Hi Jan! Thanks for looking into it, and thanks for coping with the latency!
Jan Nieuwenhuizen <jann...@gnu.org> skribis: > From e887762bd07d77b68ff19b0ced3ab41c15faa1ac Mon Sep 17 00:00:00 2001 > From: Jan Nieuwenhuizen <jann...@gnu.org> > Date: Wed, 7 Dec 2016 17:45:29 +0100 > Subject: [PATCH] doc: add Creating a New Cross Target. > > * doc/guix.texi: Add Creating a New Cross Target. [...] > +@node Creating a New Cross Target > +@section Creating a New Cross Target > + > +As a first step of making a full port, you may want to start by creating > +a cross target. A cross target in essence is a cross compiler > +@code{cross-gcc-@var{<target>}}, which depends on > +@code{cross-binutils-@var{<target>}} a @code{libc-@var{<target>}} and > +possibly @code{kernel-headers-@var{<target>}}. Several cross targets > +are available, such as @code{i586-pc-hurd}, @code{armhf-linux}, > +@code{avr}, @code{i686-w64-mingw32} and @code{mips64el-linux}. > + > +Building a full gcc cross compiler depends on a c-library for the > +target. We can build a c-library for the target once we have a cross > +compiler. To break this loop a minimal gcc compiler can be built > +without a c-library; we call this > +@code{gcc-cross-sans-libc-@var{<target>}}. With this minimal gcc > +compiler we cross compile the c-library and then we build the full cross > +gcc. > + > +In @code{gnu/packages/cross-base.scm} are functions to create these > +cross packages. Also, Guix needs to know the name of the dynamic > +linker, see @var{glibc-dynamic-linker} in > +@code{gnu/packages/bootstrap.scm}. I feel that some of it is redundant with, or should be added to the “Porting” node. WDYT? > +@menu > +* Rebuilding My World:: How to avoid rebuilding too often. > +* GCC and Cross Environment Paths:: Details on the cross build setup. > +* The MinGW Cross Target:: Some notes on MinGW. > +@end menu > + > +@node Rebuilding My World > +@subsection Rebuilding My World > + > +Why is it that we all tend love to rebuild our world, yet like it > +somewhat less when others decide do it for us? One of the great things > +of Guix is that it tracks all dependencies and will rebuild any package > +that is out of date: We never have to worry that doing a fresh, clean > +build does not reproduce. [...] > +Now it is time to check what the impact of our changes in @var{ncurses} > + > +@smallexample > +$ ./pre-inst-env guix refresh -l ncurses > +Building the following 1056 packages would ensure 2658 dependent packages > are rebuilt: @dots{} > +@end smallexample My gut feeling is that an explanation of why something gets rebuilt does not belong here (it is not specific to cross-compilation). So I would suggest putting it elsewhere; maybe we need a new “Developing with Guix” section or similar, that would include this as well as “Defining Packages” and “Programming Interface”, though I understand this goes beyond this patch. Another though is that we should provide a command to make it easier to understand why something is rebuilt (we discussed this in Berlin last week). I’m not sure exactly what that command’s output would look like, but I would welcome mockups of what people would like to see, and we can work from there. WDYT? > +@node GCC and Cross Environment Paths > +@subsection GCC and Cross Environment Paths > + > +@c See <http://gcc.gnu.org/ml/gcc/2013-02/msg00124.html> > +@c <http://bugs.gnu.org/22186> and > +@c <https://lists.gnu.org/archive/html/guix-devel/2016-04/msg00533.html> > +@c for a discussion of what follows. > +Some build systems compile and run programs at build time to generate > +host-specific data. This means we usually have two compilers installed: > +@code{gcc} and @code{<target>-gcc}. Guix does not use > +@file{/usr/include} and @file{/usr/lib} to install additional headers > +and libraries, instead it adds to environment path variables such as > +@var{C_INCLUDE_PATH} and @var{LIBRARY_PATH}. > + > +To distinguish between native build-time headers and libraries and > +cross-built target system headers and libraries, we use a patched gcc as > +cross compiler. The cross compiler instead looks at > +@var{CROSS_C_INCLUDE_PATH} and @var{CROSS_LIBRARY_PATH}. > + > +@node The MinGW Cross Target > +@subsection The MinGW Cross Target > + > +The MinGW target is somewhat special in that it does not support > +@var{glibc}. @var{Gcc} needs to be passed the @code{--with-newlib} flag > +and instead we use the combined c-library and free reïmplementation of > +Windows kernel-headers and system-libraries provided by the MinGW-w64 > +project. Also, it's not POSIX so it often needs explicit support, > +sometimes we need to create a patch ourselves. > + > +For standard libc headers and libraries that are missing in MinGW such > +as @var{libiconv} and @var{gettext} helper functions are available > + > +@example > + (inputs > + @dots{} > + ,@@(libiconv-if-needed) > + ,@@(gnu-gettext-if-needed)) > +@end example > + > +Finally, a simple example of how MinGW can be used/tested > + > +@example > +$ guix build --target=i686-w64-mingw32 hello > +@dots{} > +/gnu/store/@dots{}-hello-2.10 > +$ guix environment --ad-hoc wine > +$ wine /gnu/store/@dots{}-hello-2.10/bin/hello.exe > +Hello, world! > +@end example This sounds useful. The section about search paths is really about internals (how does it work under the hood?), while the MinGW part is more practical (how can I cross-compile to MinGW?). Should we have a new “Cross Compilation” section that would primarily focus on cross-compiling (as opposed to porting a new platform), and then have a subsection about things going on under the hood? I think my questions primarily highlight the weaknesses of the existing structure of the document, more than problems with your suggestion. Thanks, Ludo’.