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Bandhav Veluri via llvm-dev

2020-Jun-22 16:31 UTC

[llvm-dev] Restrict qualifier on class members

Hi Jeroen,

That's great! I was trying to use the patch, what's the latest version
of
the project we could apply it on?

Hi Neil,

That seems like what I can do as well! Do you happen to have some examples
lying around? Maybe a pointer to the planned presentation, if that's okay?

Thank you,
Bandhav

On Mon, Jun 22, 2020 at 1:55 AM Neil Henning <neil.henning at unity3d.com>
wrote:
> I was originally going to cover this in my now defunct EuroLLVM talk
> but... we had this exact same problem on Unity's HPC# Burst compiler -
how
> to track no-aliasing on structs. We were constrained in that we had to make
> it work with LLVM versions all the way back to shipped LLVM 6, so what we
> did was:
>
>    - Add module-level metadata that tracked whether a given struct member
>    field was no-alias.
>    - Added our own alias analysis using createExternalAAWrapperPass to
>    register it in the pass pipeline.
>
> This allowed us to have zero modifications to LLVM and do something useful
> with aliasing. The one 'issue' with it is if you have a
stack-allocated
> struct that is SROA'ed you will lose the info that it was a struct, or
if
> you are in a private/internal linkage function that has the struct as an
> argument, the opt passes can modify the function signature to lose the
> struct too. We had to do some mitigations here to get perfect aliasing on
> our usecases.
>
> Hope this helps,
> -Neil.
>
> On Mon, Jun 22, 2020 at 5:44 AM Michael Kruse via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> Unfortunately
>> https://llvm.org/docs/LangRef.html#llvm-loop-parallel-accesses-metadata
>> is not a solution here. A loop-parallel access does not imply
>> non-aliasing. The obvious case is when only reading from a location,
>> but even when a location is written to I'd be careful to deduce
that
>> they do not alias since it might be a "benign data race" or
the value
>> never used. Additionally, LLVM's loop unroller is known to now
handle
>> noalias metadata correctly as it just copies it.
>>
>> There has been a discussion here:
>> http://lists.llvm.org/pipermail/llvm-dev/2020-May/141587.html
>>
>> Michael
>>
>>
>> Am So., 21. Juni 2020 um 12:24 Uhr schrieb Johannes Doerfert via
>> llvm-dev <llvm-dev at lists.llvm.org>:
>> >
>> > Hi Bandhav,
>> >
>> >
>> > Jeroen Dobbelaere (CC'ed) is currently working on support for
restrict
>> qualified local variables and struct members.
>> >
>> > The patches exist but are not merged yet. If you want to give it a
try
>> apply https://reviews.llvm.org/D69542.
>> >
>> >
>> > Initially I could only think of this solution for your problem:
>> https://godbolt.org/z/6WtPXJ
>> >
>> > Michael (CC'ed) might now another annotation to get
`llvm.access`
>> metadata for the loop, which should do what you intend.
>> >
>> >
>> > Cheers,
>> >
>> >   Johannes
>> >
>> >
>> > On 6/21/20 11:56 AM, Bandhav Veluri via llvm-dev wrote:
>> >
>> > Hi,
>> >
>> > I'm trying to abstract some special pointers with a class,
like in the
>> > example program below:
>> >
>> >   1 #define __remote __attribute__((address_space(1)))
>> >   2 #include <stdint.h>
>> >   3
>> >   4 __remote int* A;
>> >   5 __remote int* B;
>> >   6
>> >   7 class RemotePtr {
>> >   8   private:
>> >   9     __remote int* __restrict a;
>> >  10
>> >  11   public:
>> >  12     RemotePtr(__remote int* a) : a(a) {}
>> >  13
>> >  14     __remote int& at(int n) {
>> >  15       return a[n];
>> >  16     }
>> >  17 };
>> >  18
>> >  19 int main(int argc, char** argv) {
>> >  20   RemotePtr a(A);
>> >  21   RemotePtr b(B);
>> >  22
>> >  23   #pragma unroll 4
>> >  24   for(int i=0; i<4; ++i) {
>> >  25     a.at(i) += b.at(i);
>> >  26   }
>> >  27
>> >  28   return 0;
>> >  29 }
>> >
>> > It's given that pointer a, in each object of the class
RemotePtr, is the
>> > only pointer that can access the array pointed by it. So, I tried
>> __remote
>> > int* __restrict a; (line 9) construct to tell Clang the same. This
>> doesn't
>> > seem to work and I see no noliass in the generated IR.
Specifically, I
>> want
>> > lines 23-26 optimized assuming no aliasing between A and B. Any
reason
>> why
>> > Clang shouldn't annotate memory accesses in lines 23-26 with
noaliass
>> > taking line 9 into account?
>> >
>> > The higher level problem is this: is there a way to compile lines
23-26
>> > assuming no aliasing between A and B, by just doing something in
the
>> > RemotePtr class (so that main is clear of ugly code)? If
that's not
>> > possible, is there a way to tell Clang that lines 23-26 should
assume no
>> > aliasing at all, by some pragma?
>> >
>> > Thank you,
>> > Bandhav
>> >
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> >
>> > _______________________________________________
>> > LLVM Developers mailing list
>> > llvm-dev at lists.llvm.org
>> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>
>
>
> --
> Neil Henning
> Senior Software Engineer Compiler
> unity.com
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Jeroen Dobbelaere via llvm-dev

2020-Jun-22 22:03 UTC

head link

[llvm-dev] FW: Restrict qualifier on class members

Hi Bandhav,

as mentioned in the summary of https://reviews.llvm.org/D69542  :

   The base version is
b2a37cfe2bda0bc8c4d2e981922b5ac59c429bdc<https://reviews.llvm.org/rGb2a37cfe2bda0bc8c4d2e981922b5ac59c429bdc>
(June 12, 2020)

Greetings,

Jeroen Dobbelaere



From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Bandhav
Veluri via llvm-dev
Sent: Monday, June 22, 2020 18:32
To: Neil Henning <neil.henning at unity3d.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>; Kruse, Michael
<michael.kruse at anl.gov>
Subject: Re: [llvm-dev] Restrict qualifier on class members

Hi Jeroen,

That's great! I was trying to use the patch, what's the latest version
of the project we could apply it on?

Hi Neil,

That seems like what I can do as well! Do you happen to have some examples lying
around? Maybe a pointer to the planned presentation, if that's okay?

Thank you,
Bandhav

On Mon, Jun 22, 2020 at 1:55 AM Neil Henning <neil.henning at
unity3d.com<mailto:neil.henning at unity3d.com>> wrote:
I was originally going to cover this in my now defunct EuroLLVM talk but... we
had this exact same problem on Unity's HPC# Burst compiler - how to track
no-aliasing on structs. We were constrained in that we had to make it work with
LLVM versions all the way back to shipped LLVM 6, so what we did was:

  *   Add module-level metadata that tracked whether a given struct member field
was no-alias.
  *   Added our own alias analysis using createExternalAAWrapperPass to register
it in the pass pipeline.
This allowed us to have zero modifications to LLVM and do something useful with
aliasing. The one 'issue' with it is if you have a stack-allocated
struct that is SROA'ed you will lose the info that it was a struct, or if
you are in a private/internal linkage function that has the struct as an
argument, the opt passes can modify the function signature to lose the struct
too. We had to do some mitigations here to get perfect aliasing on our usecases.

Hope this helps,
-Neil.

On Mon, Jun 22, 2020 at 5:44 AM Michael Kruse via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:
Unfortunately
https://llvm.org/docs/LangRef.html#llvm-loop-parallel-accesses-metadata<https://urldefense.com/v3/__https:/llvm.org/docs/LangRef.html*llvm-loop-parallel-accesses-metadata__;Iw!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLJxlIVzl$>
is not a solution here. A loop-parallel access does not imply
non-aliasing. The obvious case is when only reading from a location,
but even when a location is written to I'd be careful to deduce that
they do not alias since it might be a "benign data race" or the value
never used. Additionally, LLVM's loop unroller is known to now handle
noalias metadata correctly as it just copies it.

There has been a discussion here:
http://lists.llvm.org/pipermail/llvm-dev/2020-May/141587.html<https://urldefense.com/v3/__http:/lists.llvm.org/pipermail/llvm-dev/2020-May/141587.html__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLAvIBb7x$>

Michael


Am So., 21. Juni 2020 um 12:24 Uhr schrieb Johannes Doerfert via
llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>:>
> Hi Bandhav,
>
>
> Jeroen Dobbelaere (CC'ed) is currently working on support for restrict
qualified local variables and struct members.
>
> The patches exist but are not merged yet. If you want to give it a try
apply
https://reviews.llvm.org/D69542<https://urldefense.com/v3/__https:/reviews.llvm.org/D69542__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLLzXOdAP$>.
>
>
> Initially I could only think of this solution for your problem:
https://godbolt.org/z/6WtPXJ<https://urldefense.com/v3/__https:/godbolt.org/z/6WtPXJ__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLEju0wQ6$>
>
> Michael (CC'ed) might now another annotation to get `llvm.access`
metadata for the loop, which should do what you intend.
>
>
> Cheers,
>
>   Johannes
>
>
> On 6/21/20 11:56 AM, Bandhav Veluri via llvm-dev wrote:
>
> Hi,
>
> I'm trying to abstract some special pointers with a class, like in the
> example program below:
>
>   1 #define __remote __attribute__((address_space(1)))
>   2 #include <stdint.h>
>   3
>   4 __remote int* A;
>   5 __remote int* B;
>   6
>   7 class RemotePtr {
>   8   private:
>   9     __remote int* __restrict a;
>  10
>  11   public:
>  12     RemotePtr(__remote int* a) : a(a) {}
>  13
>  14     __remote int& at(int n) {
>  15       return a[n];
>  16     }
>  17 };
>  18
>  19 int main(int argc, char** argv) {
>  20   RemotePtr a(A);
>  21   RemotePtr b(B);
>  22
>  23   #pragma unroll 4
>  24   for(int i=0; i<4; ++i) {
>  25    
a.at<https://urldefense.com/v3/__http:/a.at__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLOY-9qOq$>(i)
+=
b.at<https://urldefense.com/v3/__http:/b.at__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLBIWnTL5$>(i);
>  26   }
>  27
>  28   return 0;
>  29 }
>
> It's given that pointer a, in each object of the class RemotePtr, is
the
> only pointer that can access the array pointed by it. So, I tried __remote
> int* __restrict a; (line 9) construct to tell Clang the same. This
doesn't
> seem to work and I see no noliass in the generated IR. Specifically, I want
> lines 23-26 optimized assuming no aliasing between A and B. Any reason why
> Clang shouldn't annotate memory accesses in lines 23-26 with noaliass
> taking line 9 into account?
>
> The higher level problem is this: is there a way to compile lines 23-26
> assuming no aliasing between A and B, by just doing something in the
> RemotePtr class (so that main is clear of ugly code)? If that's not
> possible, is there a way to tell Clang that lines 23-26 should assume no
> aliasing at all, by some pragma?
>
> Thank you,
> Bandhav
>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLD9-lkj8$>
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLD9-lkj8$>_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev<https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLD9-lkj8$>


--
[Image removed by sender.]
Neil Henning
Senior Software Engineer Compiler
unity.com<https://urldefense.com/v3/__http:/unity.com__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLJhMkp2Z$>

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Bandhav Veluri via llvm-dev

2020-Jun-24 08:11 UTC

head link

[llvm-dev] FW: Restrict qualifier on class members

Hi Jeroen,

Sorry, I missed that. I tried the patch, and this program:

#include <stdint.h>

#define __remote __attribute__((address_space(1)))

__remote int* A;
__remote int* B;

void vec_add(__remote int* __restrict a,
             __remote int* __restrict b,
             int n) {
  #pragma unroll 4
  for(int i=0; i<n; ++i) {
    a[i] += b[i];
  }
}

int main(int argc, char** argv) {
  __remote int* __restrict a = A;
  __remote int* __restrict b = B;

  #pragma unroll 4
  for(int i=0; i<4; ++i) {
    a[i] += b[i];
  }

  return 0;
}

vec_add give following schedule:

*** Final schedule for %bb.8 ***
SU(0):   %33:gpr = LW %56:gpr, -8 :: (load 4 from %ir.scevgep8, !tbaa !14,
!noalias !13, addrspace 1)
SU(1):   %34:gpr = LW %55:gpr, -8 :: (load 4 from %ir.scevgep14, !tbaa !14,
!noalias !13, addrspace 1)
SU(4):   %36:gpr = LW %56:gpr, -4 :: (load 4 from %ir.scevgep10, !tbaa !14,
!noalias !13, addrspace 1)
SU(5):   %37:gpr = LW %55:gpr, -4 :: (load 4 from %ir.scevgep16, !tbaa !14,
!noalias !13, addrspace 1)
SU(8):   %39:gpr = LW %56:gpr, 0 :: (load 4 from %ir.lsr.iv6, !tbaa !14,
!noalias !13, addrspace 1)
SU(9):   %40:gpr = LW %55:gpr, 0 :: (load 4 from %ir.lsr.iv12, !tbaa !14,
!noalias !13, addrspace 1)
SU(12):   %42:gpr = LW %56:gpr, 4 :: (load 4 from %ir.scevgep9, !tbaa !14,
!noalias !13, addrspace 1)
SU(13):   %43:gpr = LW %55:gpr, 4 :: (load 4 from %ir.scevgep15, !tbaa !14,
!noalias !13, addrspace 1)
SU(2):   %35:gpr = nsw ADD %34:gpr, %33:gpr
SU(3):   SW %35:gpr, %55:gpr, -8 :: (store 4 into %ir.scevgep14, !tbaa !14,
!noalias !13, addrspace 1)
SU(6):   %38:gpr = nsw ADD %37:gpr, %36:gpr
SU(7):   SW %38:gpr, %55:gpr, -4 :: (store 4 into %ir.scevgep16, !tbaa !14,
!noalias !13, addrspace 1)
SU(10):   %41:gpr = nsw ADD %40:gpr, %39:gpr
SU(11):   SW %41:gpr, %55:gpr, 0 :: (store 4 into %ir.lsr.iv12, !tbaa !14,
!noalias !13, addrspace 1)
SU(14):   %44:gpr = nsw ADD %43:gpr, %42:gpr
SU(15):   SW %44:gpr, %55:gpr, 4 :: (store 4 into %ir.scevgep15, !tbaa !14,
!noalias !13, addrspace 1)
SU(16):   %57:gpr = nuw nsw ADDI %57:gpr, 4
SU(17):   %56:gpr = ADDI %56:gpr, 16
SU(18):   %55:gpr = ADDI %55:gpr, 16

And main gives following schedule:

 *** Final schedule for %bb.0 ***
 SU(0):   %2:gpr = LUI target-flags(riscv-hi) @A
 SU(2):   %4:gpr = LUI target-flags(riscv-hi) @B
 SU(3):   %5:gpr = LW %4:gpr, target-flags(riscv-lo) @B :: (dereferenceable
load 4 from @B, !tbaa !9, !noalias !22)
 SU(1):   %3:gpr = LW %2:gpr, target-flags(riscv-lo) @A :: (dereferenceable
load 4 from @A, !tbaa !9, !noalias !22)
 SU(4):   %6:gpr = LW %5:gpr, 0 :: (load 4 from %ir.3, !tbaa !14, !noalias
!22, addrspace 1)
 SU(5):   %7:gpr = LW %3:gpr, 0 :: (load 4 from %ir.1, !tbaa !14, !noalias
!22, addrspace 1)
 SU(6):   %8:gpr = nsw ADD %7:gpr, %6:gpr
 SU(7):   SW %8:gpr, %3:gpr, 0 :: (store 4 into %ir.1, !tbaa !14, !noalias
!22, addrspace 1)
 SU(8):   %9:gpr = LW %5:gpr, 4 :: (load 4 from %ir.arrayidx.1, !tbaa !14,
!noalias !22, addrspace 1)
 SU(9):   %10:gpr = LW %3:gpr, 4 :: (load 4 from %ir.arrayidx1.1, !tbaa
!14, !noalias !22, addrspace 1)
 SU(10):   %11:gpr = nsw ADD %10:gpr, %9:gpr
 SU(11):   SW %11:gpr, %3:gpr, 4 :: (store 4 into %ir.arrayidx1.1, !tbaa
!14, !noalias !22, addrspace 1)
 SU(12):   %12:gpr = LW %5:gpr, 8 :: (load 4 from %ir.arrayidx.2, !tbaa
!14, !noalias !22, addrspace 1)
 SU(13):   %13:gpr = LW %3:gpr, 8 :: (load 4 from %ir.arrayidx1.2, !tbaa
!14, !noalias !22, addrspace 1)
 SU(14):   %14:gpr = nsw ADD %13:gpr, %12:gpr
 SU(15):   SW %14:gpr, %3:gpr, 8 :: (store 4 into %ir.arrayidx1.2, !tbaa
!14, !noalias !22, addrspace 1)
 SU(16):   %15:gpr = LW %5:gpr, 12 :: (load 4 from %ir.arrayidx.3, !tbaa
!14, !noalias !22, addrspace 1)
 SU(17):   %16:gpr = LW %3:gpr, 12 :: (load 4 from %ir.arrayidx1.3, !tbaa
!14, !noalias !22, addrspace 1)
 SU(18):   %17:gpr = nsw ADD %16:gpr, %15:gpr
 SU(20):   $x10 = COPY $x0
 SU(19):   SW %17:gpr, %3:gpr, 12 :: (store 4 into %ir.arrayidx1.3, !tbaa
!14, !noalias !22, addrspace 1)

This is great! Memory accesses are marked noalias. I wanted memory accesses
to be annotated as noalias to basically remove loop-carried dependencies so
that I can reorder them for efficient scheduling. But when I look at
Schedule DAG,

For vec_add I see something like this (note BotQ.A, scheduler can choose
any of those => no loop carried dependence):
  - Latency limited.
** ScheduleDAGMILive::schedule picking next node
Queue BotQ.P:
Queue BotQ.A: 16 15 11 7 3
  Cand SU(16) ORDER
Pick Bot ORDER

For main, at best I see something like this:
** ScheduleDAGMILive::schedule picking next node
Cycle: 45 BotQ.A
Queue BotQ.P:
Queue BotQ.A: 12 13
  Cand SU(12) ORDER
  Cand SU(13) ORDER

In theory, schedules for vec_add and main should be the same right? Is
there anything else I should do to make the __restrict remove loop-carried
dependence in main?

Attaching IR and scheduler log for reference...


On Mon, Jun 22, 2020 at 3:03 PM Jeroen Dobbelaere <
Jeroen.Dobbelaere at synopsys.com> wrote:
> Hi Bandhav,
>
>
>
> as mentioned in the summary of https://reviews.llvm.org/D69542  :
>
>
>
>    The base version is b2a37cfe2bda0bc8c4d2e981922b5ac59c429bdc
> <https://reviews.llvm.org/rGb2a37cfe2bda0bc8c4d2e981922b5ac59c429bdc>
> (June 12, 2020)
>
>
>
> Greetings,
>
>
>
> Jeroen Dobbelaere
>
>
>
>
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of
*Bandhav
> Veluri via llvm-dev
> *Sent:* Monday, June 22, 2020 18:32
> *To:* Neil Henning <neil.henning at unity3d.com>
> *Cc:* llvm-dev <llvm-dev at lists.llvm.org>; Kruse, Michael <
> michael.kruse at anl.gov>
> *Subject:* Re: [llvm-dev] Restrict qualifier on class members
>
>
>
> Hi Jeroen,
>
>
>
> That's great! I was trying to use the patch, what's the latest
version of
> the project we could apply it on?
>
>
>
> Hi Neil,
>
>
>
> That seems like what I can do as well! Do you happen to have some examples
> lying around? Maybe a pointer to the planned presentation, if that's
okay?
>
>
>
> Thank you,
>
> Bandhav
>
>
>
> On Mon, Jun 22, 2020 at 1:55 AM Neil Henning <neil.henning at
unity3d.com>
> wrote:
>
> I was originally going to cover this in my now defunct EuroLLVM talk
> but... we had this exact same problem on Unity's HPC# Burst compiler -
how
> to track no-aliasing on structs. We were constrained in that we had to make
> it work with LLVM versions all the way back to shipped LLVM 6, so what we
> did was:
>
>    - Add module-level metadata that tracked whether a given struct member
>    field was no-alias.
>    - Added our own alias analysis using createExternalAAWrapperPass to
>    register it in the pass pipeline.
>
> This allowed us to have zero modifications to LLVM and do something useful
> with aliasing. The one 'issue' with it is if you have a
stack-allocated
> struct that is SROA'ed you will lose the info that it was a struct, or
if
> you are in a private/internal linkage function that has the struct as an
> argument, the opt passes can modify the function signature to lose the
> struct too. We had to do some mitigations here to get perfect aliasing on
> our usecases.
>
>
>
> Hope this helps,
>
> -Neil.
>
>
>
> On Mon, Jun 22, 2020 at 5:44 AM Michael Kruse via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Unfortunately
> https://llvm.org/docs/LangRef.html#llvm-loop-parallel-accesses-metadata
>
<https://urldefense.com/v3/__https:/llvm.org/docs/LangRef.html*llvm-loop-parallel-accesses-metadata__;Iw!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLJxlIVzl$>
> is not a solution here. A loop-parallel access does not imply
> non-aliasing. The obvious case is when only reading from a location,
> but even when a location is written to I'd be careful to deduce that
> they do not alias since it might be a "benign data race" or the
value
> never used. Additionally, LLVM's loop unroller is known to now handle
> noalias metadata correctly as it just copies it.
>
> There has been a discussion here:
> http://lists.llvm.org/pipermail/llvm-dev/2020-May/141587.html
>
<https://urldefense.com/v3/__http:/lists.llvm.org/pipermail/llvm-dev/2020-May/141587.html__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLAvIBb7x$>
>
> Michael
>
>
> Am So., 21. Juni 2020 um 12:24 Uhr schrieb Johannes Doerfert via
> llvm-dev <llvm-dev at lists.llvm.org>:
> >
> > Hi Bandhav,
> >
> >
> > Jeroen Dobbelaere (CC'ed) is currently working on support for
restrict
> qualified local variables and struct members.
> >
> > The patches exist but are not merged yet. If you want to give it a try
> apply https://reviews.llvm.org/D69542
>
<https://urldefense.com/v3/__https:/reviews.llvm.org/D69542__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLLzXOdAP$>
> .
> >
> >
> > Initially I could only think of this solution for your problem:
> https://godbolt.org/z/6WtPXJ
>
<https://urldefense.com/v3/__https:/godbolt.org/z/6WtPXJ__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLEju0wQ6$>
> >
> > Michael (CC'ed) might now another annotation to get `llvm.access`
> metadata for the loop, which should do what you intend.
> >
> >
> > Cheers,
> >
> >   Johannes
> >
> >
> > On 6/21/20 11:56 AM, Bandhav Veluri via llvm-dev wrote:
> >
> > Hi,
> >
> > I'm trying to abstract some special pointers with a class, like in
the
> > example program below:
> >
> >   1 #define __remote __attribute__((address_space(1)))
> >   2 #include <stdint.h>
> >   3
> >   4 __remote int* A;
> >   5 __remote int* B;
> >   6
> >   7 class RemotePtr {
> >   8   private:
> >   9     __remote int* __restrict a;
> >  10
> >  11   public:
> >  12     RemotePtr(__remote int* a) : a(a) {}
> >  13
> >  14     __remote int& at(int n) {
> >  15       return a[n];
> >  16     }
> >  17 };
> >  18
> >  19 int main(int argc, char** argv) {
> >  20   RemotePtr a(A);
> >  21   RemotePtr b(B);
> >  22
> >  23   #pragma unroll 4
> >  24   for(int i=0; i<4; ++i) {
> >  25     a.at
>
<https://urldefense.com/v3/__http:/a.at__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLOY-9qOq$>(i)
> += b.at
>
<https://urldefense.com/v3/__http:/b.at__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLBIWnTL5$>
> (i);
> >  26   }
> >  27
> >  28   return 0;
> >  29 }
> >
> > It's given that pointer a, in each object of the class RemotePtr,
is the
> > only pointer that can access the array pointed by it. So, I tried
> __remote
> > int* __restrict a; (line 9) construct to tell Clang the same. This
> doesn't
> > seem to work and I see no noliass in the generated IR. Specifically, I
> want
> > lines 23-26 optimized assuming no aliasing between A and B. Any reason
> why
> > Clang shouldn't annotate memory accesses in lines 23-26 with
noaliass
> > taking line 9 into account?
> >
> > The higher level problem is this: is there a way to compile lines
23-26
> > assuming no aliasing between A and B, by just doing something in the
> > RemotePtr class (so that main is clear of ugly code)? If that's
not
> > possible, is there a way to tell Clang that lines 23-26 should assume
no
> > aliasing at all, by some pragma?
> >
> > Thank you,
> > Bandhav
> >
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
<https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLD9-lkj8$>
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
<https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLD9-lkj8$>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
<https://urldefense.com/v3/__https:/lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLD9-lkj8$>
>
>
>
> --
>
> [image: Image removed by sender.]
>
> *Neil Henning*
>
> Senior Software Engineer Compiler
>
> unity.com
>
<https://urldefense.com/v3/__http:/unity.com__;!!A4F2R9G_pg!KbQb7EC98K_vFVBRDQoSveXuMcvOvKcWvTers1QW_g1LmGPsOh-wI0r2mV_c2G1yLJhMkp2Z$>
>
>
>
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