[LLVMdev] RFC: Atomics.h
resistor at mac.com
Sun May 17 12:08:45 CDT 2009
On May 17, 2009, at 5:23 AM, Luke Dalessandro wrote:
> 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
>>> 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
> in the "Making it work with explicit memory barriers" section. As
> far as
> I know, it is still considered to work.
Our problems are actually deeper than that, because we need to
interact well with static constructors. This means that we can't use
a mutex with a non-constant initializer, or else we can't depend on it
being properly initialized before the ManagedStatic is accessed.
While this would be possible with pthread mutexes, I know of no good
way to do it for Windows CRITICAL_SECTION's.
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