PSA: GHC can now be built with Clang

[Manuel M T Chakravarty]

Austin Seipp <aseipp at pobox.com>:
> * HEAD will correctly work with Clang 3.4svn on both Linux, and OS X.
> * I have a small, 6-line patch to Clang to fix the build failure in
> primitive (Clang was too eager to stringify something.) Once this fix
> is integrated into Clang (hopefully very soon,) it will be possible to
> build GHC entirely including all stage2 libraries without any patches.
> The patch is here: http://llvm.org/bugs/show_bug.cgi?id=16371 - I am
> hoping this will also make it into XCode 5.

Xcode 5 DP2's tools are still based on 3.3svn.

> As for the infamous ticket #7602, the large performance regression on
> Mac OS X, I have some numbers finally between my fast-TLS and slow-TLS
> approach.
> 
> ./gc_bench.slow-tls 19 500000 5 22 +RTS -H180m -N7 -RTS  395.57s user
> 173.18s system 138% cpu 6:50.71 total
> 
> vs
> 
> ./gc_bench.fast-tls 19 500000 5 22 +RTS -H180m -N7 -RTS  322.98s user
> 132.37s system 132% cpu 5:44.65 total
> 
> Now, this probably looks totally awful from a scalability POV. And,
> well, yeah, it is. But I am almost 100% certain there is something
> extremely screwy going on with my machine here. I base this on the
> fact that during gc_bench, kernel_task was eating up about ~600% of my
> CPU consistently, giving user threads no time to run. I've noticed
> this with other applications that were totally unrelated too (close
> tweetbot -> 800% CPU usage,) so I guess it's time to learn DTrace. Or
> turn it on and off again or something. Ugh.
> 
> Anyway, if you look at the user times, you get a nice 30% speedup
> which is about what we expect!

I'm very glad to hear that!

Manuel