[llvm-dev] Machine Unroller patch

[via llvm-dev]

Hi all,

 

I have submitted a patch that implements machine unroller as a utility
class. https://reviews.llvm.org/D53005 

 

The intent of this patch is to provide the loop unrolling functionality at
the MI level. Currently, It handles only small inner-most loops with the
run-time trip count and a single basic block. We target such loops mostly
because they often have underutilized resources. The unrolling is not only
found to improve the resource usage in many cases but if scheduled properly
it can also help reduce stalls by hiding  the multi cycles instruction
latencies. To avoid excessive code size increase, we perform it selectively
and do it only for the loops where it's determined to improve the resource
usage. Since the Software Pipeliner already models the resource usage for
ResMII (Resoure Minimum Initiation Interval) computation, we decided to use
it for computing the unroll factor as well and thus let the pipeliner drive
the machine unroller. The unroller-pipeliner combo is found to generate much
better code for the small loops with the high latency instructions without
loop carried dependence. For now, this feature is enabled only for the
Hexagon backend. To enable it for other targets, they must extend the
MachineUnroller class and provide their own implementation of the target
specific APIs. 

 

The patch currently doesn't have any reviewers since I didn't know whom to
add. I will add some folks for the Hexagon specific changes, but I would
greatly appreciate if others can review as well and provide their feedback.
Please let me know if you any questions.

 

Thanks,

Jyotsna

 

 

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