GHC as a cross-compiler update

TL;DR:

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 :)