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

[Finkel, Hal J. via llvm-dev]

Hi, Hendrik,

A couple of thoughts:

 1. This might be related (but, perhaps, not in the way you'd prefer): https://bugs.llvm.org/show_bug.cgi?id=39282
 2. We're scheduling a call to discuss further improvements in this area, and if you might be able to join, please fill out the Doodle poll: https://doodle.com/poll/evhwr2eyfvcf8ib3

 -Hal

Hal Finkel
Lead, Compiler Technology and Programming Languages
Leadership Computing Facility
Argonne National Laboratory

________________________________
From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Hendrik Greving via llvm-dev <llvm-dev at lists.llvm.org>
Sent: Thursday, May 14, 2020 3:11 PM
To: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Cc: Marcello Maggioni <maggioni at google.com>
Subject: [llvm-dev] LLVM's loop unroller & llvm.loop.parallel_accesses

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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