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