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+ <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>
+
+<div class="doc_text">
+ <p><b>Written by Brian R. Gaeke</b></p>
+</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
+should help you understand -- from our point of view -- the way we see LLVM as
+being better than and/or different from a few other selected compilers and code
+generation systems.</p>
+
+<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
+compilation frameworks. (See our tech report on <a
+href="http://llvm.cs.uiuc.edu/pubs/2003-09-30-LifelongOptimizationTR.html">Lifelong
+Code Optimization</a> for more.)</p>
+</div>
+
+<p>GCC: Many relatively mature platform backends support assembly-language code
+generation from many source languages. No run-time compilation
+support. Relatively weak optimization support.</p>
+
+<!-- *********************************************************************** -->
+<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
+apparently necessary, but casting "int" to "void *" would not be.) No floating
+point (?!)</p>
+
+<p>LLVM: Compositional type system based on C types, supporting structures,
+opaque types, and C integer and floating point types.</p>
+
+<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>
+ <a href="http://llvm.cs.uiuc.edu">The LLVM Compiler Infrastructure</a>
+ <br>
+ Last modified: $Date$
+</div>
+
+</body>
+</html>
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