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The Clang Universal Driver Project

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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.

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We would like to solve this problem by defining a new model for how cross +compilation is done, based on the idea of a universal driver. The key +point of this model is that the user would always access the compiler through a +single entry point (e.g., /usr/bin/cc) and provide an argument +specifying the configuration 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.

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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).

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This project is in the very early (i.e., thought experiment) stages of +development. Stay tuned for more information, and of course, patches +welcome!

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Existing Solutions and Related Work

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gcc's command line arguments -V, -B, -b are + generic but limited solutions to related problems. Similarly, -m32 + and -m64 solve a small subset of the problem for specific + architectures.
gcc's multilibs + solve the part of the problem that relates to finding appropriate libraries + and include files based on particular feature support (soft float, + etc.).
Apple's "driver driver" supported by gcc and clang solve a subset of the + problem by supporting -arch. Apple also provides a tool chain which + supports universal + binaries and object files which may include data for multiple + architectures. See TN2137 + for an example of how this is used.
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.
The Debian ArmEabiPort + wiki page for their work to support the ARM EABI provide an interesting + glimpse into how related issues impact the operating system distribution.
FatELF is a proposal for bringing + Mac OS X like "Universal Binary" support to ELF based platforms.

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