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

[Rai, Deepali via llvm-dev]

I have modified the example test case for UB error, still it didn’t unroll

void foo(int x) {
  int p, i = 1;
  int mat[9][9][9];
  for (p = (x+1) ; p < (x+3) ;p++)
    mat[x][p-1][i] = mat[x][p-1][i] + 5;
}

Regard,
Deepali

From: Kevin Choi [mailto:code.kchoi at gmail.com]
Sent: Friday, September 16, 2016 1:20 PM
To: Rai, Deepali
Cc: llvm-dev at lists.llvm.org
Subject: Re: [llvm-dev] SCEV cannot compute the trip count of Simple loop

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<mailto: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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