LLVM 3.0 doesn't support llvm.memory.barrier

[bgamari]

Support for the llvm.memory.barrier intrinsic was removed in LLVM 3.0  http://llvm.org/releases/3.0/docs/Atomics.html due to the introduction of the new "fence" instruction. The LLVM code generator should be updated accordingly.

[bgamari]

In the name of release early, release often I'm attaching my hacked patch to use the "fence" instruction. The code compiles and seems to run, but this is not to say that it is correct. In fact, there are at least two outstanding issues,

1) Logic needs to be wired up to emit llvm.memory.barrier when dealing with old LLVMs 2) Someone with greater knowledge of the synchronization requirements of cmm's write barrier primative should take a look at the LLVM atomics documentation ( http://llvm.org/releases/3.0/docs/Atomics.html) and ensure that my use of "fence" is correct

[bgamari]

Preliminary patch

[dterei]

OK pushed patch + some improvements. I changed the fence to use the strongest sync of seq_cst (sequentially consistent). I'm not sure if this is too much and perhaps your use of acq_rel (acquire+release) would be enough. Please check what actual machine code is generated by GHC and maybe talk to Simon M.

[pcapriotti]

Merged as 102a5380574ed22eca32f8e63cae22f013153f0b and 932cdfd52d94cdfb074878e98767d0ff597262b6.