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| 6 | <title>Clang - Universal Driver</title> |
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| 15 | |
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| 16 | <h1>The Clang Universal Driver Project</h1> |
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| 17 | |
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| 18 | <p>Clang is inherently a cross compiler, in that it is always capable of |
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| 19 | building code for targets which are a different architecture or even operating |
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| 20 | system from the one running the compiler. However, actually cross compiling in |
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| 21 | practice involves much more than just generating the right assembly code for a |
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| 22 | target, it also requires having an appropriate tool chain (assemblers, linkers), |
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| 23 | access to header files and libraries for the target, and many other details (for |
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| 24 | example, the calling convention or whether software floating point is in |
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| 25 | use). Traditionally, compilers and development environments provide little |
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| 26 | assistance with this process, so users do not have easy access to the powerful |
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| 27 | underlying cross-compilation abilities of clang.</p> |
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| 28 | |
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| 29 | <p>We would like to solve this problem by defining a new model for how cross |
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| 30 | compilation is done, based on the idea of a <i>universal driver</i>. The key |
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| 31 | point of this model is that the user would always access the compiler through a |
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| 32 | single entry point (e.g., <tt>/usr/bin/cc</tt>) and provide an argument |
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| 33 | specifying the <i>configuration</i> they would like to target. Under the hood |
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| 34 | this entry point (the universal driver) would have access to all the information |
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| 35 | that the driver, compiler, and other tools need to build applications for that |
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| 36 | target.</p> |
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| 37 | |
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| 38 | <p>This is a large and open-ended project. It's eventual success depends not |
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| 39 | just on implementing the model, but also on getting buy-in from compiler |
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| 40 | developers, operating system distribution vendors and the development community |
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| 41 | at large. Our plan is to begin by defining a clear list of the problems we want |
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| 42 | to solve and a proposed implementation (from the user perspective).</p> |
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| 43 | |
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| 44 | <p>This project is in the very early (i.e., thought experiment) stages of |
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| 45 | development. Stay tuned for more information, and of course, patches |
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| 46 | welcome!</p> |
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| 47 | |
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| 48 | <p>See also <a href="http://llvm.org/PR4127">PR4127</a>.</p> |
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| 49 | |
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| 50 | <h2>Existing Solutions and Related Work</h2> |
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| 51 | |
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| 52 | <ul> |
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| 53 | <li>gcc's command line arguments <tt>-V</tt>, <tt>-B</tt>, <tt>-b</tt> are |
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| 54 | generic but limited solutions to related problems. Similarly, <tt>-m32</tt> |
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| 55 | and <tt>-m64</tt> solve a small subset of the problem for specific |
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| 56 | architectures.</li> |
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| 57 | |
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| 58 | <li>gcc's <a href="http://www.airs.com/ian/configure/configure_8.html">multilibs</a> |
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| 59 | solve the part of the problem that relates to finding appropriate libraries |
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| 60 | and include files based on particular feature support (soft float, |
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| 61 | etc.).</li> |
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| 62 | |
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| 63 | <li>Apple's "driver driver" supported by gcc and clang solve a subset of the |
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| 64 | problem by supporting <tt>-arch</tt>. Apple also provides a tool chain which |
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| 65 | supports <a href="http://en.wikipedia.org/wiki/Universal_binary">universal |
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| 66 | binaries</a> and object files which may include data for multiple |
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| 67 | architectures. See <a href="http://developer.apple.com/mac/library/technotes/tn2005/tn2137.html">TN2137</a> |
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| 68 | for an example of how this is used.</li> |
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| 69 | |
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| 70 | <li>Many operating systems and environments solve the problem by installing |
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| 71 | complete development environments (including the IDE, tools, header files, |
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| 72 | and libraries) for a single tool chain. This is cumbersome for users and |
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| 73 | does not match well with tools which are inherently capable of cross |
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| 74 | compiling.</li> |
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| 75 | |
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| 76 | <li>The Debian <a href="http://wiki.debian.org/ArmEabiPort">ArmEabiPort</a> |
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| 77 | wiki page for their work to support the ARM EABI provide an interesting |
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| 78 | glimpse into how related issues impact the operating system distribution.</li> |
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| 79 | |
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| 80 | <li><a href="http://icculus.org/fatelf/">FatELF</a> is a proposal for bringing |
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| 81 | Mac OS X like "Universal Binary" support to ELF based platforms.</li> |
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| 82 | |
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| 83 | </ul> |
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| 84 | |
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| 85 | </div> |
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