Dynamic libraries by default and GHC 7.8

Simon Marlow marlowsd at gmail.com
Wed Nov 28 10:20:57 CET 2012

On 27/11/12 14:52, Ian Lynagh wrote:
> Hi all,
> GHC HEAD now has support for using dynamic libraries by default (and in
> particular, using dynamic libraries and the system linker in GHCi) for a
> number of platforms.
> This has some advantages and some disadvantages, so we need to make a
> decision about what we want to do in GHC 7.8. There are also some policy
> questions we need to answer about how Cabal will work with a GHC that
> uses dynamic libraries by default. We would like to make these as soon
> as possible, so that GHC 7.6.2 can ship with a Cabal that works
> correctly.
> The various issues are described in a wiki page here:
>      http://hackage.haskell.org/trac/ghc/wiki/DynamicByDefault

Thanks for doing all the experiments and putting this page together, it
certainly helps us to make a more informed decision.

> If you have a few minutes to read it then we'd be glad to hear your
> feedback, to help us in making our decisions

My personal opinion is that we should switch to dynamic-by-default on 
all x86_64 platforms, and OS X x86. The performance penalty for 
x86/Linux is too high (30%), and there are fewer bugs affecting the 
linker on that platform than OS X.

I am slightly concerned about the GC overhead on x86_64/Linux (8%), but 
I think the benefits outweigh the penalty there, and I can probably 
investigate to find out where the overhead is coming from.


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