[cfe-dev] __sync_synchronize doesn't generate a memory barrier

[Evan Cheng]

I think Jim worked on this.

Evan

On May 22, 2011, at 9:52 AM, Chris Lattner wrote:

> Adding Owen :)
> 
> -Chris
> 
> On May 9, 2011, at 10:38 AM, Arlen Cox wrote:
> 
>> Why does __sync_synchronize() not generate a mfence instruction on x86
>> and x86_64?  I recognize that Apple gcc does not do this either, but I
>> believe this is a bug in Apple gcc as well.  More recent versions of
>> gcc implement a correct behavior (mfence on x86_64 and lock orl $0,
>> (%esp) on x86), but clang emits no code for this operation.
>> 
>> LLVM supports an instruction that emits the correct memory barrier:
>> call void @llvm.memory.barrier(i1 true, i1 true, i1 true, i1 true, i1 true)
>> but Clang uses the following, which seems to have no effect on x86:
>> call void @llvm.memory.barrier(i1 true, i1 true, i1 true, i1 true, i1 false)
>> 
>> This matters for multi-threaded code as memory barriers are the only
>> way we can force an ordering on loads and stores.
>> 
>> Thanks,
>> Arlen
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> 
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