Relocatable GHC Cross Compiler Binary Distributions
It’s been a while since the GHC Cross Compiler Binary Distributions
post. A primary issue was installation. Specifically the =./configure=
make install logic that is part of GHCs binary distributions. This
is necessary for GHC to install properly and setup the
which contains system specific information that can only be detected
on the final host. These include for example a check to see if the
host does position independent executables (PIE) by default or not;
and if not detected properly will result in a compiler that can not
produce binaries for the host.
For cross compilers this is a little different, as they are tightly coupled to the cross compilation tool chain. As such there is less freedom in how the final host will behave and a lot more knowledge about the hosts tool chain and the final target are known at build time and many of the checks performed in the =./configure= script are not suited for the installation of a cross compiler.
To that end, as I’ve mentioned in What is New in Cross Compiling Haskell: October Edition and What is New in Cross Compiling Haskell: November Edition, I’ve put a lot of effort into allowing the shake base GHC build system, and cabal to produce relocatable binary distributions of GHC.
What do I mean by relocatable binary distributions? Essentially that
you can just grab the tar or zip archive of GHC, unpack it and execute
bin/ghc with no need to actually run =./configure= and
make install if
your system is similar enough to the build system (e.g. the usual case
for cross compilers with a deterministic cross compilation tool
chain). If you still need to adapt GHC to your host, you can
run =./configure= right inside the unpacked folder prior to executing
bin/ghc. Alp Mestanogullari has done a fantastic job in bringing
the =./configure= logic to the relocatable binary distributions and is
working on getting tests and documentation working with relocatable
binary distributions as well!
One of the major benefits of the relocatable binary distributions is
that when GHC is built, it builds into
_build/stage<N>/lib and those folders can be freely moved around for
stage1 and later on macOS, linux, windows. Why only on those three
systems? This has primarily to do with how GHC figures out the
location of the executable that is invoked, as it needs to know where
lib folder is. This turns out to be a rather tricky to get right
and is rather platform dependent. /(I have not tried *BSD; but I
believe that could work as well, if someone wants to give this a try,
let me know! — If you want to add support for any other platform,
the best point to start would be to make/
base:System.Environment behave correctly for your platform.)
Another benefit is that we do not necessarily need the wrapper scripts
anymore. The wrappers used to wrap the binaries and add the
-B flag or
similar to tell the binaries where the
lib folder was. As such you can
gdb ghc or
lldb ghc. (The wrapper scripts are still generated
when installing ghc via
make install if the
are /=./configure=/d to be in different locations.)
This work is by no means concluded yet but it is sufficiently advanced that I was able to produce a set of relocatable cross compilers for iOS, Android and Raspberry Pi, which you can find at http://hackage.mobilehaskell.org. If you run into issues, please file them with the mobilehaskell hackage-overlay issue tracker.
A word of caution though: as
cabal=s =new-buildhas a rather annoying bug with respect to forwarding flags to dependencies, my advice is to unpack or add submodules for dependencies that require cabal to run
I plan to get around to fixing this; but I haven’t manage to do so just yet.
I’ve use the aforementioned binary distributions to build a stitching application where the logic part (stitching together simple screenshots into a long continuous screenshot) is written in Haskell and the UI is written in kotlin for Android and obj-c (make calling haskell via c a bit easier than swift) for iOS, which you see below.
Stitcher running on an Asus Zenfone Stitcher running on an iPhone 7 I plan to write up building Stitcher as a series starting in 2018, and the accompanying code will be available on GitHub.