[LLVMdev] [llvm-commits] [llvm] r79731 - /llvm/trunk/lib/CodeGen/SelectionDAG/SelectionDAG.cpp

[Owen Anderson]

On Aug 24, 2009, at 1:32 AM, Duncan Sands wrote:
> unfortunately, race detectors like helgrind don't like this kind of
> thing, and report it as a race.  Last time I asked about it I was
> told that fixing it would be too hard/expensive.

I know, and it's somewhat unfortunate.  However, this path is  
extremely hot in
LLC, and was significantly contended using only two threads.  I don't  
have a
non-contended solution offhand.

>>     VTs[VT.getSimpleVT().SimpleTy] = VT;
>> -    return &VTs[VT.getSimpleVT().SimpleTy];
>> +    sys::MemoryFence();
>
> I don't think this fence actually does anything useful unless
> users of the pointer do a fence before reading the contents
> of the pointer.  That said, I think alpha is the only platform
> where you have to worry about this kind of pointer vs pointee
> race.

I'm fairly sure that the only things that dereference into the array
are callers of this method, who dereference it through the returned
pointer.  The purpose of the fence is to guarantee that the valid has
been written before the pointer is returned to the caller.

--Owen
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