mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-10-29 23:12:55 +01:00
245 lines
8.1 KiB
HTML
245 lines
8.1 KiB
HTML
<!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>
|
|
<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>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<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<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>
|
|
|
|
</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>
|
|
<a href="http://validator.w3.org/check/referer"><img
|
|
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: $Date$
|
|
</address>
|
|
|
|
</body>
|
|
</html>
|