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## Build a cross-compiling GHC
In this example, our host machine is "Ubuntu 16.04.2 LTS, Linux ubuntu 4.4.0-79-generic 86_64".
We need to download necessary tools, including:
- [LLVM-4.0 source](http://releases.llvm.org/4.0.0/llvm-4.0.0.src.tar.xz), you need to build it yourself. Remember to choose release channel and use gold linker (`cmake -DCMAKE_BUILD_TYPE=Release -DLLVM_USE_LINKER=gold ..`)
- `sudo apt-get install gcc-arm-linux-gnueabihf` to install the GCC cross-compiler
- Download and install [Haskell Platform 8.0.2](https://haskell.org/platform/download/8.0.2/haskell-platform-8.0.2-unknown-posix--full-x86_64.tar.gz). Install it according to [instructions here](https://www.haskell.org/platform/linux.html#linux-generic)
After all the dependencies are in place:
- `git clone https://github.com/ghc/ghc`
- `cd ghc`
- `git clone https://github.com/snowleopard/hadrian`
- `git submodule update --init`
- `./configure --target=arm-linux-gnueabihf`
- `cd hadrian`
- Modify `src/UserSettings.hs`, set `stage1Only` to `True`.
- Build the compiler by e.g. `./build.sh --flavour=quickest --integer-simple -V -j`
After that, you should have built `inplace/bin/ghc-stage1` cross compiler. We will go to the next section to validate this.
**NOTE**: Use of `-c` to configure the target is currently not supported. Please manually run `./configure` like above.
## Test run
Write a simple hello world haskell program:
```haskell
module Main where
main = putStrLn "Hello, world!"
```
Compile it with cross-compiling GHC: `<ghc-folder>/inplace/bin/ghc-stage1 -static Main`. Note that we created a static version of it which packs together all depending libraries.
- Install QEMU: `sudo apt-get install qemu-system-arm`
- Download `vmlinuz` (kernel) and `initrd.gz` (initial ramdisk), e.g. from [this mirror](https://mirrors.tuna.tsinghua.edu.cn/ubuntu-ports/dists/xenial/main/installer-armhf/current/images/generic-lpae/cdrom/).
- Add the ARM Linux executable `Main` to the initial ramdisk so we can load it directly into memory. No need for real installation
+ `gunzip initrd.gz` to get `initrd`
+ `mkdir tmp2; cd tmp2; sudo cpio -id < ../initrd` to get a file system
+ `cp /PATH/TO/Main usr/bin`
+ `find . | cpio --create --format='newc' > /tmp/newinitrd` to pack back the `initrd`
+ `gzip /tmp/newinitrd`
+ Move `newinitrd` to where `vmlinuz` is, rename it to `newinitrd.img`
+ Run the following configured QEMU:
```bash
#!/bin/sh
qemu-system-arm \
-kernel vmlinuz \
-initrd newinitrd.img \
-append "root=/dev/vda2 rootfstype=ext4" \
-no-reboot \
-nographic \
-m 1024 \
-M virt
```
This will lead you to a installer interface. But we don't need to do that, so we can save ourself from the hassle of setting up networks etc. We just keep `Go Back`, until see a line `Execute a shell`, and select it. Now you get a shell, go find `/usr/bin/Main` and run it!
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