corresponding LLVM Function. 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<string name> : Register<name> {
<b>let</b> Namespace = "<em>Target</em>";
}
<b>class</b> IntReg<<b>bits</b><5> num, string name> : <em>Target</em>Reg<name> {
<b>field</b> <b>bits</b><5> Num = num;
}
<b>def</b> R0 : IntReg<0, "%R0">;
...
// class RegisterClass is defined in Target.td
<b>def</b> IReg : RegisterClass<i64, 64, [R0, ... ]>;
</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<<b>bits</b><6> opcode, <b>dag</b> OL, <b>string</b> asmstr> : Instruction {
<b>field</b> <b>bits</b><42> 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<42, (ops IReg:$rD, IReg:$rA, IReg:$rB), "add $rD, $rA, $rB">;
</pre>
</div>
</li>
</ul>
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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>
</ul>
</div>
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