[llvm-dev] Restrict qualifier on class members

Jeroen Dobbelaere via llvm-dev
Mon, 22 Jun 2020 08:50:33 -0000

Hi Bandhav,


> 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


This is indeed the correct way to interpret the restrict annotation.
With the full restrict patches, this code is vectorized to:

; Function Attrs: norecurse nounwind uwtable
define dso_local i32 @main(i32 %argc, i8** nocapture readnone %argv)
local_unnamed_addr #0 !noalias !2 {
entry:
  %0 = load i32 addrspace(1)*, i32 addrspace(1)** @A, align 8, !tbaa !5,
!noalias !9
  %1 = load <4 x i32> addrspace(1)*, <4 x i32> addrspace(1)**
bitcast (i32 addrspace(1)** @B to <4 x i32> addrspace(1)**), align 8,
!tbaa !5, !noalias !9
  %2 = load <4 x i32>, <4 x i32> addrspace(1)* %1, align 4, !tbaa
!13
  %3 = bitcast i32 addrspace(1)* %0 to <4 x i32> addrspace(1)*
  %4 = load <4 x i32>, <4 x i32> addrspace(1)* %3, align 4, !tbaa
!13
  %5 = add nsw <4 x i32> %4, %2
  %6 = bitcast i32 addrspace(1)* %0 to <4 x i32> addrspace(1)*
  store <4 x i32> %5, <4 x i32> addrspace(1)* %6, align 4, !tbaa !13
  ret i32 0
}

Greetings,

Jeroen Dobbelaere



From: Johannes Doerfert <johannesdoerfert at gmail.com>
Sent: Sunday, June 21, 2020 19:22
To: Bandhav Veluri <bandhav.veluri00 at gmail.com>; llvm-dev <llvm-dev
at lists.llvm.org>
Cc: Jeroen Dobbelaere <dobbel at synopsys.com>; Kruse, Michael
<michael.kruse at anl.gov>
Subject: Re: [llvm-dev] Restrict qualifier on class members


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!Pjti9CWiUzKTT2q0q845iVs3V_xoVOwy0jfjcORjShtA3AY2yyxst36qT7R2WV_qjrJfjPOL$>.



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!Pjti9CWiUzKTT2q0q845iVs3V_xoVOwy0jfjcORjShtA3AY2yyxst36qT7R2WV_qjiZydN-O$>

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





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