The tail of an off-by-one error in GHC's linker

This is the story of an odd off-by-one error in GHC’s internal static linker for the Mach-O file format on AArch64. This is our beloved macOS/iOS platform! For a while we have been observing odd non-deterministic failures in CI. We’d see a bunch of test for the aarch64-darwin runner fail, but rarely the same, and if re-run different tests would fail. The only observable correlation was that the failing tests all somehow had to do with iserv, ghc’s external interpreter.

What is New in Cross Compiling Haskell

August & September Edition Due to a lot of traveling in September, I wan’t able to write the August update in September. So what happened in the last two month? For the cross compilation pipeline through the llvm-ng, I’ve added binary serializability of cmm to a custom ghc fork, that allows to dump the cmm representation. This is helpful as it allows the decoupeling from the code generator and the ghc front end.

What is New in Cross Compiling Haskell

July Edition In July I’ve been playing around a bit more with -target. At this point I believe the best solution is to have a minimal ghc that doesn’t ship with any libraries; and all libraries are built on demand per target. We will likely want to pre-build and ship the Runtime System Library rts as we do not have a cabal package that would just build the rts. You’d have to specify the targets for which you want to build/include the rts.

What is New in Cross Compiling Haskell

June Edition This post will be quite brief. I wasn’t able to carve out much time in June as my wife an I have just welcomed a new human being. As the 8.6.1 alpha was just announced, I’m trying to finish adapting the llvm-ng backend as quickly as I can and build a fresh set of pre-built ghc 8.6.1 cross compilers.

What is New in Cross Compiling Haskell

May Edition With GHC 8.6.1 being released soon, the windows for patches was closing fast. While not necessarily cross compilation related, I was still able to get a patch in the infamous load command size limit issue on macOS. Interestingly windows suffers from a similar issue. On windows we have a command argument size limit of 32k character from Windows 7 onwards. Cabals (and to that extend also Stacks, and all other tools that use Setup.

What is New in Cross Compiling Haskell

March & April Edition Due to some travel in April, I wasn’t able to write up an update for what happened in March. So today we’ll have a combined update for March and April. Alp Mestanogullari has finished hadrian#531, which was a clean up of my earlier hadrian#445. As such we now build relocatable GHCs with hadrian by default! 🎉 I have been making progress on the Windows front to the point where we don’t even need a windows installation anymore, and run the iserv slave via WINE.

Talk: Case Study: Cross Compiling dhall-json Meetup — March 2018 At the March Meetup I gave a short presentation on how to cross compile dhall-json to raspberry pi, and the issues you might run into. I wanted to show that it is possible to cross compile non-trivial Haskell packages today, and that issues (ghc-head, build-type, and Template Haskell) have mostly trivial fixes of which most can in principle be upsteamed. The talk coincided with the release of GHC 8.

What is New in Cross Compiling Haskell

February Edition While I didn’t manage to find the time to write anything here in February, I did make some progress on the cross compilation front. cabal can now be invoked outside of the source folder, and tries to not pollute the source tree with generated files anymore (haskell/cabal#4874). The --with-PROG flag is now properly respected when using new-build; this allows new-build to work with cross compilers (haskell/cabal#5018). I’ve also spent quite some time to get cabals CI back into shape, which was hitting a rather obscure bug (haskell/cabal#5132).

What is New in Cross Compiling Haskell

January Edition After setting up with S3 and fastly backing in December, I automated the overlay generation in January. This means that any patches pushed to the overlay repository will be automatically reflected in the overlay after a few minutes. Now all that’s missing is patches for common packages! I’ve also managed to write a patch for cabal so that new-build properly propagates --with-PROG flags into dependencies. This should make it work much better for cross compilation, once the patch is merged.

Provisioning a NixOS server from macOS

Nix is a purely functional package manager and NixOS is a purely functional Linux distribution. I have been running a NixOS server for a while now, and been quite happy with the configuration. However updating it has essentially been copying the configuration.nix file over and calling nixos-rebuild switch --upgrade. This builds packages that are not in the cache on the server. NixOps is the NixOS cloud deployment tool and I should be able to use it to provision the server.

Talk: Building Android apps with Haskell Meetup — January 2018 At the January Meetup I presented building Android apps with Haskell yesterday. As we have recordings set up for a while now, you can follow it below. The talk is rather short and will only outline how to install the cross compilers, setup an Android app and call Haskell from Kotlin to display “Hello from Haskell”. It should hopefully provide a good starting point to start playing with the cross compilers.

What is New in Cross Compiling Haskell

December Edition In December I was finally able to provide new GHC cross compiler binary distribution for iOS, Android and Raspberry Pi from macOS Sierra and Linux. The only other post was Contributing to GHC via phabricator. That is not to say there has been no progress, I just found very little time to write. Axis Sivitz has been successful in cross compiling some haskell code using the cross compiler, and highlighted a few issues along the way.

Contributing to GHC

via Phabricator About two weeks ago I gave a talk on “Contriubting to GHC via Phabricator” at the local Meetup. Sadly the microphone died after a few minutes into the recording, and as such the video has no audio for most of the talk and is pretty useless. As the topic might be of interest to those who could not attend, I’ll write down the content of the talk as good as I can.

Relocatable GHC Cross Compiler Binary Distributions

*/TLDR/*/: Watch the videos at the end of this article and grab your cross compilers from/ It’s been a while since the GHC Cross Compiler Binary Distributions post. A primary issue was installation. Specifically the =./configure= and make install logic that is part of GHCs binary distributions. This is necessary for GHC to install properly and setup the settings file, which contains system specific information that can only be detected on the final host.

What is New in Cross Compiling Haskell

November Edition Wow, it’s December already! So what happened last month? As I detailed in the October Edition I have been working on a hadrian fork, that should allow us to build relocatable binary distributions for GHC, and specifically for cross compilers. This work has seen quite some improvement (in hadrian and cabal), and I am now able to build relocatable binary distributions of GHC cross compilers and if there are no more obstacles, the should be ready soon.

Building GHC: The Stages

In part one we learned about the package database. Part two then introduced the tools and programs built when building GHC. In this part we will look at how the actual stages are built. I will not go into the detail of the make based build system, but try to give an overall understanding what is happening conceptually. To build GHC usually involves running: ./boot (1) ./configure --prefix=/usr/opt (2) make -j (3) make install (4) Let’s break down what is happening here: running =.

Building GHC: The tools

After getting to know package databases we are going to look into the set of tools that are used when building the Glasgow Haskell Compiler. alex & happy alex is a tool to generate lexical analysers in Haskell, and as such can turn =.x= files into =.hs= files. happy is a parser generator, which can turn =.y= files into =.hs= files. ghc-pkg We saw during getting to know package databases. It is the command line interface to the package databases.

What is New in Cross Compiling Haskell

October Edition After the announcement and release of experimental cross compiler previews as binary distributions last month, it turned out that binary distributions produced by the make base build system were not what I had hoped for. GHC 8.4.1 is around the corner (Release in February 2018. Cut release branch in November 2017.) and will come with a new shake based build system called hadrian, which has been in development for quite some time.

Building GHC: The package database

Over the next few posts, I will explore how the Glasgow Haskell Compilers build system works. I will try not to go into too much detail how make based build system, and focus on the high level process and how to arrive at the final compiler. As we will have to deal with package databases, let us take a look at packages and package databases today. Packages While we usually build packages for use with GHC via cabal, the packages GHC knows about are those registered in the known (to GHC) package database(s).

GHC Cross Compiler Binary Distributions

As mentioned in the September Edition of What is New in Cross Compiling Haskell, I’ve been working on producing binary distributions of cross compilers, so that installing a cross compiler becomes almost as trivial as installing a GHC binary distribution. A word of caution is in order. /These are rather experimental builds, and realistically I expect there to be a few issues, that will arise. I’d still appreciate early feedback so that we can iron out the wrinkles quickly!

What is New in Cross Compiling Haskell

September Edition ICFP was a blast, and I think we made quite some progress in getting some of the cross compilation diffs merged. Facetime does pay off! The Q monad extension for template haskell (D3608) will need some adjustments going forward. I hope these will be sorted out by the end of October. Herbert Valerio Riedel announced the head.hackage hackage overlay. This allows to have a set of patches which are turned into a separete hackage repository, and the packages are picked from that repository rather than from the upstream hackage repository if a patched package exists.

What is New in Cross Compiling Haskell

August Edition The little time I manged to carve out for this project in August has been spent on building a new llvm backend. Or more precisely to start integrating the llvm bitcode backend I worked on last year into ghc. The current status of the -fllvmng backend is that it can compile GHC, but fails to validate. I believe most of these validation failures should be fairly trivial so solve.

What is New in Cross Compiling Haskell

July Edition Sadly I wasn’t able to write anything throughout July. I hope I will be able to carve out some time this month to write about the new features we have been working on. The focus has mainly been on making the compilation experience for cross compilers as pleasant as possible and figuring out how to build binary distributions of GHC cross compilers. One of the rather annoying kludges has been the need for the libtool-lite script on some platforms; this should be unnecessary soon.

What is New in Cross Compiling Haskell

June Edition After building the cross compilers Raspberry Pi and Android in May, June was only about iOS, as I’ve been traveling quite a bit in June. Building a Cross Compiler for iOS A Haskell Cross Compiler for iOS iOS and Template Haskell I hope you enjoyed the posts and are excited for more! As always you can follow us here on medium, on github, or on twitter.

iOS and Template Haskell

Similar to Android and Template Haskell we need wrap the GHCSlave (remote iserv) instance into an application for iOS. To provide the Template Haskell evaluation context on the iOS device. With the Haskell Cross Compiler for iOS from yesterday, we will now build the GHCSlave iOS application. /WARNING: Due to a bug in the x86_64 linker code, Template Haskell does not yet work with the iOS Simulator. I have marked the commands for the simulator in italic, and will remove this warning, once the linker code is fixed.

A Haskell Cross Compiler for iOS

So far we have built a Haskell Cross Compiler for Raspberry Pi, as well as a Haskell Cross Compiler for Android. To round this off, we will build a cross compiler for iOS as well. With the WWDC signaling the end of 32bit devices and the last 32bit devices are the iPad (4th gen) and iPhone 5/iPhone 5C, we will only build the 64bit cross compiler. Note that what Apple calls arm64 is called aarch64 elsewhere.

What is New in Cross Compiling Haskell

May Edition After the initial Cross Compilation Survey in April, and the results, we tried to highlight some of the progress and zw3rk made. Building a Cross Compiler for Raspberry Pi Quick Headless Raspberry Pi Setup Making a Raspbian Cross Compilation SDK A Haskell Cross Compiler for Raspberry Pi Cross Compiling Haskell Code Why use a Cross Compiler? GHC’s Cross Compilation Pipeline The Haskell Cabal and Cross Compilation Discussing Template Haskell Template Haskell Template Haskell and Cross Compilation Cross Compiling Yesod to Raspberry Pi Building a Cross Compiler for Android Building iconv for Android A Haskell Cross Compiler for Android Android and Template Haskell This should give a glimpse in what is hopefully to come with GHC 8.

Android and Template Haskell

With the Haskell Cross Compiler for Android we built yesterday, and the knowledge how to Cross Compiling Template Haskell, today we will build the slave process to run on the Android device to allow to us the use of Template Haskell in Android applications. Prerequisites As we did for the Raspberry Pi, we need to build iserv-proxy and the iserv library. Please refer to the instructions there if something is unclear.

A Haskell Cross Compiler for Android

Over the last two weeks we saw how to build a Haskell Cross Compiler for Raspberry Pi, set up Cabal for Cross Compilation, and how to Cross Compile Template Haskell. Building a Haskell cross compiler for Android is almost identical, with only minor differences. For the Raspbian Haskell cross compiler we had a single architecture only. Android runs on a plethora of architectures. We will focus on arm processors, specifically the 32bit armv7 and 64bit aarch64.

Cross Compiling Yesod to Raspberry Pi

As we have seen we can now cross compile Template Haskell. The Yesod Web Framework is one of many haskell web frameworks. By default it makes use of shakespeare templates, which happen to use Template Haskell. This is supposed to be a case study of cross compiling haskell packages to the Raspberry Pi. We will study the issues that can arise with this rather recent development and how to mitigate them.

Cross Compiling Template Haskell

As laid out yesterday, there are some interesting questions pertaining to Template Haskell and Cross Compilation. Today we will put all the pieces together and cross compile template haskell to our Raspberry Pi with file and process IO! The External Interpreter GHCs External Interpreter splits GHC into two components the haskell compiler ghc and the interpreter server iserv. Passing -fexternal-interpreter to ghc will spawn an iserv instance and run all interpreted code through it.

Template Haskell and Cross Compilation

Over the last two weeks we have seen how to build a Haskell cross compiler for the Raspberry Pi. We have also seen a high level overview of Template Haskell. Today we will look at what Template Haskell means in the context of cross compilation. As pointed out yesterday, Template Haskell requires that GHC is able to run the object code for the Template Haskell function. This in turn requires GHC to be able to load this code into memory and make it executable.

Template Haskell

After using Haskell for a while, running into Template Haskell become more and more likely. A reverse dependency lookup for template-haskell lists over one thousand packages. While I believe many have used Template Haskell via aeson, language-c-inline, lens, wreq, yesod, or any of the other packages, my gut feeling is that fewer have actually written Template Haskell code. /This post is not intended to advocate the use of Template Haskell, but rather to foster a better understanding for what it is, and how it can be used.

GHC's Cross Compilation Pipeline

Today is going to be a slight bit more technical, and less direct practical utility. We will look at the steps that GHC takes to cross compiles code via its LLVM backend. In GHC’s Compiler Commentary we can see how the front end takes a Haskell file and after Parsing, Renaming, Desugaring, it ends up in GHC’s Core language. The Core is then processed repeatedly by the Simplification pass before being translated into STG and finally Cmm.

Why use a Cross Compiler?

Since and zw3rk posted the Cross Compilation survey (and results), I have been writing about cross compiling to Raspberry Pi, and specifically building a SDK for Raspbian Jessie, and a Haskell cross compiler, one topic has been coming up repeatedly: Why use a Cross Compiler? For those who work in the mobile application space, cross compilation is very likely an essential part of their day. iOS and Android apps are naturally cross compiled, even though this fact is mostly hidden away.

The Haskell Cabal and Cross Compilation

Once you want to tap into the vast ecosystem of haskell libraries, you will run into cabal and hackage in one way or the other (stack and stackage build upon cabal as well). Over the last few days we set up the Raspberry Pi, built the Raspbian SDK and the Haskell cross compiler. Today we will look at what cabal is, and how to use it for cross compilation. Cabal, cabal-install, cabal and Hackage The *C*ommon *A*rchitecture for *B*uilding *A*pplications and *L*ibraries consists of Cabal the library.

A Haskell Cross Compiler for Raspberry Pi

After setting up our Raspberry Pi and building the raspbian SDK. Today we will build a Haskell cross compiler (GHC) for the Raspberry Pi, to be used together with the SDK. We will use a custom GHC branch, as not all necessary patches have yet landed yet in the upstream repository. Alternatively you can use the official GHC master branch and apply the patches manually with arc patch. Prerequisites Make sure you have a ghc and cabal installed.

Making a Raspbian Cross Compilation SDK

After setting up your Raspberry Pi with Raspbian jessie lite, this time we will now create the necessary cross compilation Software Development Kit (SDK). The SDK will contain the toolchain, headers and libraries to compile binaries compatible with the Raspberry Pi on a different (more powerful) machine (e.g. your laptop or desktop computer). While the Raspberry Pi 2 and Raspberry Pi 3 are armv7 and aarch64 the old Raspberry Pi 1 and Raspberry Zero are armv6; to simplify matters Raspbian is compiled for armv6hf+vfpv2 to have a single distribution supporting all.

Quick Headless Raspberry Pi Setup

To run any software on your Raspberry Pi, it clearly needs to be set up. However, without an HDMI capable device or a USB keyboard we won’t get anywhere without a headless setup. You will need a DHCP capable router, a Raspberry Pi, a SD card, and a SD card reader/writer (if you computer doesn’t already provide one). We’ll use Raspbian jessie lite and etcher. Putting Raspbian onto the SD card should present no issues.

Cross Compilation Survey Results

About two weeks ago together with, we asked about cross compiling haskell and 229 people responded to our survey. The majority (85%) noted that they would like to use GHC as a cross compiler but do not yet. The targets for cross compilation are arm (66%), aarch64 (64%) and x86_64 (42%) for android (62%), linux (60%) and iOS (43%). Build architecture is almost exclusively x86_64 (98%), with linux (82%) being twice as popular as macOS (38%), and windows a third at (16%).

Building iconv for Android

iconv (i*nternational *con*version) is provided by libiconv, and available through *GHC.IO.Encoding in the base package. As this is an external dependency, it needs to be available for a build of GHC to succeed. In the case of cross compiling GHC for iOS, we can use the libiconv.dylib provided by the iOS sdk. For android however, we’ll need to build Building libiconv version 1.15 for aarch64/android on macOS via androids ndk toolchain can be done as follows:

Hello World & A Cross Compilation Survey

we are zw3rk, the haskell focused team at Our mission is to make haskell a viable platform for mobile development. What does zw3rk stand for you might ask? It’s a simple play on words, combining the German word Zwerg (engl. dwarf/gnome) and Werk (engl. factory) with the almighty three! The current state of ghc for mobile development (Android and iOS) via cross compilation is not as smooth as we’d like.