[PATCH] D97947: [AArch64] Force runtime unrolling for in-order scheduling models

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

> It seems to be difficult to identify the remainder loop; Checking the llvm.loop.isvectorized attribute (like in ARMTargetTransformInfo) caused all gains to be negated, and caused a slight regression when compared to without this change. I've instead included a check for the llvm.loop.unroll.disable attribute, which was seen on the remainder loop IR. This check causes no difference to the benchmark numbers. (Though thinking about it now, that might be due to it being handled elsewhere, making this check redundant)

Yeah it doesn't seem worthwhile to check for something that the loop unroller will already be checking for. The vectorizer will unroll (/"interleave") at the same time as vectorizing, and we can already produce loops that are very wide compared to the input, sometimes to the point that they are never executed and we are only running the remainders. Unrolling on top of that will not be beneficial in a lot of cases, but that will depend heavily on the trip count of the loops.

The very quick benchmarks I ran didn't show this to be great because of all that extra unrolling, mostly of remainder loops by the look of it. I'm sure you have been running more benchmarks, and perhaps the ones here are not very representative of general A64 code.

> Enabling partial unrolling made no difference in the benchmark results, so I haven't enabled it here.

Partial unrolling is when the trip count is known but we can unroll a mod of it. I would be very surprised if it didn't come up in any of your testing.


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D97947/new/

https://reviews.llvm.org/D97947