llvm dev - May 2020 - LLVM's loop unroller & llvm.loop.parallel_accesses

Would you guys be open to supporting a new hint with the right semantics,
like e.g. llvm.loop.noalias_accesses?! I would need to find support in
clang however and the main point of support would be the loop unroller
behaving as stated in the OP.

On Thu, May 14, 2020 at 3:04 PM Michael Kruse <llvmdev at meinersbur.de>
wrote:
> Trivial example:
>
> #pragma clang loop vectorize(assume_safety)
> for (int i = 0; i < n; i+=1) {
>   (void)A[0];
> }
>
> I hope it is obvious that the loop is parallel and can be vectorized,
> but A[0] from iteration 0 will alias with A[0] from iteration 1.
> Replace `0` by `i*c` where c is a variable that can be 0 at runtime to
> make the fact non-obvious to the compiler.
>
> We had discussions about implementing "#pragma ivdep", but
it's
> semantics are not defined independently of the implementation. Anyway,
> even with #pragma omp ivdep, a compiler is not required to vectorize
> the loop.
>
> In LLVM, runtime/partial unrolling only takes place after
> vectorization, so there is less of an issue there.
>
> Michael
>
>
> Am Do., 14. Mai 2020 um 16:16 Uhr schrieb Hendrik Greving <
> hgreving at google.com>:
> >
> > This is interesting! So are you saying that loop.parallel_accesses
> strictly loop parallel, and says nothing about aliasing? I see, I guess we
> may have been "abusing" the hint and re-purposed it. But
isn't llvm's
> vectorizer using loop.parallel_accesses to vectorize loops including
> vectorize memory accesses that if you ignore loop-carried dependencies,
> usually means effectively re-ordering the accesses? I guess this still does
> not imply "noalias"? What about icc/gcc's #pragma ivdep?
Again here, it
> means no loop-carried dependencies, yet still doesn't say anything
about
> noalias? Another way indeed would be to propagate noalias data and indeed
> rely on the future fix that Hal mentions above.
> >
> >
> >
> > On Thu, May 14, 2020 at 1:33 PM Michael Kruse <llvmdev at
meinersbur.de>
> wrote:
> >>
> >> 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
> >> > _______________________________________________
> >> > LLVM Developers mailing list
> >> > llvm-dev at lists.llvm.org
> >> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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What would be its semantics? When would clang attach that attribute?

Michael

Am Mo., 18. Mai 2020 um 14:01 Uhr schrieb Hendrik Greving <hgreving at
google.com>:
>
> Would you guys be open to supporting a new hint with the right semantics,
like e.g. llvm.loop.noalias_accesses?! I would need to find support in clang
however and the main point of support would be the loop unroller behaving as
stated in the OP.
>
> On Thu, May 14, 2020 at 3:04 PM Michael Kruse <llvmdev at
meinersbur.de> wrote:
>>
>> Trivial example:
>>
>> #pragma clang loop vectorize(assume_safety)
>> for (int i = 0; i < n; i+=1) {
>>   (void)A[0];
>> }
>>
>> I hope it is obvious that the loop is parallel and can be vectorized,
>> but A[0] from iteration 0 will alias with A[0] from iteration 1.
>> Replace `0` by `i*c` where c is a variable that can be 0 at runtime to
>> make the fact non-obvious to the compiler.
>>
>> We had discussions about implementing "#pragma ivdep", but
it's
>> semantics are not defined independently of the implementation. Anyway,
>> even with #pragma omp ivdep, a compiler is not required to vectorize
>> the loop.
>>
>> In LLVM, runtime/partial unrolling only takes place after
>> vectorization, so there is less of an issue there.
>>
>> Michael
>>
>>
>> Am Do., 14. Mai 2020 um 16:16 Uhr schrieb Hendrik Greving <hgreving
at google.com>:
>> >
>> > This is interesting! So are you saying that loop.parallel_accesses
strictly loop parallel, and says nothing about aliasing? I see, I guess we may
have been "abusing" the hint and re-purposed it. But isn't
llvm's vectorizer using loop.parallel_accesses to vectorize loops including
vectorize memory accesses that if you ignore loop-carried dependencies, usually
means effectively re-ordering the accesses? I guess this still does not imply
"noalias"? What about icc/gcc's #pragma ivdep? Again here, it
means no loop-carried dependencies, yet still doesn't say anything about
noalias? Another way indeed would be to propagate noalias data and indeed rely
on the future fix that Hal mentions above.
>> >
>> >
>> >
>> > On Thu, May 14, 2020 at 1:33 PM Michael Kruse <llvmdev at
meinersbur.de> wrote:
>> >>
>> >> 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
>> >> > _______________________________________________
>> >> > LLVM Developers mailing list
>> >> > llvm-dev at lists.llvm.org
>> >> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

Skipping the clang question for now, this had to be a loop pragma of some
kind. One step back: what we really need is a way to express that memory
accesses between iterations can be re-ordered. The code that's being
compiled _is_ noalias, but we don't _have_ to use noalias semantics, e.g.
loop parallel semantics are sufficient. What's missing is a way to express
that past the llvm unroller. When the unroller merges iterations, loop
parallel no longer depicts the original iterations. So the obvious idea was
using noalias scope metadata for this, and llvm.loop.noalias_acccesses
would cause the unroller to propagate different scopes for each iteration.
Thinkable is also to keep the llvm.loop.parallel_accesses, and the unroller
propagates a new type of metadata analog to noalias scope, but
loop_parallel scope or something like that. We have methods to achieve this
with intrinsics, but I am looking for something more robust that also works
with clang.

On Mon, May 18, 2020 at 8:44 PM Michael Kruse <llvmdev at meinersbur.de>
wrote:
> What would be its semantics? When would clang attach that attribute?
>
> Michael
>
> Am Mo., 18. Mai 2020 um 14:01 Uhr schrieb Hendrik Greving <
> hgreving at google.com>:
> >
> > Would you guys be open to supporting a new hint with the right
> semantics, like e.g. llvm.loop.noalias_accesses?! I would need to find
> support in clang however and the main point of support would be the loop
> unroller behaving as stated in the OP.
> >
> > On Thu, May 14, 2020 at 3:04 PM Michael Kruse <llvmdev at
meinersbur.de>
> wrote:
> >>
> >> Trivial example:
> >>
> >> #pragma clang loop vectorize(assume_safety)
> >> for (int i = 0; i < n; i+=1) {
> >>   (void)A[0];
> >> }
> >>
> >> I hope it is obvious that the loop is parallel and can be
vectorized,
> >> but A[0] from iteration 0 will alias with A[0] from iteration 1.
> >> Replace `0` by `i*c` where c is a variable that can be 0 at
runtime to
> >> make the fact non-obvious to the compiler.
> >>
> >> We had discussions about implementing "#pragma ivdep",
but it's
> >> semantics are not defined independently of the implementation.
Anyway,
> >> even with #pragma omp ivdep, a compiler is not required to
vectorize
> >> the loop.
> >>
> >> In LLVM, runtime/partial unrolling only takes place after
> >> vectorization, so there is less of an issue there.
> >>
> >> Michael
> >>
> >>
> >> Am Do., 14. Mai 2020 um 16:16 Uhr schrieb Hendrik Greving <
> hgreving at google.com>:
> >> >
> >> > This is interesting! So are you saying that
loop.parallel_accesses
> strictly loop parallel, and says nothing about aliasing? I see, I guess we
> may have been "abusing" the hint and re-purposed it. But
isn't llvm's
> vectorizer using loop.parallel_accesses to vectorize loops including
> vectorize memory accesses that if you ignore loop-carried dependencies,
> usually means effectively re-ordering the accesses? I guess this still does
> not imply "noalias"? What about icc/gcc's #pragma ivdep?
Again here, it
> means no loop-carried dependencies, yet still doesn't say anything
about
> noalias? Another way indeed would be to propagate noalias data and indeed
> rely on the future fix that Hal mentions above.
> >> >
> >> >
> >> >
> >> > On Thu, May 14, 2020 at 1:33 PM Michael Kruse <llvmdev at
meinersbur.de>
> wrote:
> >> >>
> >> >> 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
> >> >> > _______________________________________________
> >> >> > LLVM Developers mailing list
> >> >> > llvm-dev at lists.llvm.org
> >> >> >
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
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