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Re: Fedora and Cross Compiling



Thought I'd follow up on my own post with some specificity on what I'm imagining:

Brendan Conoboy wrote:
Technical: There must be cross compilers before we can cross compile.

This is actually two packages: Gcc and Binutils. Both are already quite easy to build such that they use a sys-root. If you're not familiar with sys-roots, this is path prefix to /. Thus you usually have something like $sysroot/usr/lib and $sysroot/usr/include. When the compiler sees #include <stdio.h> it actually reads in $sysroot/usr/include/stdio.h. Same for -l.

Building a cross binutils requires kernel headers for the target architecture. Building gcc requires kernel headers and glibc. There are scripts to solve the initial chicken/egg problem of where to get that glibc for a new arch. At some point in the future gcc might not require glibc to build, but we aren't there yet.

The build system must be enhanced to support cross compilation.

This is a really interesting (to me) extension of the build system requiring enhancements in many areas.

First, there are primitives in Koji that need to be built up such that, for instance, x86 knows it can cross build arm tools.

Second, the x86 host needs to be able to retrieve and install arm dependencies in the arm sys-root (arm's glibc, arm's libX11-devel, etc).

Third, a native/cross hybrid environment needs to be setup to facilitate the common assumptions made by packages. Previous cross compilation efforts inside my group have spent a lot of time on this. I bet others have as well

Finally, packages must be modified to support cross compilation.

This is really a one-at-a-time process. It's often a matter of using relative paths so that #include <header> gets the right header. Or using headers instead of directly reading from /proc. Every package is different. Autotools using packages are frequently much easier than non.

Social: As a volunteer effort, it is unreasonable to expect existing package maintainers to do the work necessary to support cross compilation. There must be people to take on that responsibility and work with upstream and package maintainers to integrate the necessary changes.

I have read the Secondary Architectures proposal and thread with great interest. While there may be some overlap of interest with cross compiling, it's not inherently true. Thus, I'd suggest a cross-team who would generally be responsible for cross-compilation efforts. This might break down like so:

1. Responsible for parts of the build system dedicated to cross compilation.

2. Maintainers of the cross compilation tools (These would presumably be rebuilds of gcc and binutils for the cross-targets).

3. Make changes to existing packages in cooperation with the package maintainer.

4. Be responsible for fixing builds that fail because of cross compiling, but succeed when natively built.

Anybody whose turf I just suggested stepping on, please chime in!

--
Brendan Conoboy / Red Hat, Inc. / blc redhat com


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