GHC, Clang & XCode 4.2

Luca Ciciriello luca_ciciriello at
Wed Oct 5 15:32:44 CEST 2011

A temporary workaround could be to not remove the Xcode 4.1 and install the new Xcode 4.2 in a different location. This is just a little disk space expensive, but I've experimented with success  this workaround with the last Xcode 4.2 developer seed.

Let me know if there are news about other issue.



On Oct 5, 2011, at 10:46 AM, austin seipp wrote:

> There has been recent discussion on the Homebrew bug tracker
> concerning the upcoming XCode 4.2 release by Apple, which has
> apparently just gone GM (meaning they're going to make a real release
> on the app store Real Soon Now.)
> The primary concern is that XCode will no longer ship GCC 4.2 at all,
> it seems. XCode 4.0 & 4.1 merely set 'llvm-gcc' as the default
> compiler, and GHC's `configure` script was adjusted to find the
> `gcc-4.2` binary. If you have old XCode's installed, then you may have
> the binaries laying around, but I doubt they'll be on your $PATH, and
> anybody doing a fresh install is SOL.
> It seems Clang 3.0 will now be the default compiler, with llvm-gcc as
> a deprecated option, probably removed in XCode 4.3. It doesn't matter
> really, because both of them do not work with GHC, because of its use
> of either A) Global register variables of any kind, or B) the __thread
> storage modifier.
> David Peixotto did some work on this not too long ago as the issue of
> compiling with Clang was raised. His branches include changes which
> make the 'gct' variable use pthread_getspecific rather than __thread
> for TLS and then as an optimization use inline ASM to grab the value
> out of the variable, with an impact of about 9% it seems, but that's
> on nofib and I don't know if it was -threaded. He also included a
> version which passes the 'gct' around as a parameter to all GCC
> functions which is a bit uglier but may give some better performance I
> guess. (The discussion is from here IIRC.) I suppose the real perf
> killer here is probably -threaded code.
> Was there ever any decision on which route to take for this issue? The
> parameter passing solution looks quite uglier IMO but it may be
> necessary for performance.
> I'm just posting this here as a reminder as this is probably going to
> become a problem pretty quickly for anybody who uses Lion or modern
> XCode and also likes using GHC, so it should probably be sorted out.
> :) I'm still on SL using XCode 4 so it's not an issue for me, but it
> will be for any future Mac endeavors. Hopefully they get support for
> __thread or something equivalent soon, because nobody likes
> performance hits, but it doesn't seem like we have a choice.
> -- 
> Regards,
> Austin
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