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511 lines
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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
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<title>The LLVM Compiler Driver (llvmc)</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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<style type="text/css">
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TR, TD { border: 2px solid gray; padding: 4pt 4pt 2pt 2pt; }
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TH { border: 2px solid gray; font-weight: bold; font-size: 105%; }
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TABLE { text-align: center; border: 2px solid black;
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.td_left { border: 2px solid gray; text-align: left; }
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</style>
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<meta name="author" content="Reid Spencer" name="author">
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<meta name="description"
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content="A description of the use and design of the LLVM Compiler Driver.">
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</head>
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<body>
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<div class="doc_title">The LLVM Compiler Driver (llvmc)</div>
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<p class="doc_warning">NOTE: This document is a work in progress!</p>
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<ol>
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<li><a href="#abstract">Abstract</a></li>
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<li><a href="#introduction">Introduction</a>
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<ol>
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<li><a href="#purpose">Purpose</a></li>
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<li><a href="#operation">Operation</a></li>
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<li><a href="#phases">Phases</a></li>
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<li><a href="#actions">Actions</a></li>
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</ol>
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</li>
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<li><a href="#details">Details</a>
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<li><a href="#configuration">Configuration</a>
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<li><a href="#glossary">Glossary</a>
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</ol>
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<div class="doc_author">
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<p>Written by <a href="mailto:rspencer@x10sys.com">Reid Spencer</a>
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</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="abstract">Abstract</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This document describes the requirements, design, and configuration of the
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LLVM compiler driver, <tt>llvmc</tt>. The compiler driver knows about LLVM's
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tool set and can be configured to know about a variety of compilers for
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source languages. It uses this knowledge to execute the tools necessary
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to accomplish general compilation, optimization, and linking tasks. The main
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purpose of <tt>llvmc</tt> is to provide a simple and consistent interface to
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all compilation tasks. This reduces the burden on the end user who can just
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learn to use <tt>llvmc</tt> instead of the entire LLVM tool set and all the
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source language compilers compatible with LLVM.</p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"> <a name="introduction">Introduction</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>The <tt>llvmc</tt> <a href="def_tool">tool</a> is a configurable compiler
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<a href="def_driver">driver</a>. As such, it isn't the compiler, optimizer,
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or linker itself but it drives (invokes) other software that perform those
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tasks. If you are familiar with the GNU Compiler Collection's <tt>gcc</tt>
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tool, <tt>llvmc</tt> is very similar.</p>
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<p>The following introductory sections will help you understand why this tool
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is necessary and what it does.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="purpose">Purpose</a></div>
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<div class="doc_text">
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<p><tt>llvmc</tt> was invented to make compilation with LLVM based compilers
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easier. To accomplish this, <tt>llvmc</tt> strives to:</p>
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<ul>
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<li>Be the single point of access to most of the LLVM tool set.</li>
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<li>Hide the complexities of the LLVM tools through a single interface.</li>
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<li>Provide a consistent interface for compiling all languages.</li>
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</ul>
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<p>Additionally, <tt>llvmc</tt> makes it easier to write a compiler for use
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with LLVM, because it:</p>
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<ul>
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<li>Makes integration of existing non-LLVM tools simple.</li>
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<li>Extends the capabilities of minimal front ends by optimizing their
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output.</li>
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<li>Reduces the number of interfaces a compiler writer must know about
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before a working compiler can be completed (essentially only the VMCore
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interfaces need to be understood).</li>
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<li>Supports source language translator invocation via both dynamically
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loadable shared objects and invocation of an executable.</li>
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</ol>
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</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="operation">Operation</a></div>
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<div class="doc_text">
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<p>At a high level, <tt>llvmc</tt> operation is very simple. The basic action
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taken by <tt>llvmc</tt> is to simply invoke some tool or set of tools to fill
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the user's request for compilation. Every execution of <tt>llvmc</tt>takes the
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following sequence of steps:<br/>
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<dl>
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<dt><b>Collect Command Line Options</b></dt>
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<dd>The command line options provide the marching orders to <tt>llvmc</tt>
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on what actions it should perform. This is the request the user is making
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of <tt>llvmc</tt> and it is interpreted first. See the <tt>llvmc</tt>
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<a href="CommandGuide/html/llvmc.html">manual page</a> for details on the
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options.</dd>
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<dt><b>Read Configuration Files</b></dt>
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<dd>Based on the options and the suffixes of the filenames presented, a set
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of configuration files are read to configure the actions <tt>llvmc</tt> will
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take. Configuration files are provided by either LLVM or the front end
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compiler tools that B<llvmc> invokes. These files determine what actions
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<tt>llvmc</tt> will take in response to the user's request. See the section
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on <a href="#configuration">configuration</a> for more details.</dd>
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<dt><b>Determine Phases To Execute</b></dt>
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<dd>Based on the command line options and configuration files,
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<tt>llvmc</tt> determines the compilation <a href="#phases">phases</a> that
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must be executed by the user's request. This is the primary work of
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<tt>llvmc</tt>.</dd>
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<dt><b>Determine Actions To Execute</b></dt>
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<dd>Each <a href="#phases">phase</a> to be executed can result in the
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invocation of one or more <a href="#actions">actions</a>. An action is
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either a whole program or a function in a dynamically linked shared library.
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In this step, <tt>llvmc</tt> determines the sequence of actions that must be
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executed. Actions will always be executed in a deterministic order.</dd>
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<dt><b>Execute Actions</b></dt>
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<dd>The <a href="#actions">actions</a> necessary to support the user's
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original request are executed sequentially and deterministically. All
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actions result in either the invocation of a whole program to perform the
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action or the loading of a dynamically linkable shared library and invocation
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of a standard interface function within that library.</dd>
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<dt><b>Termination</b></dt>
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<dd>If any action fails (returns a non-zero result code), <tt>llvmc</tt>
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also fails and returns the result code from the failing action. If
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everything succeeds, <tt>llvmc</tt> will return a zero result code.</dd>
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</dl></p>
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<p><tt>llvmc</tt>'s operation must be simple, regular and predictable.
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Developers need to be able to rely on it to take a consistent approach to
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compilation. For example, the invocation:</p>
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<tt><pre>
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llvmc -O2 x.c y.c z.c -o xyz</pre></tt>
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<p>must produce <i>exactly</i> the same results as:</p>
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<tt><pre>
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llvmc -O2 x.c
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llvmc -O2 y.c
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llvmc -O2 z.c
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llvmc -O2 x.o y.o z.o -o xyz</pre></tt>
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<p>To accomplish this, <tt>llvmc</tt> uses a very simple goal oriented
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procedure to do its work. The overall goal is to produce a functioning
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executable. To accomplish this, <tt>llvmc</tt> always attempts to execute a
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series of compilation <a href="#def_phase">phases</a> in the same sequence.
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However, the user's options to <tt>llvmc</tt> can cause the sequence of phases
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to start in the middle or finish early.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="phases"></a>Phases </div>
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<div class="doc_text">
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<p><tt>llvmc</tt> breaks every compilation task into the following five
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distinct phases:</p>
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<dl><dt><b>Preprocessing</b></dt><dd>Not all languages support preprocessing;
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but for those that do, this phase can be invoked. This phase is for
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languages that provide combining, filtering, or otherwise altering with the
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source language input before the translator parses it. Although C and C++
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are the most common users of this phase, other languages may provide their
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own preprocessor (whether its the C pre-processor or not).</dd>
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</dl>
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<dl><dt><b>Translation</b></dt><dd>The translation phase converts the source
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language input into something that LLVM can interpret and use for
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downstream phases. The translation is essentially from "non-LLVM form" to
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"LLVM form".</dd>
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</dl>
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<dl><dt><b>Optimization</b></dt><dd>Once an LLVM Module has been obtained from
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the translation phase, the program enters the optimization phase. This phase
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attempts to optimize all of the input provided on the command line according
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to the options provided.</dd>
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</dl>
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<dl><dt><b>Linking</b></dt><dd>The inputs are combined to form a complete
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program.</dd>
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</dl>
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<p>The following table shows the inputs, outputs, and command line options
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applicabe to each phase.</p>
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<table>
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<tr>
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<th style="width: 10%">Phase</th>
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<th style="width: 25%">Inputs</th>
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<th style="width: 25%">Outputs</th>
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<th style="width: 40%">Options</th>
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</tr>
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<tr><td><b>Preprocessing</b></td>
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<td class="td_left"><ul><li>Source Language File</li></ul></td>
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<td class="td_left"><ul><li>Source Language File</li></ul></td>
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<td class="td_left"><dl>
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<dt><tt>-E</tt></dt>
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<dd>Stops the compilation after preprocessing</dd>
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</dl></td>
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</tr>
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<tr>
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<td><b>Translation</b></td>
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<td class="td_left"><ul>
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<li>Source Language File</li>
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</ul></td>
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<td class="td_left"><ul>
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<li>LLVM Assembly</li>
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<li>LLVM Bytecode</li>
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<li>LLVM C++ IR</li>
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</ul></td>
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<td class="td_left"><dl>
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<dt><tt>-c</tt></dt>
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<dd>Stops the compilation after translation so that optimization and
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linking are not done.</dd>
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<dt><tt>-S</tt></dt>
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<dd>Stops the compilation before object code is written so that only
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assembly code remains.</dd>
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</dl></td>
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</tr>
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<tr>
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<td><b>Optimization</b></td>
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<td class="td_left"><ul>
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<li>LLVM Assembly</li>
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<li>LLVM Bytecode</li>
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</ul></td>
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<td class="td_left"><ul>
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<li>LLVM Bytecode</li>
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</ul></td>
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<td class="td_left"><dl>
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<dt><tt>-Ox</tt>
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<dd>This group of options affects the amount of optimization
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performed.</dd>
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</dl></td>
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</tr>
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<tr>
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<td><b>Linking</b></td>
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<td class="td_left"><ul>
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<li>LLVM Bytecode</li>
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<li>Native Object Code</li>
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<li>LLVM Library</li>
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<li>Native Library</li>
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</ul></td>
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<td class="td_left"><ul>
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<li>LLVM Bytecode Executable</li>
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<li>Native Executable</li>
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</ul></td>
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<td class="td_left"><dl>
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<dt><tt>-L</tt></dt><dd>Specifies a path for library search.</dd>
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<dt><tt>-l</tt></dt><dd>Specifies a library to link in.</dd>
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</dl></td>
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</tr>
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</table>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="actions"></a>Actions</div>
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<div class="doc_text">
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<p>An action, with regard to <tt>llvmc</tt> is a basic operation that it takes
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in order to fulfill the user's request. Each phase of compilation will invoke
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zero or more actions in order to accomplish that phase.</p>
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<p>Actions come in two forms:<ol>
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<li>Invokable Executables</li>
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<li>Functions in a shared library</li>
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</ul></p>
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="details">Details</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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</div>
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<!-- *********************************************************************** -->
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<div class="doc_section"><a name="configuration">Configuration</a></div>
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<!-- *********************************************************************** -->
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<div class="doc_text">
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<p>This section of the document describes the configuration files used by
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<tt>llvmc</tt>. Configuration information is relatively static for a
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given release of LLVM and a front end compiler. However, the details may
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change from release to release of either. Users are encouraged to simply use
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the various options of the B<llvmc> command and ignore the configuration of
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the tool. These configuration files are for compiler writers and LLVM
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developers. Those wishing to simply use B<llvmc> don't need to understand
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this section but it may be instructive on how the tool works.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="overview"></a>Overview</div>
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<div class="doc_text">
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<p><tt>llvmc</tt> is highly configurable both on the command line and in
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configuration files. The options it understands are generic, consistent and
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simple by design. Furthermore, the <tt>llvmc</tt> options apply to the
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compilation of any LLVM enabled programming language. To be enabled as a
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supported source language compiler, a compiler writer must provide a
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configuration file that tells <tt>llvmc</tt> how to invoke the compiler
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and what its capabilities are. The purpose of the configuration files then
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is to allow compiler writers to specify to <tt>llvmc</tt> how the compiler
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should be invoked. Users may but are not advised to alter the compiler's
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<tt>llvmc</tt> configuration.</p>
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<p>Because <tt>llvmc</tt> just invokes other programs, it must deal with the
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available command line options for those programs regardless of whether they
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were written for LLVM or not. Furthermore, not all compilation front ends will
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have the same capabilities. Some front ends will simply generate LLVM assembly
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code, others will be able to generate fully optimized byte code. In general,
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<tt>llvmc</tt> doesn't make any assumptions about the capabilities or command
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line options of a sub-tool. It simply uses the details found in the configuration
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files and leaves it to the compiler writer to specify the configuration
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correctly.</p>
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<p>This approach means that new compiler front ends can be up and working very
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quickly. As a first cut, a front end can simply compile its source to raw
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(unoptimized) bytecode or LLVM assembly and <tt>llvmc</tt> can be configured
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to pick up the slack (translate LLVM assembly to bytecode, optimize the
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bytecode, generate native assembly, link, etc.). In fact, the front end need
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not use any LLVM libraries, and it could be written in any language (instead of
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C++). The configuration data will allow the full range of optimization,
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assembly, and linking capabilities that LLVM provides to be added to these kinds
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of tools. Enabling the rapid development of front-ends is one of the primary
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goals of <tt>llvmc</tt>.</p>
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<p>As a compiler front end matures, it may utilize the LLVM libraries and tools
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to more efficiently produce optimized bytecode directly in a single compilation
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and optimization program. In these cases, multiple tools would not be needed
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and the configuration data for the compiler would change.</p>
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<p>Configuring <tt>llvmc</tt> to the needs and capabilities of a source language
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compiler is relatively straight forward. A compiler writer must provide a
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definition of what to do for each of the five compilation phases for each of
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the optimization levels. The specification consists simply of prototypical
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command lines into which <tt>llvmc</tt> can substitute command line
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arguments and file names. Note that any given phase can be completely blank if
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the source language's compiler combines multiple phases into a single program.
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For example, quite often pre-processing, translation, and optimization are
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combined into a single program. The specification for such a compiler would have
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blank entries for pre-processing and translation but a full command line for
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optimization.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="filetypes"></a>Configuration Files</div>
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<div class="doc_text">
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<h3>File Types</h3>
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<p>There are two types of configuration files: the master configuration file
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and the language specific configuration file. The master configuration file
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contains the general configuration of <tt>llvmc</tt> itself and is supplied
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with the tool. It contains information that is source language agnostic.
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Language specific configuration files tell <tt>llvmc</tt> how to invoke the
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language's compiler for a variety of different tasks and what other tools
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are needed to backfill the compiler's missing features (e.g.
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optimization).</p>
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<h3>Directory Search</h3>
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<p><tt>llvmc</tt> always looks for files of a specific name. It uses the
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first file with the name its looking for by searching directories in the
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following order:<br/>
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<ol>
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<li>Any directory specified by the <tt>--config-dir</tt> option will be
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checked first.</li>
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<li>If the environment variable LLVM_CONFIG_DIR is set, and it contains
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the name of a valid directory, that directory will be searched next.</li>
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<li>If the user's home directory (typically <tt>/home/user</tt> contains
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a sub-directory named <tt>.llvm</tt> and that directory contains a
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sub-directory named <tt>etc</tt> then that directory will be tried
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next.</li>
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<li>If the LLVM installation directory (typically <tt>/usr/local/llvm</tt>
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contains a sub-directory named <tt>etc</tt> then that directory will be
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tried last.</li>
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<li>If the configuration file sought still can't be found, <tt>llvmc</tt>
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will print an error message and exit.</li>
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</ol>
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The first file found in this search will be used. Other files with the same
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name will be ignored even if they exist in one of the subsequent search
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locations.</p>
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<h3>File Names</h3>
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<p>In the directories searched, a file named <tt>master</tt> will be
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recognized as the master configuration file for <tt>llvmc</tt>. Note that
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users <i>may</i> override the master file with a copy in their home directory
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but they are advised not to. This capability is only useful for compiler
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implementers needing to alter the master configuration while developing
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their compiler front end. When reading the configuration files, the master
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files are always read first.</p>
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<p>Language specific configuration files are given specific names to foster
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faster lookup. The name of a given language specific configuration file is
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the same as the suffix used to identify files containing source in that
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language. For example, a configuration file for C++ source might be named
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<tt>cpp</tt>, <tt>C</tt>, or <tt>cxx</tt>.</p>
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<h3>What Gets Read</h3>
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<p>The master configuration file is always read. Which language specific
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configuration files are read depends on the command line options and the
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suffixes of the file names provided on <tt>llvmc</tt>'s command line. Note
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that the <tt>--x LANGUAGE</tt> option alters the language that <tt>llvmc</tt>
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uses for the subsequent files on the command line. Only the language
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specific configuration files actually needed to complete <tt>llvmc</tt>'s
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task are read. Other language specific files will be ignored.</p>
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</div>
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<!-- _______________________________________________________________________ -->
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<div class="doc_subsection"><a name="syntax"></a>Syntax</div>
|
|
<div class="doc_text">
|
|
<p>The syntax of the configuration files is yet to be determined. There are
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two viable options remaining:<br/>
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<ul>
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|
<li>XML DTD Specific To <tt>llvmc</tt></li>
|
|
<li>Windows .ini style file with numerous sections</li>
|
|
</ul></p>
|
|
</div>
|
|
|
|
<!-- _______________________________________________________________________ -->
|
|
<div class="doc_subsection"><a name="master_items">Configuration Items</a></div>
|
|
<div class="doc_text">
|
|
<p>The following description of configuration items is syntax-less and simply
|
|
uses a naming hierarchy to describe the configuration items. Whatever
|
|
syntax is chosen will need to map the hierarchy to the given syntax.</p>
|
|
<table>
|
|
<tr>
|
|
<th>Name</th>
|
|
<th>Value Type</th>
|
|
<th>Description</th>
|
|
</tr>
|
|
<tr>
|
|
<td><b>Capabilities.hasPreProcessor</b></td>
|
|
<td>boolean</td>
|
|
<td class="td_left">This item specifies whether the language has a
|
|
pre-processing phase or not. This controls whether the B<-E> option works
|
|
for the language or not.</td>
|
|
</tr>
|
|
<tr>
|
|
<td><b>Capabilities.outputFormat</b></td>
|
|
<td>"bc" or "ll"</td>
|
|
<td class="td_left">This item specifies the kind of output the language's
|
|
compiler generates. The choices are either bytecode (<tt>bc</tt>) or LLVM
|
|
assembly (<tt>ll</tt>).</td>
|
|
</tr>
|
|
<tr>
|
|
<td><b>Capabilities.understandsOptimization</b></td>
|
|
<td>boolean</td>
|
|
<td>Indicates whether the compiler for this language understands the
|
|
<tt>-O</tt> options or not</td>
|
|
</tr>
|
|
</table>
|
|
</div>
|
|
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_section"><a name="glossary">Glossary</a></div>
|
|
<!-- *********************************************************************** -->
|
|
<div class="doc_text">
|
|
<p>This document uses precise terms in reference to the various artifacts and
|
|
concepts related to compilation. The terms used throughout this document are
|
|
defined below.</p>
|
|
<dl>
|
|
<dt><a name="def_assembly"><b>assembly</b></a></dt>
|
|
<dd>A compilation <a href="#def_phase">phase</a> in which LLVM bytecode or
|
|
LLVM assembly code is assembled to a native code format (either target
|
|
specific aseembly language or the platform's native object file format).
|
|
</dd>
|
|
|
|
<dt><a name="def_compiler"><b>compiler</b></a></dt>
|
|
<dd>Refers to any program that can be invoked by <tt>llvmc</tt> to accomplish
|
|
the work of one or more compilation <a href="#def_phase">phases</a>.</dd>
|
|
|
|
<dt><a name="def_driver"><b>driver</b></a></dt>
|
|
<dd>Refers to <tt>llvmc</tt> itself.</dd>
|
|
|
|
<dt><a name="def_linking"><b>linking</b></a></dt>
|
|
<dd>A compilation <a href="#def_phase">phase</a> in which LLVM bytecode files
|
|
and (optionally) native system libraries are combined to form a complete
|
|
executable program.</dd>
|
|
|
|
<dt><a name="def_optimization"><b>optimization</b></a></dt>
|
|
<dd>A compilation <a href="#def_phase">phase</a> in which LLVM bytecode is
|
|
optimized.</dd>
|
|
|
|
<dt><a name="def_phase"><b>phase</b></a></dt>
|
|
<dd>Refers to any one of the five compilation phases that that
|
|
<tt>llvmc</tt> supports. The five phases are:
|
|
<a href="#def_preprocessing">preprocessing</a>,
|
|
<a href="#def_translation">translation</a>,
|
|
<a href="#def_optimization">optimization</a>,
|
|
<a href="#def_assembly">assembly</a>,
|
|
<a href="#def_linking">linking</a>.</dd>
|
|
|
|
<dt><a name="def_sourcelanguage"><b>source language</b></a></dt>
|
|
<dd>Any common programming language (e.g. C, C++, Java, Stacker, ML,
|
|
FORTRAN). These languages are distinguished from any of the lower level
|
|
languages (such as LLVM or native assembly), by the fact that a
|
|
<a href="#def_translation">translation</a> <a href="#def_phase">phase</a>
|
|
is required before LLVM can be applied.</dd>
|
|
|
|
<dt><a name="def_tool"><b>tool</b></a></dt>
|
|
<dd>Refers to any program in the LLVM tool set.</dd>
|
|
|
|
<dt><a name="def_translation"><b>translation</b></a></dt>
|
|
<dd>A compilation <a href="#def_phase">phase</a> in which
|
|
<a href="#def_sourcelanguage">source language</a> code is translated into
|
|
either LLVM assembly language or LLVM bytecode.</dd>
|
|
</dl>
|
|
</div>
|
|
<!-- *********************************************************************** -->
|
|
<hr>
|
|
<address> <a href="http://jigsaw.w3.org/css-validator/check/referer"><img
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src="http://jigsaw.w3.org/css-validator/images/vcss" alt="Valid CSS!"></a><a
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href="http://validator.w3.org/check/referer"><img
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src="http://www.w3.org/Icons/valid-html401" alt="Valid HTML 4.01!"></a><a
|
|
href="mailto:rspencer@x10sys.com">Reid Spencer</a><br>
|
|
<a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a><br>
|
|
Last modified: $Date$
|
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</address>
|
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