---
title: "Very smaller Haskell DLLs"
author: "Stéphane Laurent"
date: "2017-12-18"
output:
  md_document:
    variant: markdown
    preserve_yaml: true
  html_document:
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prettify: yes
prettifycss: twitter-bootstrap
tags: haskell
highlighter: kate
---

I've found a way to reduce the size of one of my Haskell DLLs from 12.8 MB to 1 MB. 
There are two steps: passing the option `-s` to the linker and using [UPX](https://upx.github.io/) to compress the DLL. In [a previous post](https://laustep.github.io/stlahblog/posts/HaskellDLL.html), I said that my Haskell DLLs do not work after compressing them with UPX. But thanks to this `-s` option, they work.

Well, more exactly, I should say that *my* DLL works... Unfortunately, I have tried this method on a minimal example and the DLL does not work anymore after the UPX compression. That's strange.


Let me share now. If you pass the verbose option `-v` to `ghc -shared`, you can see that there are two linker phases. The first one generates the `o` file. The second one generates the DLL. When I pass the option `-s` to this second phase, the size of my DLL is 4.7 MB, while it is 12.8 MB without the `-s` option. 

You can pass the `-s` option to the linker like this:

```bash
ghc -shared -optl-s ...
```

But if I do like this, the compilation does not work. This command passes the `-s` option to the two linker phases. In order to pass this option to the second phase only, I proceed in two steps. I firstly run this command:

```bash
ghc -c myhaskellscript.hs
```

This generates the file `myhaskellscript.o`. Then I run this command:

```bash
ghc -shared -optl-s myhaskellscript.hs StartEnd.c -o mydll.dll
```

In this way, since `myhaskellscript.o` already exists, the first linker phase is skipped, and thus the `-s` option is passed to the second phase only.


Then I can compress the DLL with the UPX program. As I previously said, the `-s` linking option has a role here: without this option, I can compress the DLL with UPX but the DLL does not work.