[LLVMdev] confusion w.r.t. scalar evolution and nuw

[Sanjoy Das]

Here is a small program that illustrates the bug:

define void @f(i32* %loc) {
 entry:
  br label %loop

 loop:
  %idx = phi i32 [ 6, %entry ], [ %idx.dec, %loop ]
  store i32 %idx, i32* %loc
  %idx.dec = sub nuw i32 %idx, 1
  %cond = icmp uge i32 %idx.dec, 5
  br i1 %cond, label %loop, label %exit

 exit:
  ret void
}

The store is supposed to happen exactly twice.  But opt -indvars turns
it into an infinite loop:

define void @f(i32* %loc) {
entry:
  br label %loop

loop:                                             ; preds = %loop, %entry
  %idx = phi i32 [ 6, %entry ], [ %idx.dec, %loop ]
  store i32 %idx, i32* %loc
  %idx.dec = sub nuw nsw i32 %idx, 1
  br i1 true, label %loop, label %exit

exit:                                             ; preds = %loop
  ret void
}


because %idx.dec, which is {5,+,-1}<nuw><%loop> can never be ult 5.

On Wed, Jan 14, 2015 at 10:28 PM, David Majnemer
<david.majnemer at gmail.com> wrote:
> On Wed, Jan 14, 2015 at 10:14 PM, Sanjoy Das
> <sanjoy at playingwithpointers.com> wrote:
>>
>> > We are permitted to turn 'sub nsw i32 %x, 1' into 'add nsw i32 %x, -1'
>>
>> nsw also has the same problem:
>>
>> sub nsw int_min, int_min is 0 but
>> add nsw int_min, (-int_min) is poison
>
>
> Right, it doesn't work with all values.  I was specifically referring to the
> %x and -1 case.
>
>>
>>
>> -- Sanjoy
>
>