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6 <title>LLVM vs. the World - Comparing Compilers to Compilers</title>
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11<div class="doc_title">
12 LLVM vs. the World - Comparing Compilers to Compilers
13</div>
14
15<ol>
16 <li><a href="#introduction">Introduction</a></li>
17 <li><a href="#generalapplicability">General Applicability</a></li>
18 <li><a href="#typesystem">Type System</a></li>
19 <li><a href="#dataflowinformation">Control-flow and Data-flow Information</a></li>
20 <li><a href="#registers">Registers</a></li>
21 <li><a href="#programmerinterface">Programmer Interface</a></li>
22 <li><a href="#codeemission">Machine Code Emission</a></li>
23</ol>
24
25<div class="doc_text">
26 <p><b>Written by Brian R. Gaeke</b></p>
27</div>
28
29<!-- *********************************************************************** -->
30<div class="doc_section">
31 <a name="introduction">Introduction</a>
32</div>
33<!-- *********************************************************************** -->
34
35<div class="doc_text">
36<p>Whether you are a stranger to LLVM or not, and whether you are considering
37using it for your projects or not, you may find it useful to understand how we
38compare ourselves to other well-known compilers. The following list of points
39should help you understand -- from our point of view -- the way we see LLVM as
40being better than and/or different from a few other selected compilers and code
41generation systems.</p>
42
43<p>At the moment, we only compare ourselves below to <a
44href="http://gcc.gnu.org/">GCC</a> and <a
45href="http://www.gnu.org/software/lightning/">GNU lightning</a>, but we will try
46to revise and expand it as our knowledge and experience permit. Contributions are
47welcome.</p>
48</div>
49
50<!-- *********************************************************************** -->
51<div class="doc_section">
52 <a name="generalapplicability">General Applicability</a>
53</div>
54<!-- *********************************************************************** -->
55
56<div class="doc_text">
57<p>GNU lightning: Only currently usable for dynamic runtime emission of binary
58machine code to memory. Supports one backend at a time.</p>
59
60<p>LLVM: Supports compilation of C and C++ (with more languages coming soon),
61strong SSA-based optimization at compile-time, link-time, run-time, and
62off-line, and multiple platform backends with Just-in-Time and ahead-of-time
63compilation frameworks. (See our tech report on <a
64href="http://llvm.cs.uiuc.edu/pubs/2003-09-30-LifelongOptimizationTR.html">Lifelong
65Code Optimization</a> for more.)</p>
66</div>
67
68<p>GCC: Many relatively mature platform backends support assembly-language code
69generation from many source languages. No run-time compilation
70support. Relatively weak optimization support.</p>
71
72<!-- *********************************************************************** -->
73<div class="doc_section">
74 <a name="typesystem">Type System</a>
75</div>
76<!-- *********************************************************************** -->
77
78<div class="doc_text">
79<p>GNU lightning: C integer types and "void *" are supported. No type checking
80is performed. Explicit type casts are not typically necessary unless the
81underlying machine-specific types are distinct (e.g., sign- or zero-extension is
82apparently necessary, but casting "int" to "void *" would not be.) No floating
83point (?!)</p>
84
85<p>LLVM: Compositional type system based on C types, supporting structures,
86opaque types, and C integer and floating point types.</p>
87
88<p>GCC: Union of high-level types including those used in Pascal, C, C++, Ada,
89Java, and FORTRAN.</p>
90</div>
91
92<!-- *********************************************************************** -->
93<div class="doc_section">
94 <a name="dataflowinformation">Control-flow and Data-flow Information</a>
95</div>
96<!-- *********************************************************************** -->
97
98<div class="doc_text">
99<p>GNU lightning: No data-flow information encoded in the generated program. No
100support for calculating CFG or def-use chains over generated programs.</p>
101
102<p>LLVM: Scalar values in Static Single-Assignment form; def-use chains and CFG
103always implicitly available and automatically kept up to date.</p>
104
105<p>GCC: Trees and RTL do not directly encode data-flow info; but def-use chains
106and CFGs can be calculated on the side. They are not automatically kept up to
107date.</p>
108</div>
109
110<!-- *********************************************************************** -->
111<div class="doc_section">
112 <a name="registers">Registers</a>
113</div>
114<!-- *********************************************************************** -->
115
116<div class="doc_text">
117<p>GNU lightning: Very small fixed register set -- it takes the least common
118denominator of supported platforms; basically it inherits its tiny register set
119from IA-32, unnecessarily crippling targets like PowerPC with a large register
120set.</p>
121
122<p>LLVM: An infinite register set, reduced to a particular platform's finite
123register set by register allocator.</p>
124
125<p>GCC: Trees and RTL provide an arbitrarily large set of values. Reduced to a
126particular platform's finite register set by register allocator.</p>
127</div>
128
129<!-- *********************************************************************** -->
130<div class="doc_section">
131 <a name="programmerinterface">Programmer Interface</a>
132</div>
133<!-- *********************************************************************** -->
134
135<div class="doc_text">
136<p>GNU lightning: Library interface based on C preprocessor macros that emit
137binary code for a particular instruction to memory. No support for manipulating
138code before emission.</p>
139
140<p>LLVM: Library interface based on classes representing platform-independent
141intermediate code (Instruction) and platform-dependent code (MachineInstr) which
142can be manipulated arbitrarily and then emitted to memory.</p>
143
144<p>GCC: Internal header file interface (tree.h) to abstract syntax trees,
145representing roughly the union of all possible supported source-language
146constructs; also, an internal header file interface (rtl.h, rtl.def) to a
147low-level IR called RTL which represents roughly the union of all possible
148target machine instructions.</p>
149</div>
150
151<!-- *********************************************************************** -->
152<div class="doc_section">
153 <a name="codeemission">Machine Code Emission</a>
154</div>
155<!-- *********************************************************************** -->
156
157<div class="doc_text">
158<p>GNU lightning: Only supports binary machine code emission to memory.</p>
159
160<p>LLVM: Supports writing out assembly language to a file, and binary machine
161code to memory, from the same back-end.</p>
162
163<p>GCC: Supports writing out assembly language to a file. No support for
164emitting machine code to memory.</p>
165</div>
166
167<!-- *********************************************************************** -->
168
169<hr>
170<div class="doc_footer">
171 <address>Brian R. Gaeke</address>
172 <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
173 <br>
174 Last modified: $Date$
175</div>
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