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