[LLVMdev] RFC: Atomics.h

[Luke Dalessandro]

Jeffrey Yasskin wrote:
> What would you do with a just-hardware memory fence? If the compiler's
> free to move operations over the hardware fence, that seems to defeat
> the purpose.

If your compiler memory fence (gcc's __sync_synchronize()) doesn't 
distinguish different kinds of fencing requirements, and your platform 
has different hardware fences with different costs (isync, lwsync, 
eieio, etc), then it can make sense to separate the two using something 
like asm volatile(::"memory") for the compiler fence. It gets pretty 
architecture dependent though.

Luke

> 
> C++0X provides a compiler-only fence, and a hardware+compiler fence,
> but no hardware-only fence, I believe for this reason. See
> <http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2857.pdf>,
> section 29.8.
> 
> On Sat, May 16, 2009 at 7:33 PM, Owen Anderson <resistor at mac.com> wrote:
>> Surprisingly enough, libatomic_ops doesn't define just a hardware memory
>> fence call as far as I can tell.
>> --Owen
>> On May 16, 2009, at 3:00 PM, Zoltan Varga wrote:
>>
>> Hi,
>>
>>   You might want to use this:
>>
>> http://www.hpl.hp.com/research/linux/atomic_ops/
>>
>>                              Zoltan
>>
>> On Sat, May 16, 2009 at 11:11 PM, Owen Anderson <resistor at mac.com> wrote:
>>> Some of you may have noticed that I addedd include/llvm/System/Atomics.h
>>> to the repository briefly, which will be used for adding support for
>>> threading in LLVM.
>>>
>>> I have tried to provided appropriate implementations of the atomic ops
>>> (currently memory fence and CAS) for platforms we care about, but my
>>> knowledge of these, and my ability to test them, is limited.  So, please, if
>>> you run on any less common platform, test out the file, and send me patches
>>> to improve it if it doesn't work verbatim on your system.
>>>
>>> Thanks,
>>>
>>> --Owen
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
>> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev
>>
>>
> 
> _______________________________________________
> LLVM Developers mailing list
> LLVMdev at cs.uiuc.edu         http://llvm.cs.uiuc.edu
> http://lists.cs.uiuc.edu/mailman/listinfo/llvmdev