Threaded RTS performing badly on recent OS X (10.8?)

This ticket is to remind us about the following problem: OS X is now using llvm-gcc, and as a result GHC's garbage collector with -threaded is much slower than it should be (approx 30% slower overall runtime). Some results here: http://www.haskell.org/pipermail/cvs-ghc/2011-July/063552.html

This is because the GC code relies on having fast access to thread-local state. It uses one of two methods: either a register variable (gcc only) or __thread variables (which aren't supported on OS X). To make things work on OS X, we use calls to pthread_getspecific instead (see #5634 (closed)), which is quite slow, even though it compiles to inline assembly.

I don't recall which OS X / XCode versions are affected, maybe a Mac expert could fill in the details.

We have tried other fixes, such as passing around the thread-local state as extra arguments, but performance wasn't good. Ideally Apple will implement TLS in OS X at some point and we can start to use it.

A workaround is to install a real gcc (using homebrew?) and use that to compile GHC. Whoever builds the GHC distributions for OS X should probably do it that way, so everyone benefits.

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
Test case
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Operating system	Unknown/Multiple
Architecture	Unknown/Multiple