[llvm-dev] SCEV cannot compute the trip count of Simple loop

[Kevin Choi via llvm-dev]

void foo(int x) {

  int p, i = 1;

  int mat[6][6][6];

  for (p = x+3 ; p<= x+6 ;p++)

    mat[x][p][i] = mat[x][p][i] + 5;

}


When x=0, max(p)=6, which is outside of allocated 3d array, which is UB.

On Fri, Sep 16, 2016 at 12:31 AM, Rai, Deepali via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi All,
>
>
>
> I am trying to unroll the below loop, but couldn’t as SCEV returns
> TripCount as 0.
>
>
>
> void foo(int x) {
>
>   int p, i = 1;
>
>   int mat[6][6][6];
>
>   for (p = x+3 ; p<= x+6 ;p++)
>
>     mat[x][p][i] = mat[x][p][i] + 5;
>
> }
>
>
>
> For a quick reference I have added the generated IR compiled with clang
> using –O3.
>
> Please let me know if this is an known issue in SCEV or I am missing
> something here ?
>
>
>
> ; Function Attrs: nounwind readnone uwtable
>
> define void @_Z3fooi(i32 %x) local_unnamed_addr #0 {
>
> entry:
>
>   %mat = alloca [6 x [6 x [6 x i32]]], align 16
>
>   %0 = bitcast [6 x [6 x [6 x i32]]]* %mat to i8*
>
>   call void @llvm.lifetime.start(i64 864, i8* %0) #2
>
>   %add = add nsw i32 %x, 3
>
>   %add1 = add nsw i32 %x, 6
>
>   %idxprom3 = sext i32 %x to i64
>
>   %1 = sext i32 %add to i64
>
>   %2 = sext i32 %add1 to i64
>
>   br label %for.body
>
>
>
> for.body:                                         ; preds = %for.body,
> %entry
>
>   %indvars.iv = phi i64 [ %indvars.iv.next, %for.body ], [ %1, %entry ]
>
>   %arrayidx5 = getelementptr inbounds [6 x [6 x [6 x i32]]], [6 x [6 x [6
> x i32]]]* %mat, i64 0, i64 %idxprom3, i64 %indvars.iv, i64 1
>
>   %3 = load i32, i32* %arrayidx5, align 4, !tbaa !1
>
>   %add6 = add nsw i32 %3, 5
>
>   store i32 %add6, i32* %arrayidx5, align 4, !tbaa !1
>
>   %indvars.iv.next = add nsw i64 %indvars.iv, 1
>
>   %cmp = icmp slt i64 %indvars.iv, %2
>
>   br i1 %cmp, label %for.body, label %for.end
>
>
>
> for.end:                                          ; preds = %for.body
>
>   call void @llvm.lifetime.end(i64 864, i8* nonnull %0) #2
>
>   ret void
>
> }
>
>
>
> Thanks,
>
> Deepali
>
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>
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