[LLVMdev] llvm memory barrier as a builtin

[Luke Dalessandro]

Mon Ping Wang wrote:
> Thanks for the info.  My impression is that __sync_synchronize takes  
> no arguments and is the memory barrier, i.e.,
>   "llvm.memory.barrier(i1 true,i1 true,i1 true,i1 true,i1 true)".  Is  
> that right?  

That's my understanding as well.

> I would like a little finer control to express just a  
> write barrier (st-st) or a read barrier.

My understanding is that the only types of finer grained control in gcc 
are the __sync_lock_test_and_set and __sync_lock_release which appear to 
implement acquire/release style barriers.

http://gcc.gnu.org/onlinedocs/gcc-4.3.2/gcc/Atomic-Builtins.html#Atomic-Builtins

I expect that there will be large changes once the memory model for 
C++0X is released, and there may be things implemented in gcc branches 
or even undocumented in the mainline that give you the kind of control 
you want.

It might make sense to allow __sync_synchronize to be overloaded in the 
same way that LLVM's builtin is for use in clang, with the default 
version being the full memory barrier.

Luke

> 
>    -- Mon Ping
> 
> 
> On Sep 17, 2008, at 5:50 AM, Andrew Lenharth wrote:
> 
>> On Tue, Sep 16, 2008 at 5:42 PM, Mon Ping Wang <wangmp at apple.com>  
>> wrote:
>>> I would like access to LLVM memory barrier instruction as a built-in
>>> from clang, which means that I need a name for it. In gcc, I see  
>>> names
>>> like __builtin_ia32_mfence but those refers to X86 SSE instruction
>>> that we support.  I don't see a gcc name that has the same semantics
>>> as our barrier instruction.  For a name,  I was thinking of
>>> __builtin_memory_barrier or __builtin_llvm_memory_barrier.  Does
>>> anyone object of adding it as built-in or have a better idea for a  
>>> name?
>> __sync_synchronize is the gcc builtin for a memory barrier.
>>
>> Andrew
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