[LLVMdev] Pointer Context Metadata (was: Parallel Loop Metadata)

[Pekka Jääskeläinen]

On 02/17/2013 11:15 PM, Hal Finkel wrote:
> If the unroller somehow differentiates the metadata coming from different
> loop iterations, then BBVectorize can use this information as well. Even
> better, we could make BasicAA understand that appropriately marked loads
> and stores from different iterations don't alias. Then the AA-based
> dependency breaker in the scheduler could also make use of the information.
> Thoughts?

This is roughly what we did in our first version of work-group
autovectorization in pocl that works on "fully unrolled wi-loops"
(we call it the 'replication' work group generation method).

We forked the BBVectorize to the pocl code base and added explicit
knowledge of the separate work-items (that are really just parallel loop
iterations) so it tries to pair the matching instructions from the
different iterations (WIs) directly.

We also have an AA that exploits the independent iterations (WIs)
information along with the other OpenCL AA helping features (disjoint
address spaces). We use this AA down to the custom instruction
scheduler of ours with the TCE target to help the VLIW-style
scheduling/bundling of multiple WIs.

I have hoped to get the BBVectorizer and the "unrolled parallel loop AA"
functionality upstreamed as it applies to all fully parallel loops, not just
the OpenCL "work-item loops", and I hate to have the forked BBVectorizer in
pocl.

The metadata scheme should be thought through, however, to make it cleaner
than our OpenCL-specific hackish attempt, and possibly usable for other
similar "context-dependent scenarios".

The earlier idea I had was to attach "context information" to the
memory accesses. In this case it would communicate that the mem access
belongs (or belonged, if fully unrolled) to a loop and it can alias only with
the accesses from the same iteration, or with accesses without the metadata.

Something like:

llvm.mem.parallel_loop_iteration [loopid] [iteration_id_integer]

This can help the "pairing" of the BBVectorizer: it can try to pair with
the different iterations first. The ParallelLoopIterationAA can look at this
metadata and if the other instruction has also a parallel_loop_iteration MD
that points to the same loopid (the self-referencing id metadata from the
llvm.loop.parallel patch), check their iteration identifier, and if it's 
different, return NO ALIAS.

The similar idea could be applied to preserve the 'restrict' info across
function inlines:

llvm.mem.restricted_access [funcid] [pointerid]

Similarly, if the RestrictedPointerAA finds that both of the accesses are
marked with this metadata and point to the same funcid, and the [pointerid]
is different, it can return NO ALIAS.

-- 
Pekka