GHC, Clang & XCode 4.2

Simon Marlow marlowsd at
Wed Oct 12 12:26:19 CEST 2011

On 11/10/2011 18:45, David Peixotto wrote:
> Ok, I have attached a set of patches to support building the GHC
> runtime with llvm-gcc. The patches are based off of commit
> 29a97fded4010bd01aa0a17945c84258e285d421 which was last Friday's HEAD.
> These patches are also available from my github repository on the
> llvm-gcc branch at
> 	  git://
> There are three patches:
> 	0001- Uses pthread_getspecific and pthread_setspecfic to
> 	      	   access gct when the `llvm_CC_FLAVOR` CPP macro is
> 	      	   set
> 	0002- Modifies the configure scripts to set the
> 		   `llvm_CC_FLAVOR` macro when compiling with an llvm
> 		   based compile (either llvm-gcc or clang)
> 	0003- Passes the gct variable as a parameter in the GC. This
> 	      	   change is parameterized with CPP macros so that it
> 	      	   is only in effect when compiling for an llvm-based
> 	      	   compiler.
> The patches 0001 and 0002 provide the minimal support needed to build
> GHC with llvm-gcc. The 0003 patch is there to limit the performance
> hit we get by going through pthread functions to access the gct. I
> think the 0001 and 0002 patches should not be very controversial, but
> the 0003 patch is a more invasive change and perhaps Simon Marlow will
> want to clean it up before it is applied.

Thanks, I'll take a look at these soon.

Just a thought, but someone might want to write a blog post about how 
Apple's choice to move to llvm-gcc is imposing a performance penalty on 
us here, and get it up on Reddit.  That would give the issue some 
publicity (they love that sort of thing on Reddit), and might result in 
some action.  I'd be happy to proof read a blog post before publication. 
  Some simple benchmarks would be needed - one option is to use GHC 
itself with the various combinations of compilers + RTS changes, or 
there are a small set of GC benchmarks in nofib/gc.


> I ran a validate with the patches, and found one additional failure
> when going through an llvm-based compiler. There were no additional
> failures when using a gcc compiler even with my patches applied. The
> additional failure is the cgrun071 test which tests the popCnt
> primitives. I'm going to look into why that test fails, but I think
> the patches should be safe to apply as it would only show up when
> compiling with llvm-gcc, which is currently impossible without these
> patches.
> -David
> On Oct 7, 2011, at 10:30 AM, David Peixotto wrote:
>> On Oct 6, 2011, at 7:32 AM, Simon Marlow wrote:
>>> On 05/10/2011 09:46, 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 happy to incorporate the parameter-passing changes if necessary.  I think it should only be important in the inner loop of the GC (scavenge_block/evacuate and the functions called from there).  If someone sends me a working patch, I can clean it up as much as possible.
>> I will take a look at getting the patches to work with GHC head. To support llvm-gcc we need two basic things:
>> 1. autoconf support to detect when we are compiling with llvm-gcc
>> 2. a work-around for tls in the gc
>> My old patches take care of both 1 and 2. For #2 the easy approach is using pthread_getspecific which is a pretty small change. Passing gct as a parameter is much uglier more invasive change, but I had it working at some point. It could perhaps be made less ugly by only passing it around in the "important" functions.
>> Supporting clang is going to require more changes. The last time I tried there were problems with the preprocessor that made it not work. I just tried again yesterday and ran into a problem generating makefile dependencies because apparently clang does not allow both the -MM and -MF flags (
>>> Note that some of the overhead measured by David is coming from using the LLVM backend to gcc instead of gcc's own backend.  In my own measurements a few months ago I found LLVM generated slower (but smaller) code for the GC.  Maybe this will change over time.  It's also possible that the current GC code is tuned for gcc - at various times in the past I've gone through and tweaked the code to get good assembly out (much as we teak Haskell to get good Core :-).
>>> Cheers,
>>> 	Simon
>>>> 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.
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