[polly] scop detection: detect regions bottom up

[Sebastian Pop]

Tobias Grosser wrote:
> Also, in terms of speeding up scop detection. My guess is that the
> largest speedup would come from detecting scops bottom up, starting
> from small regions to bigger regions. We could then skip analyzing
> regions as soon as we found a construct in a smaller region that
> would
> also invalidate all regions containing the smaller region.

Here is a patch that makes scop detection to work bottom up.  On my huge C++
benchmark, the attached patch gets the overall sequential time spent in Polly
from 2151 seconds to 1573 seconds.

Also note that there still are redundant computations in the case of nested
valid regions: we would start calling isValidRegion on the innermost region
containing a loop, then supposing that the parent region is composed of only
valid regions, we would call again isValidRegion and that would walk again over
all the subregions, validating the CFG, loops, and every instruction again.

With the attached patch there are 4 fails:

Failing Tests (4):
    Polly :: polybench/linear-algebra/solvers/lu/lu_with_param.ll
    Polly :: polybench/linear-algebra/solvers/lu/lu_without_param.ll
    Polly :: polybench/linear-algebra/solvers/ludcmp/ludcmp_without_param.ll
    Polly :: polybench/stencils/jacobi-2d-imper/jacobi-2d-imper_with_param.ll

These fails are due to the fact that we do not detect a maximal region anymore:
first, scop detection splits region exit blocks when the exit block has multiple
exit edges, and then these new basic blocks that we inserted do not belong to
any region, and so we end up not discovering a maximal region anymore.

I think we can add the newly created blocks to the region info.

Another solution, and probably more difficult, is to avoid invalidating the
region info by trying to not split the blocks ending a region.  I'm not sure
about the impact of such a change on the codegen region versioning.

Sebastian
-- 
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
hosted by The Linux Foundation
-------------- next part --------------
A non-text attachment was scrubbed...
Name: 0001-scop-detection-detect-regions-bottom-up.patch
Type: text/x-diff
Size: 2961 bytes
Desc: not available
URL: <http://lists.llvm.org/pipermail/llvm-commits/attachments/20130612/ee753562/attachment.patch>