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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
"http://www.w3.org/TR/html4/strict.dtd">
<html>
<head>
<title>The LLVM Target-Independent Code Generator</title>
<link rel="stylesheet" href="llvm.css" type="text/css">
</head>
<body>
<div class="doc_title">
The LLVM Target-Independent Code Generator
</div>
<ol>
<li><a href="#introduction">Introduction</a>
<ul>
<li><a href="#required">Required components in the code generator</a></li>
<li><a href="#high-level-design">The high-level design of the code generator</a></li>
<li><a href="#tablegen">Using TableGen for target description</a></li>
</ul>
</li>
<li><a href="#targetdesc">Target description classes</a>
<ul>
<li><a href="#targetmachine">The <tt>TargetMachine</tt> class</a></li>
<li><a href="#targetdata">The <tt>TargetData</tt> class</a></li>
<li><a href="#mregisterinfo">The <tt>MRegisterInfo</tt> class</a></li>
<li><a href="#targetinstrinfo">The <tt>TargetInstrInfo</tt> class</a></li>
<li><a href="#targetframeinfo">The <tt>TargetFrameInfo</tt> class</a></li>
<li><a href="#targetjitinfo">The <tt>TargetJITInfo</tt> class</a></li>
</ul>
</li>
<li><a href="#codegendesc">Machine code description classes</a>
<ul>
<li><a href="#machineinstr">The <tt>MachineInstr</tt> class</a></li>
</ul>
</li>
<li><a href="#codegenalgs">Target-independent code generation algorithms</a>
</li>
<li><a href="#targetimpls">Target description implementations</a>
<ul>
<li><a href="#x86">The X86 backend</a></li>
</ul>
</li>
</ol>
<div class="doc_author">
<p>Written by <a href="mailto:sabre@nondot.org">Chris Lattner</a></p>
</div>
<div class="doc_warning">
<p>Warning: This is a work in progress.</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="introduction">Introduction</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The LLVM target-independent code generator is a framework that provides a
suite of reusable components for translating the LLVM internal representation to
the machine code for a specified target -- either in assembly form (suitable for
a static compiler) or in binary machine code format (usable for a JIT compiler).
The LLVM target-independent code generator consists of five main components:</p>
<ol>
<li><a href="#targetdesc">Abstract target description</a> interfaces which
capture important properties about various aspects of the machine, independently
of how they will be used. These interfaces are defined in
<tt>include/llvm/Target/</tt>.</li>
<li>Classes used to represent the <a href="#codegendesc">machine code</a> being
generated for a target. These classes are intended to be abstract enough to
represent the machine code for <i>any</i> target machine. These classes are
defined in <tt>include/llvm/CodeGen/</tt>.</li>
<li><a href="#codegenalgs">Target-independent algorithms</a> used to implement
various phases of native code generation (register allocation, scheduling, stack
frame representation, etc). This code lives in <tt>lib/CodeGen/</tt>.</li>
<li><a href="#targetimpls">Implementations of the abstract target description
interfaces</a> for particular targets. These machine descriptions make use of
the components provided by LLVM, and can optionally provide custom
target-specific passes, to build complete code generators for a specific target.
Target descriptions live in <tt>lib/Target/</tt>.</li>
<li><a href="#jit">The target-independent JIT components</a>. The LLVM JIT is
completely target independent (it uses the <tt>TargetJITInfo</tt> structure to
interface for target-specific issues. The code for the target-independent
JIT lives in <tt>lib/ExecutionEngine/JIT</tt>.</li>
</ol>
<p>
Depending on which part of the code generator you are interested in working on,
different pieces of this will be useful to you. In any case, you should be
familiar with the <a href="#targetdesc">target description</a> and <a
href="#codegendesc">machine code representation</a> classes. If you want to add
a backend for a new target, you will need to <a href="#targetimpls">implement the
target description</a> classes for your new target and understand the <a
href="LangRef.html">LLVM code representation</a>. If you are interested in
implementing a new <a href="#codegenalgs">code generation algorithm</a>, it
should only depend on the target-description and machine code representation
classes, ensuring that it is portable.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="required">Required components in the code generator</a>
</div>
<div class="doc_text">
<p>The two pieces of the LLVM code generator are the high-level interface to the
code generator and the set of reusable components that can be used to build
target-specific backends. The two most important interfaces (<a
href="#targetmachine"><tt>TargetMachine</tt></a> and <a
href="#targetdata"><tt>TargetData</tt></a> classes) are the only ones that are
required to be defined for a backend to fit into the LLVM system, but the others
must be defined if the reusable code generator components are going to be
used.</p>
<p>This design has two important implications. The first is that LLVM can
support completely non-traditional code generation targets. For example, the C
backend does not require register allocation, instruction selection, or any of
the other standard components provided by the system. As such, it only
implements these two interfaces, and does its own thing. Another example of a
code generator like this is a (purely hypothetical) backend that converts LLVM
to the GCC RTL form and uses GCC to emit machine code for a target.</p>
<p>This design also implies that it is possible to design and
implement radically different code generators in the LLVM system that do not
make use of any of the built-in components. Doing so is not recommended at all,
but could be required for radically different targets that do not fit into the
LLVM machine description model: programmable FPGAs for example.</p>
<p><b>Important Note:</b> For historical reasons, the LLVM SparcV9 code
generator uses almost entirely different code paths than described in this
document. For this reason, there are some deprecated interfaces (such as
<tt>TargetRegInfo</tt> and <tt>TargetSchedInfo</tt>), which are only used by the
V9 backend and should not be used by any other targets. Also, all code in the
<tt>lib/Target/SparcV9</tt> directory and subdirectories should be considered
deprecated, and should not be used as the basis for future code generator work.
The SparcV9 backend is slowly being merged into the rest of the
target-independent code generators, but this is a low-priority process with no
predictable completion date.</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="high-level-design">The high-level design of the code generator</a>
</div>
<div class="doc_text">
<p>The LLVM target-indendent code generator is designed to support efficient and
quality code generation for standard register-based microprocessors. Code
generation in this model is divided into the following stages:</p>
<ol>
<li><b>Instruction Selection</b> - Determining an efficient implementation of the
input LLVM code in the target instruction set. This stage produces the initial
code for the program in the target instruction set, then makes use of virtual
registers in SSA form and physical registers that represent any required
register assignments due to target constraints or calling conventions.</li>
<li><b>SSA-based Machine Code Optimizations</b> - This (optional) stage consists
of a series of machine-code optimizations that operate on the SSA-form produced
by the instruction selector. Optimizations like modulo-scheduling, normal
scheduling, or peephole optimization work here.</li>
<li><b>Register Allocation</b> - The target code is transformed from an infinite
virtual register file in SSA form to the concrete register file used by the
target. This phase introduces spill code and eliminates all virtual register
references from the program.</li>
<li><b>Prolog/Epilog Code Insertion</b> - Once the machine code has been
generated for the function and the amount of stack space required is known (used
for LLVM alloca's and spill slots), the prolog and epilog code for the function
can be inserted and "abstract stack location references" can be eliminated.
This stage is responsible for implementing optimizations like frame-pointer
elimination and stack packing.</li>
<li><b>Late Machine Code Optimizations</b> - Optimizations that operate on
"final" machine code can go here, such as spill code scheduling and peephole
optimizations.</li>
<li><b>Code Emission</b> - The final stage actually outputs the code for
the current function, either in the target assembler format or in machine
code.</li>
</ol>
<p>
The code generator is based on the assumption that the instruction selector will
use an optimal pattern matching selector to create high-quality sequences of
native instructions. Alternative code generator designs based on pattern
expansion and
aggressive iterative peephole optimization are much slower. This design
permits efficient compilation (important for JIT environments) and
aggressive optimization (used when generating code offline) by allowing
components of varying levels of sophisication to be used for any step of
compilation.</p>
<p>
In addition to these stages, target implementations can insert arbitrary
target-specific passes into the flow. For example, the X86 target uses a
special pass to handle the 80x87 floating point stack architecture. Other
targets with unusual requirements can be supported with custom passes as needed.
</p>
</div>
<!-- ======================================================================= -->
<div class="doc_subsection">
<a name="tablegen">Using TableGen for target description</a>
</div>
<div class="doc_text">
<p>The target description classes require a detailed description of the target
architecture. These target descriptions often have a large amount of common
information (e.g., an add instruction is almost identical to a sub instruction).
In order to allow the maximum amount of commonality to be factored out, the LLVM
code generator uses the <a href="TableGenFundamentals.html">TableGen</a> tool to
describe big chunks of the target machine, which allows the use of domain- and
target-specific abstractions to reduce the amount of repetition.
</p>
</div>
<!-- *********************************************************************** -->
<div class="doc_section">
<a name="targetdesc">Target description classes</a>
</div>
<!-- *********************************************************************** -->
<div class="doc_text">
<p>The LLVM target description classes (which are located in the
<tt>include/llvm/Target</tt> directory) provide an abstract description of the
target machine, independent of any particular client. These classes are
designed to capture the <i>abstract</i> properties of the target (such as what
instruction and registers it has), and do not incorporate any particular pieces
of code generation algorithms (these interfaces do not take interference graphs
as inputs or other algorithm-specific
|
