[llvm-commits] CVS: llvm/docs/WritingAnLLVMBackend.html
Misha Brukman
brukman at cs.uiuc.edu
Mon Sep 6 15:58:23 PDT 2004
Changes in directory llvm/docs:
WritingAnLLVMBackend.html added (r1.1)
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Log message:
How to write an LLVM backend, first draft.
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+ <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
+ "http://www.w3.org/TR/html4/strict.dtd">
+ <html>
+ <head>
+ <title>Writing an LLVM backend</title>
+ <link rel="stylesheet" href="llvm.css" type="text/css">
+ </head>
+
+ <body>
+
+ <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></li>
+ <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>
+
+ <!-- *********************************************************************** -->
+ <div class="doc_section">
+ <a name="intro">Introduction</a>
+ </div>
+ <!-- *********************************************************************** -->
+
+ <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>
+
+ <!-- *********************************************************************** -->
+ <div class="doc_section">
+ <a name="backends">Writing a backend</a>
+ </div>
+ <!-- *********************************************************************** -->
+
+ <!-- ======================================================================= -->
+ <div class="doc_subsection">
+ <a name="machine">Machine backends</a>
+ </div>
+
+ <!-- _______________________________________________________________________ -->
+ <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>
+
+ <!-- _______________________________________________________________________ -->
+ <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 : Register {
+ <b>let</b> Namespace = "<em>Target</em>";
+ }
+
+ <b>class</b> IntReg<<b>bits</b><5> num> : <em>Target</em>Reg {
+ <b>field</b> <b>bits</b><5> Num = num;
+ }
+
+ <b>def</b> R0 : IntReg<0>;
+ ...
+
+ // 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>
+
+ </div>
+
+ <!-- ======================================================================= -->
+ <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>
+
+ <!-- *********************************************************************** -->
+ <div class="doc_section">
+ <a name="related">Related reading material</a>
+ </div>
+ <!-- *********************************************************************** -->
+
+ <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>
+
+ <!-- *********************************************************************** -->
+
+ <hr>
+ <address>
+ <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
+ src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a>
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+ src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!" /></a>
+
+ <a href="http://misha.brukman.net">Misha Brukman</a><br>
+ <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
+ <br>
+ Last modified: $Date: 2004/09/06 22:58:13 $
+ </address>
+
+ </body>
+ </html>
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