On 01/06/2011 13:30, Manuel M T Chakravarty wrote:
Simon Marlow:
On 01/06/2011 07:11, Manuel M T Chakravarty wrote:
Simon Marlow:
On 30/05/2011 14:59, Manuel M T Chakravarty wrote:
It is no secret that Apple moves away from the traditional GCC
backend to LLVM. In fact, Xcode (which bundles all command line
developer tools on the Mac) today comes with two flavours of gcc:
'gcc' and 'llvm-gcc', which AFAIK only differ in the backend that is
being used. Currently, the default is the traditional GCC backend,
but it takes no precognition to realise that this will eventually
change. The 'gcc' executable will use the LLVM backend and, at least
for a while, the traditional backend will still be available under a
different name.
Unfortunately, GHC will break at this point as the LLVM backend does
not support pinned global registers. ('llvm-gcc' happily accepts the
register assignment, but fails with a runtime error during code
generation.)
This shouldn't be a problem. We don't use pinned global registers any more,
except in one place - the GC (see rts/sm/GCTDecl.h). There it's optional, but
you lose a bit of performance by not using a pinned register. It's not a huge
deal.
Have you tried building GHC with llvm-gcc? I think I tried it on the RTS a
year or so ago to check the LLVM output against gcc (LLVM wasn't quite as good
at the time).
Yes, I tried and it failed, while compiling the RTS, with
sorry, unimplemented: LLVM cannot handle register variable ‘R1’, report
a bug
This was using the 64bit version of GHC. I'll have a closer look.
Perhaps that was when compiling StgCRun.c? It doesn't actually need register
variables (on x86_64 at least), but it does include the header files, so that
probably needs some #ifdefery somewhere for llvm-gcc.
Yes, it's in 'StgCRun.c'. Ok, and how about on i386 (or do you want
to phase that arch out)?
It doesn't look like the x86 code in StgCRun.c uses registers either.
The sparc version does, but it could be rewritten.
The other place, as I mentioned above, is rts/sm/GCTDecl.h, which will need to
use a different method for declaring the garbage collector's thread-local state
variable, gct. On x86_64 I found that using a fixed register was the fastest,
but using a thread-local variable (the __thread modifier) also works.
Just to make sure I understand correctly, are you saying that using a
thread-local variable is already implemented as an option,
Yes - look at the series of #ifdefs in that file, it's pretty
straightforward to change how gct is declared for a particular platform.
However, I've just done some poking around and it seems that __thread is
not supported on OS X:
http://lifecs.likai.org/2010/05/mac-os-x-thread-local-storage.html
see also this thread about Clang:
http://lists.cs.uiuc.edu/pipermail/cfe-dev/2011-March/013673.html
It seems there might be support for __thread in the future, but not in
the short term.
It seems our very own David Peixotto tried building GHC with Clang a
year ago and ran into the same thing:
http://www.dmpots.com/blog/2010/05/08/building-ghc-with-clang.html
So this is less than ideal. The short term fix would be to #define gct
to be a called to pthread_getspecific(). The call will be inlined - the
OS X headers define pthread_getspecific in terms of some inline
assembly, but the optimiser won't know anything about the inline
assembly so it won't be able to common up multiple loads of gct, and
that probably means it won't perform well. If that's the case, then the
solution is to load up gct into a temporary in the performance-critical
functions in the GC (evacuate(), scavenge_block()), and add it as an
argument to inline functions. I'd rather avoid having to do all that if
possible.
If you want to benchmark the GC, there are some good programs in nofib/gc.
Cheers,
Simon
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