[PATCH] doc: add Creating a New Cross Target.

[Jan Nieuwenhuizen <janneke@gnu.org>]

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Hi,

Here's my doc draft on the cross build system.  Suggestions/additions
welcome!

Greetings,
Jan


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From e887762bd07d77b68ff19b0ced3ab41c15faa1ac Mon Sep 17 00:00:00 2001
From: Jan Nieuwenhuizen <janneke@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.
---
 doc/guix.texi          | 170 +++++++++++++++++++++++++++++++++++++++++++++++++
 gnu/packages/mingw.scm |   6 +-
 2 files changed, 171 insertions(+), 5 deletions(-)

diff --git a/doc/guix.texi b/doc/guix.texi
index 738b7fb..91a83e9 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -182,6 +182,7 @@ GNU Distribution
 * Packaging Guidelines::        Growing the distribution.
 * Bootstrapping::               GNU/Linux built from scratch.
 * Porting::                     Targeting another platform or kernel.
+* Creating a New Cross Target::
 
 System Installation
 
@@ -259,6 +260,12 @@ Coding Style
 * Data Types and Pattern Matching::  Implementing data structures.
 * Formatting Code::             Writing conventions.
 
+Creating a New Cross Target
+
+* 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 detailmenu
 @end menu
 
@@ -6457,6 +6464,7 @@ For information on porting to other architectures or kernels,
 * Packaging Guidelines::        Growing the distribution.
 * Bootstrapping::               GNU/Linux built from scratch.
 * Porting::                     Targeting another platform or kernel.
+* Creating a New Cross Target::
 @end menu
 
 Building this distribution is a cooperative effort, and you are invited
@@ -14361,6 +14369,168 @@ Second, some of the required packages could fail to build for that
 platform.  Lastly, the generated binaries could be broken for some
 reason.
 
+@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}.
+
+@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.
+
+However, if we make the tiniest change for our cross build to the
+@var{ncurses} package then Guix will first rebuild world before starting
+the cross-build.  If we made a silly typo in the cross-build recipe, it
+takes a long time to get feedback on that.
+
+Say our target is to cross-build @var{Guile} which needs a cross-built
+@var{readline} which requires making a change to the @var{ncurses}
+package.  What we can do is create a temporary alternative package
+hierarchy.  We copy @var{ncurses} to @var{cross-ncurses}
+
+@example
+(define-public cross-ncurses
+   @dots{}
+   (name "cross-ncurses")
+   @dots{})
+@end example
+
+@noindent
+and because we are really aiming for @var{readline}, we copy that too
+
+@example
+(define-public cross-readline
+   @dots{}
+   (name "cross-readline")
+   @dots{}
+   (propagated-inputs `(("ncurses" ,cross-ncurses)))
+   @dots{})
+@end example
+
+@noindent
+which we then use in our copied @var{cross-guile} package.  We replace
+the original packages descriptions with the @var{cross-} variants and
+remove the @var{cross-} prefixes.
+
+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
+
+When we are satisfied with our changes, we can enjoy our favorite
+beverage while we having our worlds rebuild.  If we decide the impact is
+too great we can rewrite our changes in a way so that they do not change
+the regular package recipe, e.g.
+
+@smallexample
+    (patches (search-patches "ncurses-mingw.patch"))))
+    @dots{}
+    (inputs
+     `(;; breaks with MinGW...not needed? ("coreutils" ,coreutils)
+       ("zlib" ,zlib)))
+@end smallexample
+
+can be written as
+
+@smallexample
+    (patches (if (target-mingw?)
+                 (search-patches "ncurses-mingw.patch")
+                 '()))))
+    @dots{}
+    (inputs
+     `(;; fails with MinGW and could be removed unconditionally
+       ;; but that triggers a rebuild
+       ,@@(if (target-mingw?)
+             '()
+             `(("coreutils" ,coreutils)))
+       ("zlib" ,zlib))
+@end smallexample
+
+and the original non-cross build recipe remains identical to the
+builder.
+
+@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
+
 @c *********************************************************************
 @include contributing.texi
 
diff --git a/gnu/packages/mingw.scm b/gnu/packages/mingw.scm
index 4eb2c8f..dd9b597 100644
--- a/gnu/packages/mingw.scm
+++ b/gnu/packages/mingw.scm
@@ -21,14 +21,10 @@
   #:use-module (gnu packages)
   #:use-module (gnu packages base)
   #:use-module (gnu packages cross-base)
-  #:use-module (gnu packages gcc)
-  #:use-module (gnu packages compression)
-  #:use-module (gnu packages multiprecision)
   #:use-module (guix build-system gnu)
   #:use-module (guix packages)
   #:use-module (guix download)
-  #:use-module (guix utils)
-  #:use-module (ice-9 match))
+  #:use-module (guix utils))
 
 (define-public mingw-w64
   (package
-- 
2.10.2


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Jan Nieuwenhuizen <janneke@gnu.org> | GNU LilyPond http://lilypond.org
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