Gentoo’s dev-lang/ghc-8.2.1_rc1 supports both cross-building and cross-compiling modes! It’s useful for cross-compiling haskell software and initial porting of GHC itself on a new gentoo target.

Building a GHC crossompiler on Gentoo

Getting ${CTARGET}-ghc (crosscompiler) on Gentoo:

# # convenience variables:
CTARGET=powerpc64-unknown-linux-gnu
#
# # Installing a target toolchain: gcc, glibc, binutils
crossdev ${CTARGET}
# # Installing ghc dependencies:
emerge-${CTARGET} -1 libffi ncurses gmp
#
# # adding 'ghc' symlink to cross-overlay:
ln -s path/to/haskell/overlay/dev-lang/ghc part/to/cross/overlay/cross-${CTARGET}/ghc
#
# # Building ghc crosscompiler:
emerge -1 cross-${CTARGET}/ghc
#
powerpc64-unknown-linux-gnu-ghc --info | grep Target
# ,("Target platform","powerpc64-unknown-linux")

Cross-building GHC on Gentoo

Cross-building ghc on ${CTARGET}:

# # convenience variables:
CTARGET=powerpc64-unknown-linux-gnu
#
# # Installing a target toolchain: gcc, glibc, binutils
crossdev ${CTARGET}
# # Installing ghc dependencies:
emerge-${CTARGET} -1 libffi ncurses gmp
#
# # Cross-building ghc crosscompiler:
emerge-${CTARGET} --buildpkg -1 dev-lang/ghc
#
# # Now built packages can be used on a target to install
# # built ghc as: emerge --usepkg -1 dev-lang/ghc

Building a GHC crossompiler (generic)

That’s how you get a powerpc64 crosscompiler in a fresh git checkout:

$ ./configure --target=powerpc64-unknown-linux-gnu
$ cat mk/build.mk
HADDOCK_DOCS=NO
BUILD_SPHINX_HTML=NO
BUILD_SPHINX_PDF=NO
# to speed things up
BUILD_PROF_LIBS=NO
$ make -j$(nproc)
$ inplace/bin/ghc-stage1 --info | grep Target
,("Target platform","powerpc64-unknown-linux")

Simple!

Below are details that have only historical (or backporting) value.

How did we get there?

Cross-compiling support in GHC is not a new thing. GHC wiki has a detailed section on how to build a crosscompiler. That works quite good. You can even target ghc at m68k: porting example.

What did not work so well is the attempt to install the result! In some places GHC build system tried to run ghc-pkg built for ${CBUILD}, in some places for ${CHOST}.

I never really tried to install a crosscompiler before. I think mostly because I was usually happy to make cross-compiler build at all: making GHC build for a rare target usually required a patch or two.

But one day I’ve decided to give full install a run. Original motivation was a bit unusual: I wanted to free space on my hard drive.

The build tree for GHC usually takes about 6-8GB. I had about 15 GHC source trees lying around. All in all it took about 10% of all space on my hard drive. Fixing make install would allow me to install only final result and get rid of all intermediate files.

I’ve decided to test make install code on Gentoo‘s dev-lang/ghc package as a proper package.

As a result a bunch of minor cleanups happened:

• fixed NCG/llvm presence for target
• marked aarch64 as NCG/llvm target
• fixed ${CTARGET}-prefixing for hp2ps
• fixed ${CTARGET}-prefixing for noncanonical targets
• dropped ${CTARGET}-prefixing for stage2 installs (crossbuilds)
• added ${CTARGET}-prefixing for ghci
• fixed stage2 install to run only ${CBUILD} tools
• fixed all stage2 binaries to run on ${CHOST}, not ${CBUILD}

What works?

It allowed me to test various targets. Namely:

I am running all of this on x86_64 (64-bit LE platform)

Quite a list! With help of qemu we can even test whether cross-compiler produces something that works:

$ cat hi.hs 
main = print "hello!"
$ powerpc64le-unknown-linux-gnu-ghc hi.hs -o hi.ppc64le
[1 of 1] Compiling Main             ( hi.hs, hi.o )
Linking hi.ppc64le ...
$ file hi.ppc64le 
hi.ppc64le: ELF 64-bit LSB executable, 64-bit PowerPC or cisco 7500, version 1 (SYSV), dynamically linked, interpreter /lib64/ld64.so.2, for GNU/Linux 3.2.0, not stripped
$ qemu-ppc64le -L /usr/powerpc64le-unknown-linux-gnu/ ./hi.ppc64le 
"hello!"

Many qemu targets are slightly buggy and usually are very easy to fix!

A few recent examples:

• epoll syscall is not wired properly on qemu-alpha: patch
• CPU initialization code on qemu-s390x
• thread creation fails on qemu-sparc32plus due to simple mmap() emulation bug
• tcg on qemu-sparc64 crashes at runtime in static_code_gen_buffer()

Tweaking qemu is fun

• use --autounmask=n
• use --backtrack=1000 (or more)
• package.mask all outdated packages that cause conflicts (usually requires more iterations)
• run world update

The problem

Occasionally (more frequently on haskel packages) portage starts taking long time to only tell you it was not able to figure out the upgrade path.

Some people suggest "wipe-blockers-and-reinstall" solution. This post will try to explain how to actually upgrade (or find out why it’s not possible) without actually destroying your system.

Real-world example

I’ll take a real-world example in Gentoo’s bugzilla: bug 579520 where original upgrade error looked like that:

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:
x11-libs/gtk+:3
  (x11-libs/gtk+-3.18.7:3/3::gentoo, ebuild scheduled for merge) pulled in by
    (no parents that aren't satisfied by other packages in this slot)
  (x11-libs/gtk+-3.20.0:3/3::gnome, installed) pulled in by
    >=x11-libs/gtk+-3.19.12:3[introspection?] required by (gnome-base/nautilus-3.20.0:0/0::gnome, installed)
    ^^              ^^^^^^^^^
    >=x11-libs/gtk+-3.20.0:3[cups?] required by (gnome-base/gnome-core-libs-3.20.0:3.0/3.0::gnome, installed)
    ^^              ^^^^^^^^
    >=x11-libs/gtk+-3.19.4:3[introspection?] required by (media-video/totem-3.20.0:0/0::gnome, ebuild scheduled for merge)
    ^^              ^^^^^^^^
    >=x11-libs/gtk+-3.19.0:3[introspection?] required by (app-editors/gedit-3.20.0:0/0::gnome, ebuild scheduled for merge)
    ^^              ^^^^^^^^
    >=x11-libs/gtk+-3.19.5:3 required by (gnome-base/dconf-editor-3.20.0:0/0::gnome, ebuild scheduled for merge)
    ^^              ^^^^^^^^
    >=x11-libs/gtk+-3.19.6:3[introspection?] required by (x11-libs/gtksourceview-3.20.0:3.0/3::gnome, ebuild scheduled for merge)
    ^^              ^^^^^^^^
    >=x11-libs/gtk+-3.19.3:3[introspection,X] required by (media-gfx/eog-3.20.0:1/1::gnome, ebuild scheduled for merge)
    ^^              ^^^^^^^^
    >=x11-libs/gtk+-3.19.8:3[X,introspection?] required by (x11-wm/mutter-3.20.0:0/0::gnome, installed)
    ^^              ^^^^^^^^
    >=x11-libs/gtk+-3.19.12:3[X,wayland?] required by (gnome-base/gnome-control-center-3.20.0:2/2::gnome, installed)
    ^^              ^^^^^^^^^
    >=x11-libs/gtk+-3.19.11:3[introspection?] required by (app-text/gspell-1.0.0:0/0::gnome, ebuild scheduled for merge)
    ^^              ^^^^^^^^^
x11-base/xorg-server:0
  (x11-base/xorg-server-1.18.3:0/1.18.3::gentoo, installed) pulled in by
    x11-base/xorg-server:0/1.18.3= required by (x11-drivers/xf86-video-nouveau-1.0.12:0/0::gentoo, installed)
                        ^^^^^^^^^^
    x11-base/xorg-server:0/1.18.3= required by (x11-drivers/xf86-video-fbdev-0.4.4:0/0::gentoo, installed)
                        ^^^^^^^^^^
    x11-base/xorg-server:0/1.18.3= required by (x11-drivers/xf86-input-evdev-2.10.1:0/0::gentoo, installed)
                        ^^^^^^^^^^
  (x11-base/xorg-server-1.18.2:0/1.18.2::gentoo, ebuild scheduled for merge) pulled in by
    x11-base/xorg-server:0/1.18.2= required by (x11-drivers/xf86-video-vesa-2.3.4:0/0::gentoo, installed)
                        ^^^^^^^^^^
    x11-base/xorg-server:0/1.18.2= required by (x11-drivers/xf86-input-synaptics-1.8.2:0/0::gentoo, installed)
                        ^^^^^^^^^^
    x11-base/xorg-server:0/1.18.2= required by (x11-drivers/xf86-input-mouse-1.9.1:0/0::gentoo, installed)
                        ^^^^^^^^^^
app-text/poppler:0
  (app-text/poppler-0.32.0:0/51::gentoo, ebuild scheduled for merge) pulled in by
    >=app-text/poppler-0.32:0/51=[cxx,jpeg,lcms,tiff,xpdf-headers(+)] required by (net-print/cups-filters-1.5.0:0/0::gentoo, installed)
                           ^^^^^^
    >=app-text/poppler-0.16:0/51=[cxx] required by (app-office/libreoffice-5.0.5.2:0/0::gentoo, installed)
                           ^^^^^^
    >=app-text/poppler-0.12.3-r3:0/51= required by (app-text/texlive-core-2014-r4:0/0::gentoo, installed)
                                ^^^^^^
  (app-text/poppler-0.42.0:0/59::gentoo, ebuild scheduled for merge) pulled in by
    >=app-text/poppler-0.33[cairo] required by (app-text/evince-3.20.0:0/evd3.4-evv3.3::gnome, ebuild scheduled for merge)
    ^^                 ^^^^
net-fs/samba:0
  (net-fs/samba-4.2.9:0/0::gentoo, installed) pulled in by
    (no parents that aren't satisfied by other packages in this slot)
  (net-fs/samba-3.6.25:0/0::gentoo, ebuild scheduled for merge) pulled in by
    net-fs/samba[smbclient] required by (media-sound/xmms2-0.8-r2:0/0::gentoo, ebuild scheduled for merge)
                 ^^^^^^^^^
It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously.  If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously.
For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.
emerge: there are no ebuilds to satisfy ">=dev-libs/boost-1.55:0/1.57.0=".
(dependency required by "app-office/libreoffice-5.0.5.2::gentoo" [installed])
(dependency required by "@selected" [set])
(dependency required by "@world" [argument])

A lot of text! Let’s trim it down to essential detail first (AKA how to actually read it). I’ve dropped the "cause" of conflcts from previous listing and left only problematic packages:

!!! Multiple package instances within a single package slot have been pulled
!!! into the dependency graph, resulting in a slot conflict:
x11-libs/gtk+:3
  (x11-libs/gtk+-3.18.7:3/3::gentoo, ebuild scheduled for merge) pulled in by
  (x11-libs/gtk+-3.20.0:3/3::gnome, installed) pulled in by
x11-base/xorg-server:0
  (x11-base/xorg-server-1.18.3:0/1.18.3::gentoo, installed) pulled in by
  (x11-base/xorg-server-1.18.2:0/1.18.2::gentoo, ebuild scheduled for merge) pulled in by
app-text/poppler:0
  (app-text/poppler-0.32.0:0/51::gentoo, ebuild scheduled for merge) pulled in by
  (app-text/poppler-0.42.0:0/59::gentoo, ebuild scheduled for merge) pulled in by
net-fs/samba:0
  (net-fs/samba-4.2.9:0/0::gentoo, installed) pulled in by
  (net-fs/samba-3.6.25:0/0::gentoo, ebuild scheduled for merge) pulled in by
emerge: there are no ebuilds to satisfy ">=dev-libs/boost-1.55:0/1.57.0=".

That is more manageable. We have 4 "conflicts" here and one "missing" package.

Note: all the listed requirements under "conflicts" (the previous listing) suggest these are >= depends. Thus the dependencies themselves can’t block upgrade path and error message is misleading.

For us it means that portage leaves old versions of gtk+ (and others) for some unknown reason.

To get an idea on how to explore that situation we need to somehow hide outdated packages from portage and retry an update. It will be the same as uninstalling and reinstalling a package but not actually doing it

package.mask does exactly that. To make package hidden for real we will also need to disable autounmask: --autounmask=n (default is y).

Let’s hide outdated x11-libs/gtk+-3.18.7 package from portage:

# /etc/portage/package.mask
<x11-libs/gtk+-3.20.0:3

Blocker list became shorter but still does not make sense:

x11-base/xorg-server:0
  (x11-base/xorg-server-1.18.2:0/1.18.2::gentoo, ebuild scheduled for merge) pulled in by
  (x11-base/xorg-server-1.18.3:0/1.18.3::gentoo, installed) pulled in by
                        ^^^^^^^^^^
app-text/poppler:0
  (app-text/poppler-0.32.0:0/51::gentoo, ebuild scheduled for merge) pulled in by
  (app-text/poppler-0.42.0:0/59::gentoo, ebuild scheduled for merge) pulled in by

Blocking more old stuff:

# /etc/portage/package.mask
<x11-libs/gtk+-3.20.0:3
<x11-base/xorg-server-1.18.3
<app-text/poppler-0.42.0

The output is:

[blocks B      ] <dev-util/gdbus-codegen-2.48.0 ("<dev-util/gdbus-codegen-2.48.0" is blocking dev-libs/glib-2.48.0)
* Error: The above package list contains packages which cannot be
* installed at the same time on the same system.
 (dev-libs/glib-2.48.0:2/2::gentoo, ebuild scheduled for merge) pulled in by
 (dev-util/gdbus-codegen-2.46.2:0/0::gentoo, ebuild scheduled for merge) pulled in by

That’s our blocker! Stable <dev-util/gdbus-codegen-2.48.0 blocks unstable blocking dev-libs/glib-2.48.0.

The solution is to ~arch keyword dev-util/gdbus-codegen-2.48.0:

# /etc/portage/package.accept_keywords
dev-util/gdbus-codegen

And run world update.

Simple!

The most interesting projects are:

• pandoc
• git-annex
• gitit
• yi
• xmonad

Thanks!