[LLVMdev] RFC: Atomics.h

Luke Dalessandro luked at cs.rochester.edu
Sun May 17 07:23:09 CDT 2009


Owen Anderson wrote:
> 
> On May 16, 2009, at 7:47 PM, Luke Dalessandro wrote:
>
>> Also, atomic ops are usually pretty low level things used for
>> nonblocking algorithms or to build higher level locking constructs. Is
>> that the plan here too? It seems like you'd want to avoid anything too
>> fancy since LLVM has to run on so many different architectures with
>> their variety of memory semantics, etc.
> 
> I totally agree.  However, at least one case of thread-unsafety 
> (ManagedStatic), has proven very-difficult-to-impossible to implement 
> correctly without using lower-level operations.
> 

Yes, double-checked locking is a pain. There's a C++ safe implementation 
in 
http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html 
in the "Making it work with explicit memory barriers" section. As far as 
I know, it is still considered to work.

Luke

> --Owen
> 
> 
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