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