2003-11-12 21:19:40 +01:00
|
|
|
<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN"
|
|
|
|
|
"http://www.w3.org/TR/html4/strict.dtd">
|
|
|
|
|
<html>
|
|
|
|
|
<head>
|
|
|
|
|
<link rel="stylesheet" href="llvm.css" type="text/css">
|
|
|
|
|
<title>LLVM vs. the World - Comparing Compilers to Compilers</title>
|
|
|
|
|
</head>
|
|
|
|
|
|
|
|
|
|
<body>
|
|
|
|
|
|
|
|
|
|
<div class="doc_title">
|
|
|
|
|
LLVM vs. the World - Comparing Compilers to Compilers
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<ol>
|
|
|
|
|
<li><a href="#introduction">Introduction</a></li>
|
|
|
|
|
<li><a href="#generalapplicability">General Applicability</a></li>
|
|
|
|
|
<li><a href="#typesystem">Type System</a></li>
|
|
|
|
|
<li><a href="#dataflowinformation">Control-flow and Data-flow Information</a></li>
|
|
|
|
|
<li><a href="#registers">Registers</a></li>
|
|
|
|
|
<li><a href="#programmerinterface">Programmer Interface</a></li>
|
|
|
|
|
<li><a href="#codeemission">Machine Code Emission</a></li>
|
|
|
|
|
</ol>
|
|
|
|
|
|
2004-05-23 23:07:27 +02:00
|
|
|
<div class="doc_author">
|
|
|
|
|
<p>Written by Brian R. Gaeke</p>
|
2003-11-12 21:19:40 +01:00
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="introduction">Introduction</a>
|
|
|
|
|
</div>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>Whether you are a stranger to LLVM or not, and whether you are considering
|
|
|
|
|
using it for your projects or not, you may find it useful to understand how we
|
|
|
|
|
compare ourselves to other well-known compilers. The following list of points
|
2003-11-12 22:36:29 +01:00
|
|
|
should help you understand -- from our point of view -- some of the important
|
|
|
|
|
ways in which we see LLVM as different from other selected compilers and
|
|
|
|
|
code generation systems.</p>
|
2003-11-12 21:19:40 +01:00
|
|
|
|
|
|
|
|
<p>At the moment, we only compare ourselves below to <a
|
|
|
|
|
href="http://gcc.gnu.org/">GCC</a> and <a
|
|
|
|
|
href="http://www.gnu.org/software/lightning/">GNU lightning</a>, but we will try
|
|
|
|
|
to revise and expand it as our knowledge and experience permit. Contributions are
|
|
|
|
|
welcome.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="generalapplicability">General Applicability</a>
|
|
|
|
|
</div>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>GNU lightning: Only currently usable for dynamic runtime emission of binary
|
|
|
|
|
machine code to memory. Supports one backend at a time.</p>
|
|
|
|
|
|
|
|
|
|
<p>LLVM: Supports compilation of C and C++ (with more languages coming soon),
|
|
|
|
|
strong SSA-based optimization at compile-time, link-time, run-time, and
|
|
|
|
|
off-line, and multiple platform backends with Just-in-Time and ahead-of-time
|
2004-02-22 06:45:02 +01:00
|
|
|
compilation frameworks. (See our document on <a
|
2006-03-14 06:39:39 +01:00
|
|
|
href="http://llvm.org/pubs/2004-01-30-CGO-LLVM.html">Lifelong
|
2003-11-12 21:19:40 +01:00
|
|
|
Code Optimization</a> for more.)</p>
|
|
|
|
|
|
|
|
|
|
<p>GCC: Many relatively mature platform backends support assembly-language code
|
|
|
|
|
generation from many source languages. No run-time compilation
|
2004-03-09 08:20:26 +01:00
|
|
|
support.</p>
|
2003-11-12 21:20:55 +01:00
|
|
|
</div>
|
2003-11-12 21:19:40 +01:00
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="typesystem">Type System</a>
|
|
|
|
|
</div>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>GNU lightning: C integer types and "void *" are supported. No type checking
|
|
|
|
|
is performed. Explicit type casts are not typically necessary unless the
|
|
|
|
|
underlying machine-specific types are distinct (e.g., sign- or zero-extension is
|
2003-11-12 22:38:50 +01:00
|
|
|
apparently necessary, but casting "int" to "void *" would not be.)
|
|
|
|
|
Floating-point support may not work on all platforms (it does not appear to be
|
|
|
|
|
documented in the latest release).</p>
|
2003-11-12 21:19:40 +01:00
|
|
|
|
|
|
|
|
<p>LLVM: Compositional type system based on C types, supporting structures,
|
2003-11-12 22:39:31 +01:00
|
|
|
opaque types, and C integer and floating point types. Explicit cast instructions
|
|
|
|
|
are required to transform a value from one type to another.</p>
|
2003-11-12 21:19:40 +01:00
|
|
|
|
|
|
|
|
<p>GCC: Union of high-level types including those used in Pascal, C, C++, Ada,
|
|
|
|
|
Java, and FORTRAN.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="dataflowinformation">Control-flow and Data-flow Information</a>
|
|
|
|
|
</div>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>GNU lightning: No data-flow information encoded in the generated program. No
|
|
|
|
|
support for calculating CFG or def-use chains over generated programs.</p>
|
|
|
|
|
|
|
|
|
|
<p>LLVM: Scalar values in Static Single-Assignment form; def-use chains and CFG
|
|
|
|
|
always implicitly available and automatically kept up to date.</p>
|
|
|
|
|
|
|
|
|
|
<p>GCC: Trees and RTL do not directly encode data-flow info; but def-use chains
|
|
|
|
|
and CFGs can be calculated on the side. They are not automatically kept up to
|
|
|
|
|
date.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="registers">Registers</a>
|
|
|
|
|
</div>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>GNU lightning: Very small fixed register set -- it takes the least common
|
|
|
|
|
denominator of supported platforms; basically it inherits its tiny register set
|
|
|
|
|
from IA-32, unnecessarily crippling targets like PowerPC with a large register
|
|
|
|
|
set.</p>
|
|
|
|
|
|
|
|
|
|
<p>LLVM: An infinite register set, reduced to a particular platform's finite
|
|
|
|
|
register set by register allocator.</p>
|
|
|
|
|
|
|
|
|
|
<p>GCC: Trees and RTL provide an arbitrarily large set of values. Reduced to a
|
|
|
|
|
particular platform's finite register set by register allocator.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="programmerinterface">Programmer Interface</a>
|
|
|
|
|
</div>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>GNU lightning: Library interface based on C preprocessor macros that emit
|
|
|
|
|
binary code for a particular instruction to memory. No support for manipulating
|
|
|
|
|
code before emission.</p>
|
|
|
|
|
|
|
|
|
|
<p>LLVM: Library interface based on classes representing platform-independent
|
|
|
|
|
intermediate code (Instruction) and platform-dependent code (MachineInstr) which
|
|
|
|
|
can be manipulated arbitrarily and then emitted to memory.</p>
|
|
|
|
|
|
|
|
|
|
<p>GCC: Internal header file interface (tree.h) to abstract syntax trees,
|
|
|
|
|
representing roughly the union of all possible supported source-language
|
|
|
|
|
constructs; also, an internal header file interface (rtl.h, rtl.def) to a
|
|
|
|
|
low-level IR called RTL which represents roughly the union of all possible
|
|
|
|
|
target machine instructions.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
<div class="doc_section">
|
|
|
|
|
<a name="codeemission">Machine Code Emission</a>
|
|
|
|
|
</div>
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<div class="doc_text">
|
|
|
|
|
<p>GNU lightning: Only supports binary machine code emission to memory.</p>
|
|
|
|
|
|
|
|
|
|
<p>LLVM: Supports writing out assembly language to a file, and binary machine
|
|
|
|
|
code to memory, from the same back-end.</p>
|
|
|
|
|
|
|
|
|
|
<p>GCC: Supports writing out assembly language to a file. No support for
|
|
|
|
|
emitting machine code to memory.</p>
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
<!-- *********************************************************************** -->
|
|
|
|
|
|
|
|
|
|
<hr>
|
|
|
|
|
<div class="doc_footer">
|
|
|
|
|
<address>Brian R. Gaeke</address>
|
2006-03-14 06:39:39 +01:00
|
|
|
<a href="http://llvm.org">The LLVM Compiler Infrastructure</a>
|
2003-11-12 21:19:40 +01:00
|
|
|
<br>
|
|
|
|
|
Last modified: $Date$
|
|
|
|
|
</div>
|
|
|
|
|
|
|
|
|
|
</body>
|
|
|
|
|
</html>
|
