[llvm-dev] SCEV related question

[vivek pandya via llvm-dev]

Here is original C code:
  void topup(int a[], unsigned long i) {

     for (; i < 16; i++) {

       a[i] = 1;

     }

   }

Here is the IR before the pass where I expect SCEV to return trip-count
value

 ; Function Attrs: nofree norecurse nounwind uwtable writeonly
 define dso_local void @topup(i32* nocapture %a, i64 %i) local_unnamed_addr
#0 {
 entry:
   %cmp3 = icmp ult i64 %i, 16
   br i1 %cmp3, label %for.body.preheader, label %for.end

 for.body.preheader:                               ; preds = %entry
   br label %for.body

 for.body:                                         ; preds =
%for.body.preheader, %for.body
   %i.addr.04 = phi i64 [ %inc, %for.body ], [ %i, %for.body.preheader ]
   %arrayidx = getelementptr inbounds i32, i32* %a, i64 %i.addr.04
   store i32 1, i32* %arrayidx, align 4, !tbaa !2
   %inc = add nuw nsw i64 %i.addr.04, 1
   %exitcond = icmp eq i64 %inc, 16
   br i1 %exitcond, label %for.end.loopexit, label %for.body

 for.end.loopexit:                                 ; preds = %for.body
   br label %for.end

 for.end:                                          ; preds =
%for.end.loopexit, %entry
   ret void
}

I have also checked that at a pass before above pass SCEV works and returns
16.

Here is the IR before a pass where SCEV works as per the expectations:
 ; Preheader:

 for.body.preheader:                               ; preds = %entry

   br label %for.body



 ; Loop:

 for.body:                                         ; preds =
%for.body.preheader, %for.body
   %i.addr.04 = phi i64 [ %inc, %for.body ], [ %i, %for.body.preheader ]

   %arrayidx = getelementptr inbounds i32, i32* %a, i64 %i.addr.04

   store i32 1, i32* %arrayidx, align 4, !tbaa !2

   %inc = add nuw nsw i64 %i.addr.04, 1

   %cmp = icmp ult i64 %inc, 16

   br i1 %cmp, label %for.body, label %for.end.loopexit



 ; Exit blocks

 for.end.loopexit:                                 ; preds = %for.body

   br label %for.end

- Vivek

On Mon, Aug 26, 2019 at 2:09 AM Sanjoy Das <sanjoy at playingwithpointers.com>
wrote:

> On Sun, Aug 25, 2019 at 4:49 AM vivek pandya via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> >
> > Hello,
> >
> > I am first time paying with SCEV codebase.
> > I am trying to find out why ScalarEvolution is not able to give correct
> back edge taken count for an expression.
> >
> > So in my case flow reaches to howFarToZero() and in that function, I
> have following expressions as SCEV
> >
> > Start = (15 + (-1 * %i) (which is set to Distance SCEV)
> > Step = 1
> >
> > now, first of all, should I expect Start as ConstantSCEV (15) instead of
> the whole expression
> > the problem here is getUnsignedRangeMax(Distance) returns very large
> number because of -1 in the SCEV.
>
> Given you have have above, in howFarToZero SCEV is reasoning about the
> loop as if it were:
>
> unsigned i = ...;
> unsigned Start = 15 - i;
> while (Start != 0) {
>   unsigned Step = 1;
>   Start = Start + Step;
> }
>
> > How we can make this work? Here we can clearly say that after 15 steps
> this expression will be 0
>
> and so I don't think this assertion ("after 15 steps this expression
> will be 0") is correct.
>
> If you post a minimal example we can try to figure out if SCEV can be
> smarter here.
>
> -- Sanjoy
>
> > and thus we have a value for backedge taken count.
> >
> > Any help will be appriciated.
> >
> > Thanks,
> > Vivek
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20190826/013b56bc/attachment.html>