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

[Mehdi Amini via llvm-dev]

> On Sep 16, 2016, at 4:45 AM, Philip Pfaffe via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> There is no way to express the trip count in a closed expression.
> 
> E.g., if I call foo(MAX_INT - 2), the loop condition will evaluate to false immediately.

Are you implying that because of wrapping?
Signed integer wrapping is UB.

— 
Mehdi


> 
> Best,
> Philip
> 
> 2016-09-16 10:44 GMT+02:00 Rai, Deepali via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>:
> 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 <mailto:code.kchoi at gmail.com>] 
> Sent: Friday, September 16, 2016 1:20 PM
> To: Rai, Deepali
> Cc: llvm-dev at lists.llvm.org <mailto: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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