[llvm-dev] LLVM's loop unroller & llvm.loop.parallel_accesses

[Michael Kruse via llvm-dev]

llvm.loop.parallel_accesses does not imply that these accesses from
different iterations are not aliasing. Examples where an access are
parallel are that the accesses are atomic or read-only from a specific
location.

The LoopUnrollPass might deduce that non-atomic stores are necessarily
not aliasing (when not using transactional memory), but I don't think
we can do this for all the read accesses. Would that be sufficiently
useful?

Michael


Am Do., 14. Mai 2020 um 15:11 Uhr schrieb Hendrik Greving via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> Hi, in our backend, which is unfortunately not upstreamed, we are relying on llvm.loop.parallel_accesses metadata for certain passes like software pipelining so we can re-order instructions. Ideally, we would want the loop unroller to support the notion of the loop's parallelism in its pre-unrolled version. This probably should happen by propagating !alias.scope and !alias metadata. Is there any plan or open patch for supporting this?
>
> Simplified example:
>
> for.body:
> %0 = load [..]
>  store %0 [..]
> br label %for.cond, !llvm.loop !2
>
> !1 = distinct !{}
> !2 = distinct !{!2, !3, !4, !5, !6, !7}
> !3 = !{!"llvm.loop.parallel_accesses", !1}
> !4 = !{!"llvm.loop.vectorize.width", i32 1}
> !5 = !{!"llvm.loop.interleave.count", i32 1}
> !6 = !{!"llvm.loop.vectorize.enable", i1 true}
> !7 = !{!"llvm.loop.vectorize.followup_all", !8}
> !8 = !{!"llvm.loop.unroll.count", i32 2}
>
> (unroll by 2) =>
>
> for.body:
> %0 = load [..] !alias.scope !9 !noalias !11
> store %0 [..] !alias.scope !9 !noalias !11
> %1 = load [..] !alias.scope !10 !noalias !12
>  store %1 [..] !alias.scope !10 !noalias !12
> br label %for.cond, !llvm.loop !2
>
> [..]
>
> !9 = distinct !{!9, !"iteration0"}
> !10 = distinct !{!10, !"iteration1"}
> !11 = !{!10}
> !12 = !{!9}
>
> Thanks, Hendrik
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