[LLVMdev] Parallel Loop Metadata

[Sebastian Pop]

Nadav Rotem wrote:
> 
> 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.


Ok.  A solution similar to Pekka's, using fewer annotations as you suggested, is
to reference in 'llvm.loop.parallel' all the datarefs to which the parallel
access semantics applies.  When the list of datarefs in 'llvm.loop.parallel'
differs from the datarefs in the loop, the annotation is deemed invalid.

Sebastian
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