[LLVMdev] Parallel Loop Metadata

[Nadav Rotem]

On Feb 7, 2013, at 10:55 AM, Pekka Jääskeläinen <pekka.jaaskelainen at tut.fi> wrote:

> Hi Nadav,
> 
> On 02/07/2013 07:46 PM, Nadav Rotem wrote:
>> Pekka suggested that we add two kind of metadata: llvm.loop.parallel
>> (attached to each loop latch) and llvm.mem.parallel (attached to each memory
>> instruction!).  I think that the motivation for the first metadata is clear -
>> it says that the loop is data-parallel. I can also see us adding additional
>> metadata such as llvm.loop.unrollcnt to allow the users to control the unroll
>> count of loops using pragmas. That's fine. Pekka, can you think of
>> transformations that may need invalidate or take this metadata into
>> consideration ?
> 
> Any pass that introduces new non-parallel memory instructions to the loop,
> because they think the loop is sequential and it's ok to do so. I do not know
> any other such pass than the one pointed out earlier, reg2mem (if the
> variables inside the loop body reuse stack slots). E.g., inlining
> should be safe. So should be unrolling an inner loop inside a parallel loop.
> 

I suggest that we only add the 'llvm.loop.parallel' metadata and not llvm.mem.parallem.  I believe that it should be the job of the consumer pass (e.g loop-vectorizer) to scan the loop and detect parallelism violations. This is also the approach that we use when we optimize stack slots using lifetime markers. I understand that the consumer passes will have to be more conservative and miss some optimizations. But I still think that this is better than forcing different passes in the compiler to know about parallel metadata.