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5 <title>Writing an LLVM backend</title>
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11 <div class="doc_title">
12 Writing an LLVM backend
16 <li><a href="#intro">Introduction</a>
17 <li><a href="#backends">Writing a backend</a>
19 <li><a href="#machine">Machine backends</a>
21 <li><a href="#machineTOC">Outline</a></li>
22 <li><a href="#machineDetails">Implementation details</a></li>
24 <li><a href="#lang">Language backends</a></li>
26 <li><a href="#related">Related reading material</a>
29 <div class="doc_author">
30 <p>Written by <a href="http://misha.brukman.net">Misha Brukman</a></p>
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34 <div class="doc_section">
35 <a name="intro">Introduction</a>
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39 <div class="doc_text">
41 <p>This document describes techniques for writing backends for LLVM which
42 convert the LLVM representation to machine assembly code or other languages.</p>
46 <!-- *********************************************************************** -->
47 <div class="doc_section">
48 <a name="backends">Writing a backend</a>
50 <!-- *********************************************************************** -->
52 <!-- ======================================================================= -->
53 <div class="doc_subsection">
54 <a name="machine">Machine backends</a>
57 <!-- _______________________________________________________________________ -->
58 <div class="doc_subsubsection">
59 <a name="machineTOC">Outline</a>
62 <div class="doc_text">
64 <p>In general, you want to follow the format of X86 or PowerPC (in
65 <tt>lib/Target</tt>).</p>
67 <p>To create a static compiler (one that emits text assembly), you need to
68 implement the following:</p>
71 <li>Describe the register set
73 <li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
74 the register set and register classes</li>
75 <li>Implement a subclass of <tt><a
76 href="CodeGenerator.html#mregisterinfo">MRegisterInfo</a></tt></li>
78 <li>Describe the instruction set
80 <li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
81 the instruction set</li>
82 <li>Implement a subclass of <tt><a
83 href="CodeGenerator.html#targetinstrinfo">TargetInstrInfo</a></tt></li>
85 <li>Describe the target machine
87 <li>Create a <a href="TableGenFundamentals.html">TableGen</a> description of
88 the target that describes the pointer size and references the instruction
90 <li>Implement a subclass of <tt><a
91 href="CodeGenerator.html#targetmachine">TargetMachine</a></tt>, which
92 configures <tt><a href="CodeGenerator.html#targetdata">TargetData</a></tt>
94 <li>Register your new target using the <tt>RegisterTarget</tt>
96 <div class="doc_code"><pre>
97 RegisterTarget<<em>MyTargetMachine</em>> M("short_name", " Target name");
99 <br>Here, <em>MyTargetMachine</em> is the name of your implemented
101 href="CodeGenerator.html#targetmachine">TargetMachine</a></tt>,
102 <em>short_name</em> is the option that will be active following
103 <tt>-march=</tt> to select a target in llc and lli, and the last string
104 is the description of your target to appear in <tt>-help</tt>
107 <li>Implement the assembly printer for the architecture. Usually, if you have
108 described the instruction set with the assembly printer generator in mind, that
109 step can be almost automated.</li>
112 <p>Now, for static code generation you also need to write an instruction
113 selector for your platform: see <tt>lib/Target/*/*ISelSimple.cpp</tt> which
114 is no longer "simple" but it gives you the idea: you have to be able to create
115 MachineInstrs for any given LLVM instruction using the <tt>InstVisitor</tt>
116 pattern, and produce a <tt>MachineFunction</tt> with
117 <tt>MachineBasicBlock</tt>s full of <tt><a
118 href="CodeGenerator.html#machineinstr">MachineInstr</a></tt>s for a
119 corresponding LLVM Function. Creating an instruction selector is perhaps the
120 most time-consuming part of creating a back-end.</p>
122 <p>To create a JIT for your platform:</p>
125 <li>Create a subclass of <tt><a
126 href="CodeGenerator.html#targetjitinfo">TargetJITInfo</a></tt></li>
127 <li>Create a machine code emitter that will be used to emit binary code
128 directly into memory, given <tt>MachineInstr</tt>s</li>
131 <p>Note that <tt>lib/target/Skeleton</tt> is a clean skeleton for a new target,
132 so you might want to start with that and adapt it for your target, and if you
133 are wondering how things are done, peek in the X86 or PowerPC target.</p>
135 <p>The Skeleton target is non-functional but provides the basic building blocks
136 you will need for your endeavor.</p>
140 <!-- _______________________________________________________________________ -->
141 <div class="doc_subsubsection">
142 <a name="machineDetails">Implementation details</a>
145 <div class="doc_text">
149 <li><p><b>TableGen register info description</b> - describe a class which
150 will store the register's number in the binary encoding of the instruction
151 (e.g., for JIT purposes).</p>
153 <p>You also need to define register classes to contain these registers, such as
154 the integer register class and floating-point register class, so that you can
155 allocate virtual registers to instructions from these sets, and let the
156 target-independent register allocator automatically choose the actual
157 architected registers.</p>
159 <div class="doc_code">
161 // class Register is defined in Target.td
162 <b>class</b> <em>Target</em>Reg<string name> : Register<name> {
163 <b>let</b> Namespace = "<em>Target</em>";
166 <b>class</b> IntReg<<b>bits</b><5> num, string name> : <em>Target</em>Reg<name> {
167 <b>field</b> <b>bits</b><5> Num = num;
170 <b>def</b> R0 : IntReg<0, "%R0">;
173 // class RegisterClass is defined in Target.td
174 <b>def</b> IReg : RegisterClass<i64, 64, [R0, ... ]>;
179 <li><p><b>TableGen instruction info description</b> - break up instructions into
180 classes, usually that's already done by the manufacturer (see instruction
181 manual). Define a class for each instruction category. Define each opcode as a
182 subclass of the category, with appropriate parameters such as the fixed binary
183 encoding of opcodes and extended opcodes, and map the register bits to the bits
184 of the instruction which they are encoded in (for the JIT). Also specify how
185 the instruction should be printed so it can use the automatic assembly printer,
188 <div class="doc_code">
190 // class Instruction is defined in Target.td
191 <b>class</b> Form<<b>bits</b><6> opcode, <b>dag</b> OL, <b>string</b> asmstr> : Instruction {
192 <b>field</b> <b>bits</b><42> Inst;
194 <b>let</b> Namespace = "<em>Target</em>";
195 <b>let</b> Inst{0-6} = opcode;
196 <b>let</b> OperandList = OL;
197 <b>let</b> AsmString = asmstr;
200 <b>def</b> ADD : Form<42, (ops IReg:$rD, IReg:$rA, IReg:$rB), "add $rD, $rA, $rB">;
209 <!-- ======================================================================= -->
210 <div class="doc_subsection">
211 <a name="lang">Language backends</a>
214 <div class="doc_text">
216 <p>For now, just take a look at <tt>lib/Target/CBackend</tt> for an example of
217 how the C backend is written.</p>
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222 <div class="doc_section">
223 <a name="related">Related reading material</a>
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227 <div class="doc_text">
230 <li><a href="CodeGenerator.html">Code generator</a> -
231 describes some of the classes in code generation at a high level, but
232 it is not (yet) complete.</li>
233 <li><a href="TableGenFundamentals.html">TableGen fundamentals</a> -
234 describes how to use TableGen to describe your target information succinctly
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249 <a href="http://misha.brukman.net">Misha Brukman</a><br>
250 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
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