[llvm-dev] LLVM's loop unroller & llvm.loop.parallel_accesses (original) (raw)

Hendrik Greving via llvm-dev llvm-dev at lists.llvm.org
Thu May 14 13:11:04 PDT 2020

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}

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