[Libclc-dev] [PATCH 1/1] r600: Add fence implementation, rework barrier

[Jeroen Ketema]

Hi,

>> My original approach implemented read/write fences by just calling
>> mem_fence(). That should take care of both using only seq_cst and
>> duplicate labels. I can post is as a v2 if you are ok with a patch that
>> is not really useful on its own.
>> 
>> regards,
>> Jan
>> 
> You can't use the LLVM atomic fence instruction for this. For reasons I do not understand, the atomic fence LLVM instruction only impacts the ordering of other atomic instructions, but OpenCL mem_fence is for all memory accesses

I don’t think this is totally accurate, as the LLVM atomic orderings — and the fence takes an ordering argument — are intended to implement C++11 atomic orderings and those put requirements on the writes to non-atomic memory locations. The IR language reference heavily leans on the C++11 spec for this reason [0]. However, the OpenCL mem_fence also has a requirement on the reads that were issued before the fence, which is something that is not required by any of the C++11 orderings. I’m not totally confident in stating this, but because of this read “issue” it seems that the llvm fence cannot be used to implement OpenCL’s mem_fence.

Jeroen

[0] http://llvm.org/docs/LangRef.html#ordering